Next Article in Journal
Analysis of the Impact of Environmental and Agronomic Variables on Agronomic Parameters in Soybean Cultivation Based on Long-Term Data
Next Article in Special Issue
Research Progress of Grassland Ecosystem Structure and Stability and Inspiration for Improving Its Service Capacity in the Karst Desertification Control
Previous Article in Journal
Microencapsulation of Bacillus Strains for Improving Wheat (Triticum turgidum Subsp. durum) Growth and Development
Previous Article in Special Issue
Agents Affecting the Plant Functional Traits in National Soil and Water Conservation Demonstration Park (China)
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Plants
Volume 11
Issue 21
10.3390/plants11212921
plants-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Leaf Photosynthetic and Functional Traits of Grassland Dominant Species in Response to Nutrient Addition on the Chinese Loess Plateau

by
Yuan Jin
1,†,
Shuaibin Lai
1,†,
Zhifei Chen
2,
Chunxia Jian
1,
Junjie Zhou
1,
Furong Niu
3 and
Bingcheng Xu
1,4,*
1
State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Northwest A&F University, Xianyang 712100, China
2
College of Life Sciences, Guizhou University, Guiyang 550025, China
3
College of Forestry, Gansu Agricultural University, Lanzhou 730070, China
4
Institute of Soil and Water Conservation, Chinese Academy of Sciences and Ministry of Water Resources, Xianyang 712100, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Plants 2022, 11(21), 2921; https://doi.org/10.3390/plants11212921
Submission received: 11 June 2022 / Revised: 19 October 2022 / Accepted: 27 October 2022 / Published: 30 October 2022
(This article belongs to the Special Issue Ecophysiology and Ecology of Grassland)
Browse Figures
Versions Notes

Abstract

:
Leaf photosynthetic and functional traits of dominant species are important for understanding grassland community dynamics under imbalanced nitrogen (N) and phosphorus (P) inputs. Here, the effects of N (N0, N50, and N100, corresponding to 0, 50, and 100 kg ha−1 yr−1, respectively) or/and P additions (P0, P40, and P80, corresponding to 0, 40, and 80 kg ha–1 yr–1) on photosynthetic characteristics and leaf economic traits of three dominant species (two grasses: Bothriochloa ischaemum and Stipa bungeana; a leguminous subshrub: Lespedeza davurica) were investigated in a semiarid grassland community on the Loess Plateau of China. Results showed that, after a three-year N addition, all three species had higher specific leaf area (SLA), leaf chlorophyll content (SPAD value), maximum net photosynthetic rate (PNmax), and leaf instantaneous water use efficiency (WUE), while also having a lower leaf dry matter content (LDMC). The two grasses, B. ischaemum and S. bungeana, showed greater increases in PNmax and SLA than the subshrub L. davurica. P addition alone had no noticeable effect on the PNmax of the two grasses while it significantly increased the PNmax of L. davurica. There was an evident synergetic effect of the addition of N and P combined on photosynthetic traits and most leaf economic traits in the three species. All species had relatively high PNmax and SLA under the addition of N50 combined with P40. Overall, this study suggests that N and P addition shifted leaf economic traits towards a greater light harvesting ability and, thus, elevated photosynthesis in the three dominant species of a semiarid grassland community, and this was achieved by species–specific responses in leaf functional traits. These results may provide insights into grassland restoration and the assessment of community development in the context of atmospheric N deposition and intensive agricultural fertilization.

1. Introduction

The semiarid Loess Plateau region in China is characterized by severe water scarcity, soil erosion, and nutrient-poor soils, which all greatly limit vegetation growth [1]. Grassland, with a size of ~2.7 × 105 km2, accounts for ca. 43% of the regional total land area and is the dominant vegetation type on the Plateau [2]. It provides essential ecosystem functions and services such as carbon sequestration, soil and water conservation, and biodiversity [3,4]. Grassland management and restoration are of great ecological and economic significance in the region. Fertilization, as an effective management practice to increase grassland productivity and promote grassland restoration, is adopted in many natural/semi-natural grasslands, e.g., in the Inner Mongolian steppe [5], the alpine meadow on the Qinghai–Tibet Plateau [6] in China, and in the European alpine grasslands [7] and the North American Great Plains [8]. However, fertilization may also lead to undesirable consequences such as exacerbated eutrophication [9], reduced ecosystem resilience (e.g., increased grassland drought sensitivity [8,10]), and biodiversity loss [11,12,13], which subsequently alter community composition and structure, as well as decrease the effects of diversity stability on maintaining grassland productivity [14]. Soils in the semiarid Loess Plateau are commonly deficient in both nitrogen (N) and phosphorus (P) [15]. Previous studies in the region have shown that appropriate N and P fertilization could improve soil N and P availability and play a positive role in increasing grassland productivity and recovery of degraded grasslands [16,17,18]. On the other hand, the effects of the N and P addition on photosynthetic and leaf functional traits of dominant species, which are important for understanding underlying mechanisms of grassland community dynamics under N and P inputs, have only received limited attention in the regional grassland.
Plant photosynthesis is the basis of plant growth, and its diurnal patterns reflect the sustained carbon assimilation ability of plants and have been extensively studied across a wide range of arid and semiarid grassland species, particularly in North America (e.g., [19,20,21,22]). Previous studies have explored the photosynthetic diurnal dynamics—under elevated CO2 conditions—in dry and wet years [22,23], the diurnal photosynthetic performance of grasses with contrasting functional types (e.g., C3 vs. C4, invasive vs. native) [20,21], and biotic and abiotic controls of photosynthetic diurnal courses [19,24]. However, there is limited information on the impacts of nutrient addition on photosynthetic diurnal courses in dryland grassland species. As an essential element of all proteins in plants (e.g., nucleic acid, enzymes, and chlorophyll), N primarily determines plant photosynthetic performance [25,26]. Extensive studies have documented that N addition increases plant photosynthetic rate and promotes plant growth in grasslands [27,28], and this promotion may be mediated by soil moisture [10]. However, when N addition exceeds a threshold, it will not continue to increase plant photosynthesis or, even, inhibit plant growth [26,29]. P, as another essential macronutrient, is also vital for plant photosynthesis, and it is the main component of ATP, NADPH, and phospholipids, which all play important roles in regulating photosynthesis machinery and electron transport activities [30,31]. Apart from the direct regulation of N/P on plant photosynthesis, N and P addition could indirectly or interactively affect plant photosynthesis. For instance, P addition could improve photosynthesis by increasing leaf area and stomatal aperture, particularly under soil water deficit conditions [32,33]. P addition could increase the activity of N-fixing bacteria, nodule biomass, and nitrogenase activity in legumes, which subsequently increases leaf N and P content and photosynthetic rate [34]. The combined fertilization of N and P is, thereby, often considered an effective management tool for sustaining productivity in many grassland communities, while such effects need to be evaluated in the regional semiarid grassland on the Loess Plateau.
Besides photosynthetic characteristics, other important leaf functional traits, such as specific leaf area (SLA), leaf dry matter content (LDMC), leaf nitrogen mass (Nmass), and leaf phosphorus mass (Pmass), are also the intuitive representation of strategies adopted by plants to cope with environmental changes [35,36]. According to the leaf economic spectrum (LES) theory [37], angiosperm plants could generally be divided into a rapid/slow growing strategy according to a set of leaf functional traits, with the rapid-growing ones having low LDMC and high Pn, SLA, Nmass, and Pmass; contrarily, the slow-growing ones have the opposite leaf traits. This kind of functional trait-based theory, from leaf to plant levels, provides great insights into understanding species resource utilization and species distribution [37]. Nevertheless, species-specific responses in these leaf economic traits exist under varied environmental (e.g., under different soil water and nutrient availability) conditions, which should be systematically assessed [38]. Efforts have been made to quantify the variation of LES of grassland species under differed soil nutrient availability, which confirmed species-specific patterns [39], while the assessment of species-specific responses to varying N and P fertilization conditions is seldom conducted on the semiarid Loess Plateau, which is needed to better understand grassland community development.
The dominant species occupy important ecological niches and play vital roles in maintaining community structure and function [40,41]. Biomass increases after N/P additions tend to be achieved by decreasing species diversity or increasing the biomass of dominant species [42]. Quantifying physiological and growth characteristics of dominant species under N and/or P addition could, thereby, be important for the evaluation of community productivity and dynamics, as well as provide valuable information for grassland management and restoration. Bothriochloa ischaemum (L.) Keng (a C4 perennial grass), Stipa bungeana Trin. (a C3 perennial grass), and Lespedeza davurica (Laxm.) Schindl. (a C3 N-fixing subshrub) are co-dominant species in the natural/restored grasslands on the semiarid Loess Plateau of China [43]. The previous study on a regional grassland community, targeting these species, has shown that the addition of N and P combined improved grassland productivity and decreased species diversity, primarily via effects on tall clonal and annual species [44], which, once again, suggested species or functional-type-specific responses within a community, while the variation of leaf functional traits in these dominant species, after N and P fertilization, have not been fully assessed. Thus, we examined photosynthetic diurnal change, SPAD value, and leaf economic traits, including Nmass, Pmass, Nmass/Pmass ratio, SLA, and LDMC of the three dominant species, following a three-year N and P addition experiment in a typical semiarid grassland community on the Loess Plateau. We tested the hypotheses that: (1) N/P addition would increase photosynthetic rates and alter the photosynthetic diurnal dynamics of the three dominant species in the peak growing season, and these photosynthetic responses would be related to species-specific shifts in leaf functional traits; (2) addition of N and P combined would further promote photosynthesis compared with N/P additions alone.

2. Results

2.1. Environmental Factors

Photosynthetically active radiation (PAR) and air temperature (Ta) showed a single-peaked diurnal curve during the experimental period, and the maximum values appeared at 12:00 h and 14:00 h, with the values of 1854 μmol·m−2·s−1 and 30.2 °C, respectively (Figure 1). The relative humidity (RH) remained stable during the daytime (~13%) (Figure 1).

2.2. Diurnal Changes in Photosynthesis

The diurnal changes of net photosynthetic rate (Pn) and leaf instantaneous water use efficiency (WUE) of the three dominant species showed a double-peak curve under different N and P addition treatments. The first peak appeared at 10:00 h, the second at 14:00 h, and the midday depression of the photosynthesis (so-called “noon break”) appeared at around 12:00 h (Figure 2). The leaf transpiration rate (Tr) of B. ischaemum mostly showed a double-peak diurnal course. While diurnal changes of Tr in S. bungeana and L. davurica showed a single peak.
Compared with CK (i.e., N0P0), N addition alone and addition of N and P combined significantly increased the Pn values at 10:00 h and 14:00 h in the three species (except under N50P40 and N50P80 treatments in S. bungeana). The greatest Pn values appeared at 10:00 h under N and P combined treatments (i.e., N50P40 and N50P80) for the three species. The WUE of the three species significantly increased by N addition alone compared with CK (Table 1). The WUE of L. davurica increased significantly under all levels of P alone additions, while the WUE of the two grasses only significantly increased under N0P40 (Table 1). Under N and P combined addition, the maximum WUE values of B. ischaemum, S. bungeana, and L. davurica were 1.17, 1.09, and 1.47 μmol mmol−1, respectively (Table 1).
N addition, alone, significantly increased the Ls values of B. ischaemum and L. davurica (Table 1). P addition, alone, increased (p < 0.05) and decreased (p < 0.05) the Ls of L davurica and S. bungeana, respectively, while only significantly increasing the Ls of B. ischaemum under N0P80 treatment. N and P interaction significantly affected the Ls values of the three species (Table 1). Under the addition of N50 combined with P, the Ls of L. davurica increased, and those of B. ischaemum decreased (both p < 0.05). Under the addition of N100 combined with P, the Ls of B. ischaemum significantly increased, while those of S. bungeana decreased significantly (p < 0.05; Table 1).
N addition, alone, increased the PNmax values of the three species (p < 0.05), while there was no difference between them under N50 and N100 (Figure 3). The increase in PNmax in the two grasses was about two times larger than those of L. davurica under N addition alone (Figure 3). P addition, alone, increased the PNmax values of L. davurica and S. bungeana (p < 0.05; Figure 3). N and P combined addition only significantly affected the PNmax values of S. bungeana and L. davurica (p < 0.05; Figure 3).

2.3. Leaf SPAD Value

N addition, alone, significantly increased the SPAD values of the three species (Figure 4). P addition, alone, significantly increased the SPAD values of B. ischaemum and L. davurica (p < 0.05), while SPAD values only increased under low levels of P addition alone (i.e., N0P40) in S. bungeana (p < 0.05). Addition of N and P combined significantly increased the SPAD values of both B. ischaemum and L. davurica (Figure 4).

2.4. Leaf N and P Concentration (Nmass and Pmass) and Nmass/Pmass Ratio

N addition, alone, significantly increased the Nmass values of the two grasses, but it had no effects on Nmass of L. davurica. The high level of P addition, alone (N0P80), significantly increased Pmass of all species except L. davurica (Figure 5). N and P interaction significantly affected Nmass of the three species, while it only significantly affected Pmass of the two grasses. Under the addition of N50 combined with P, the Pmass of B. ischaemum and S. bungeana, as well as the Nmass of L. davurica, increased significantly. Under the addition of N100 combined with P, the Pmass of S. bungeana increased significantly, while those of L. davurica decreased significantly (Figure 5).
N addition, alone, significantly affected the Nmass/Pmass ratios of all three species. The low level of N addition alone (N50P0) significantly increased the Nmass/Pmass of the two grasses; the high level of N addition, alone (N100P0), only increased the Nmass/Pmass of S. bungeana grass (p < 0.05, Figure 5). N and P addition interaction significantly affects the Nmass/Pmass of the two grasses but has no effect on the subshrub. Under the addition of N50 combined with P, the Nmass/ Pmass of B. ischaemum and S. bungeana decreased significantly, while the Nmass/Pmass of L. davurica increased under N50P40 treatment (p < 0.05). Under the addition of N100 combined with P, the Nmass/ Pmass of S. bungeana decreased significantly, while the Nmass/Pmass of B. ischaemum and L. davurica had no significant changes (Figure 5).

2.5. Specific Leaf Area (SLA) and Leaf Dry Matter Content (LDMC)

N addition, alone, significantly increased the SLA values of the three species, and the low level of N addition alone (N50P0) decreased the LDMC of the two grasses (p < 0.05, Figure 6). P addition, alone, significantly increased SLA of L. davurica, while it had limited effects on the two grasses (Figure 6). N and P addition interaction significantly affected both the SLA and LDMC values of all three species. Under the addition of N50 combined with P, the SLA of B. ischaemum and L. davurica increased significantly (p < 0.05), and the SLA of S. bungeana increased under N50P40, while it decreased significantly under N50P80 (p < 0.05); LDMC was comparable between different levels of P additions in the two grasses, but it decreased in L. davurica. Under the addition of N100 combined with P, the SLA values of B. ischaemum and L. davurica increased significantly, and the SLA of S. bungeana increased (p < 0.05) only under the high level of N addition (N100P80); LDMC, among different levels of P addition (i.e., N100P40 and N100P80), was comparable in B. ischaemum and L. davurica, while it significantly decreased under N100P80 in S. bungeana (Figure 6).
PNmax was positively correlated with WUE, SPAD, Nmass, and SLA in all three species, while it was negatively correlated with LDMC in S. bungeana and L. davurica (Figure 7). PCA analysis showed that the variance explained by the first and second principal components was 37.5% and 23.5%, respectively, with a total value of 61% (Figure 8). The first principal component had a high correlation with PNmax, WUE, Pmass, and SLA; the second principal component had high correlation with LDMC. SPAD, Nmass, and Nmass/ Pmass ratio are correlated with both principal components (Figure 8). In a score plot of PCA analysis, under N addition alone, B. ischaemum and L. davurica gradually moved to the right with N addition level, and S. bungeana gradually moved to the upper right. Under P addition, alone, L. davurica gradually moved to the right with P addition level, and B. ischaemum slightly moved to the upper right. Under the addition of N combined with P, with the increase in fertilizer application, all three species moved towards higher PNmax, WUE, SPAD, and SLA values (Figure 9).

3. Discussion

The diurnal dynamics of photosynthesis reflect plants’ sustained ability to carry out physiological metabolism and biomass accumulation throughout the daytime [45,46], which have been extensively studied in numerous dryland species (e.g., [19,20,21,22]). Our results, corroborated with others, showed that the diurnal course of photosynthetic rate showed a double-peaked curve in the three grassland dominant species, and they showed an evident “noon break” of photosynthesis. The noon break is a mechanism to avoid stresses such as excess light, high temperature, and water deficit during the midday [47], and it is a result of stomatal or non-stomatal restriction [48]. Our measurements showed that, during the period of 10:00–12:00 h, the Pn values gradually decreased while the Ls values increased (Figure 2 and Table 1), suggesting the “noon break” was likely caused by stomatal limitation [46,48]. In line with our first hypothesis and consistent with others (e.g., [49,50]), the Pn values of the three species considerably increased by nutrient addition, particularly during the peak photosynthetic period (~10:00 h). This is expected since environmental conditions (e.g., light and temperature) are relatively optimal for photosynthesis during this period, hence the promotion of nutrient addition would be most effective. In addition, during the noon time with high air temperature and light radiation, the Pn was slightly increased after fertilization, which may be due to increased stomatal conductance due to N and P addition, and this ostensibly alleviated the “noon break” [51]. Our study indicated that N and P fertilization could improve the photosynthetic ability of the three species at the diurnal scale and increase the daily accumulative carbon assimilation. However, we only focused on short-term responses during the peak growing season (i.e., July), so future studies should be taken to further assess intra- or interannual patterns of their photosynthesis to better understand the long-term effects of fertilization.
Both N and P are essential elements of key compounds involved in the photosynthetic process, and appropriate N and P additions would increase the content of these compounds and, subsequently, the photosynthetic rate [17,26,30]. This was observed in our study: the PNmax, SPAD, and Nmass of the three species increased significantly after N addition alone, and there were strong positive correlations between Nmass, SPAD, and PNmax (p < 0.05; Figure 3, Figure 4 and Figure 5 and Figure 7). The increase in PNmax and WUE with the N addition level was much greater (larger regression slopes) in the two grass species than in the legume L. davurica (Table 2), with the greatest increase (the largest slope) of PNmax and WUE, along with the N addition level, in C4 grass B. ischaemum (Table 2). Together, this suggested that the two grasses were more sensitive to N addition alone than the legume. This is consistent with our previous study quantifying the plant biomass of B. ischaemum and L. davurica mixtures under varying soil moisture and nutrient supplies [52]. We suspect that the subshrub L. davurica may not be N-limited due to its N fixation ability and is, thereby, insensitive to exogenous N fertilization. On the other hand, the photosynthetic rate does not continuously increase with N addition amounts after passing a threshold [29,53], which was also observed, here, as the PNmax values of the three species were not significantly different between N50 and N100 (Figure 3 and Figure 6; Table 1). Fossil fuel combustion and extensive fertilization have greatly increased atmospheric N deposition globally in recent decades [54]. Chronic N input by long-term N deposition may, hence, alleviate N limitation and promote plant photosynthesis and growth of regional grassland species, but it may, meanwhile, intensify plant P limitation by increasing P demand [55,56].
Here, the P addition, alone, had greater effects on PNmax, SPAD, and SLA of the leguminous L. davurica among the three species (Figure 3, Figure 4 and Figure 6; Table 2). This may be ascribed to P, as it could promote the activity of nitrogenase in the root nodules of legumes and enhance their photosynthesis [34,57], and elevated leaf P content can also directly improve photosynthetic capacity by promoting ATP and NADPH synthesis, as well as regeneration of RuBP [33,58]. Compared with N or P addition alone, the three species had higher PNmax, WUE, and SLA values under N and P combined additions, suggesting a synergetic effect of N and P on plant photosynthesis (Figure 5 and Figure 6). This confirms our second hypothesis and suggests that appropriate N and P combined fertilization should be considered to maintain regional grassland productivity. A myriad of studies have documented this synergetic effect in grasslands worldwide (e.g., [59,60]). Previous studies in the Loess Plateau grasslands also reported the N and P combination had synergetic effects on community productivity [44]. A recent long-term (over 66 years) nutrient addition study in a mesic grassland in South Africa also highlighted that N and P combined addition promoted plant P acquisition and uptake (e.g., increased organic P storage, P recycling, and plant P utilization), which may contribute to the synergetic effect of N and P combined addition [59].
Drylands (e.g., the semiarid Loess Plateau) are often co-limited by water and nutrients [8], as well as characterized by frequent drought events, which greatly impact plant N and P uptake [61]. Nutrient addition, such as N, at an appropriate rate could improve post-drought recovery of grassland and increase the aboveground biomass production [62]. Contrarily, some studies reported that nutrient addition increased grassland drought sensitivity and constrained its recovery from drought events [10]. Besides, grass species with different photosynthetic pathways (C3 vs. C4) may respond differentially to drought and rewatering under nutrient addition conditions [63]. For the regional grassland, previous studies have quantified the photosynthetic responses of dominant species following rainfall events and reported species-specific patterns [64]. Nevertheless, the interaction of soil moisture (especially drought) and fertilization on dominant species performance remains less understood in the regional grassland and should be assessed, considering recurrent drought events, under future climate scenarios [65].
Leaf functional traits, particularly those so-called economic traits, are invoked to explain plant resource acquisition and utilization [36]. Among them, the leaf Nmass/ Pmass ratio indicates environmental N and P availability where the plant grows [56]. In general, Nmass/ Pmass ratio less than 10 indicates the N limitation, and greater than 20 indicates the P limitation [56]. The leaf Nmass/ Pmass ratio of the three species, averaged across treatments, was 18.8 (B. ischaemum), 11.3 (S. bungeana), and 24.3 (L. davurica), respectively (Figure 5), suggesting species-specific N and P limitations. The Nmass/Pmass ratio of the two grasses increased significantly with N addition, while no noticeable change was found in L. davurica (Figure 5). This indicates that N addition may lead to P limitation in the two grass species. Meanwhile, increased soil N and P availability would release plants from nutrient competition to other resource competition, such as light and water [66]. Grassland dominant species may accordingly alter their leaf functional traits to maximize light harvesting to maintain dominance. According to the LES theory, plants with higher light capture, resource acquisition, and turnover capacity show higher SLA, Nmass, and Pmass in contrast to the slow-growth ones with higher LDMC and conservative nutrient resource use [36]. Similar to other studies (e.g., [66]), the three species studied here shifted to a fast-growth strategy after N addition with larger, thinner, and N-rich leaves (higher SLA, Nmass, and SPAD), as well as higher assimilation rate per unit leaf area (higher PNmax). Though score plots from PCA analysis indicated that, under N addition, three species adopted different strategies to improve their light harvesting: C4 grass B. ischaemum mainly by increasing SLA and PNmax, while C3 grass S. bungeana and C3 subshrub L. davurica primarily increased leaf N content and SPAD (Figure 9), and only L. davurica had notable shifts in photosynthetic and leaf functional traits under P additions (Figure 9B), which suggests that the three species had different trade-off strategies in photosynthetic performance and leaf economic traits in response to N and/or P addition [17,66], and these should be considered when assessing N and P fertilization effects on community structure and functions. The PNmax values of the three species were mostly highest under the ‘N50P40′ treatment among all treatments, indicating that it could be considered an optimal fertilization measure for improving grassland production.

4. Materials and Methods

4.1. Site Description

This work was conducted at the Zhifanggou watershed (109°13’46’’–109°16’03’’ E, 36°42’42’’–36°46’28’’ N), located in the Ansai District, Yan’an City, Shaanxi Province, China. It has a semiarid continental monsoon climate. The mean annual temperature is 8.8 °C, with the lowest temperature being −6.9 °C in January and the highest being 22.6 °C in July. The mean annual rainfall is 507 mm. The soil is classified as Calcaric Cambisol. Rainfall shows a highly seasonal variability with ca. 82% occurring from May to September (the growing season). The soil available N, P, and K were 20.9–71.3 mg kg−1, 1.6–2.8 mg kg−1, and 10.07–30.97 g kg−1, respectively, and soil pH was 8.4–8.8 [67]. The targeted grassland is dominated by xerophytic plants, e.g., B. ischaemum, S. bungeana, L. davurica, Artemisia sacrorum, and Artemisia giraldii.

4.2. N and P addition

A grassland community (20 × 30 m) was fenced to exclude grazing since May 2017. A randomized split-plot design with three N addition rates at the main plot level and three P addition rates at the subplot level was carried out. The main plot was 4 × 4 m, and N addition rates were N0 (0 kg N), N50 (50 kg N ha−1 yr−1), and N100 (100 kg N ha−1 yr−1). The N50 and N100 treatments were about 2 and 4 times the annual average N deposition rate in the loess hilly area [~21.76 kg (N) ha−1 yr−1] [68]. N was applied as calcium ammonium nitrate [5Ca(NO3)2 NH4NO3 10H2O] (15.5% of N). Each main plot was divided into four subplots (2 × 2 m). P was applied as triple superphosphate [Ca(H2PO4)2·H2O] (45% of P), and the addition rates were set to 0, 1, and 2 times the local fertilization rate, corresponding to P0 (0 kg P2O5), P40 (40 kg P2O5 ha−1 yr−1), and P80 (80 kg P2O5 ha−1 yr−1) [44].
Totally, there were 9 treatments, including a control (N0P0), two N addition alone treatments (N50P0, N100P0), two P addition alone treatments (N0P40, N0P80), four N and P combined addition treatments (N50P40, N50P80, N100P40, N100P80), and three replicates per treatment. N and P additions were conducted once a year, on rainy days, from 2017–2019 (4 June 2017, 21 May 2018, and 13 June 2019).

4.3. Ecophysiological Measurements

4.3.1. Diurnal Variations of Photosynthesis

The portable photosynthesis system (CIRAS-2, PP Systems, Amesbury, MA, USA) was used to measure the diurnal changes of photosynthesis of B. ischaemum, S. bungeana, and L. davurica, successively, and all measurements were conducted on three consecutive sunny days from 20–22 July 2019 (one species per day). The measurement was taken on one newly fully-expanded healthy leaf per species per treatment from 8:00–18:00 h with 2 h intervals. The measured parameters include net photosynthetic rate (Pn, μmol·m−2·s−1), transpiration rate (Tr, mmol·m−2·s−1), intercellular CO2 concentration (Ci, μmol·mol−1), and environmental factors, including photosynthetically active radiation (PAR, μmol·m−2·s−1), air temperature (Ta, °C), and relative humidity (RH, %). The photosynthetic rate at 10:00 h was taken as the maximum net photosynthetic rate (PNmax, μmol·m−2·s−1). Instantaneous water use efficiency (WUE, μmol mmol−1) was calculated as Pn/Tr. Stomatal limitation value (Ls) was derived by 1−Ci/Ca [48].

4.3.2. Leaf SPAD Value

Leaf SPAD value (a measure of leaf relative chlorophyll content) was measured on three newly fully-expanded healthy leaves per species per treatment using a chlorophyll meter (SPAD-502 model, Konica-Minolta, Osaka, Japan) on 20–22 July 2019.

4.3.3. Leaf Functional Traits

The 10–20 newly-fully expanded healthy leaves were randomly sampled per species per treatment, stored in zipped plastic bags, and quickly taken back to the laboratory, in an insulated box with ice packs, for leaf functional traits measurements. Leaves were weighed with an analytical balance (d = 0.0001 g). The fresh leaves were scanned (Epson duplex scanner, Epson, Tokyo, Japan), and the leaf area was derived using ImageJ (National Institutes of Health, Bethesda, MD, USA). Then, leaves were oven-dried at 75 °C for 24 h and ground with a high-throughput tissue grinder (MM-400, Retsch, Haan, Germany). Specific leaf area (SLA, m2 g–1) was calculated as leaf area divided by leaf dry mass. Leaf dry matter content (LDMC, g g–1) was calculated as leaf dry mass divided by fresh mass. After digestion with H2SO4-HClO4, the mass-based leaf N concentration (Nmass) was obtained using a Kjeldahl N analyzer (FOSS-8400, Foss, Höganäs, Denmark). The mass-based leaf P concentration (Pmass) was determined by a molybdenum blue colorimetry (UV-2600 ultraviolet-visible spectrophotometer, Shimadzu, Kyoto, Japan). Nmass/Pmass ratio was then calculated.

4.4. Statistical Analysis

All statistical analyses were performed with SPSS 20.0. One-way analysis of variance (ANOVA) was used to compare the differences in leaf photosynthetic characteristics (WUE, Ls, and PNmax) and leaf functional traits (SPAD, Nmass, Pmass, Nmass/Pmass, SLA, and LDMC) of the three species under different N and P addition treatments. Tukey’s HSD test was used for multiple comparisons. Two-way ANOVA was used to test the effects of N addition, P addition, and their interaction on PNmax, SPAD value, Nmass, Pmass, SLA, and LDMC. Pearson correlation was used to explore the relationship between leaf photosynthetic characteristics (PNmax, WUE) and leaf functional traits (SPAD value, Nmass, Pmass, Nmass/ Pmass, SLA, and LDMC). Multiple linear regression was used to explore the relationship between N addition, P addition, and their interaction, as well as PNmax and WUE. Principal component analysis (PCA) was conducted on photosynthetic characteristics and leaf functional traits. Graphing was performed with Origin 2021 (Origin Lab Software, Chicago, IL, USA).

5. Conclusions

Our three-year field fertilization study suggested that N addition—alone or combined with P—improved the photosynthesis of the three grassland dominant species on the semiarid Loess Plateau of China. All three species shifted to a fast-growth strategy with increased PNmax, SLA, and Nmass, as well as reduced LDMC under N and/or P addition. Furthermore, species-specific shifts in leaf functional traits were observed among the three species following N and/or P addition, of which C4 grass B. ischaemum increased SLA and PNmax, and C3 grass S. bungeana and subshrub L. davurica mainly increased leaf N and SPAD. P addition seems to only effectively impact the Pn of L. davurica. Evident N and P synergetic effects on the photosynthetic performance in all three species were observed, and a combination of 50 kg ha−1 yr−1 N and 40 kg ha−1 yr−1 P addition could be considered optimal fertilization for improving grassland productivity locally.

Author Contributions

Conceptualization: B.X.; Methodology: Y.J., S.L. and Z.C.; data collection and curation: Y.J. and S.L.; data analysis: Y.J. and S.L.; field investigation: Y.J., S.L., C.J. and J.Z.; writing-review and editing: Y.J., F.N. and B.X. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by National Key Research and Development Program of China (2016YFC0501703).

Data Availability Statement

The data presented in this study are available on reasonable request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Huang, L.; Shao, M. Advances and perspectives on soil water research in China’s Loess Plateau. Earth Sci. Rev. 2019, 199, 102962. [Google Scholar] [CrossRef]
  2. Gang, C.C.; Zhao, W.; Zhao, T.; Zhang, Y.; Gao, X.R.; Wen, Z.G. The impacts of land conversion and management measures on the grassland net primary productivity over the Loess Plateau, Northern China. Sci. Total Environ. 2018, 645, 827–836. [Google Scholar] [CrossRef] [PubMed]
  3. Sun, C.J.; Hou, H.X.; Chen, W. Effects of vegetation cover and slope on soil erosion in the Eastern Chinese Loess Plateau under different rainfall regimes. Peer J. 2021, 9, e11226. [Google Scholar] [CrossRef] [PubMed]
  4. Yang, Y.; Liu, H.; Yang, X.; Yao, H.J.; Deng, X.Q.; Wang, Y.Q.; An, S.S.; Kuzyakov, Y.; Chang, S.X. Plant and soil elemental C:N:P ratios are linked to soil microbial diversity during grassland restoration on the Loess Plateau, China. Sci. Total Environ. 2022, 806, 150557. [Google Scholar] [CrossRef] [PubMed]
  5. Bai, Y.; Wu, J.; Clark, C.M.; Naeem, S.; Pan, Q.; Huang, J.; Zhang, L.; Han, X. Tradeoffs and thresholds in the effects of nitrogen addition on biodiversity and ecosystem functioning: Evidence from inner Mongolia Grasslands. Glob. Change Biol. 2010, 16, 358–372. [Google Scholar] [CrossRef]
  6. Wang, Y.; Sun, Y.; Chang, S.; Wang, Z.; Fu, H.; Zhang, W.; Hou, F. Restoration practices affect alpine meadow ecosystem coupling and functions. Rangel. Ecol. Manag. 2020, 73, 441–451. [Google Scholar] [CrossRef]
  7. Botter, M.; Zeeman, M.; Burlando, P.; Fatichi, S. Impacts of fertilization on grassland productivity and water quality across the European Alps under current and warming climate: Insights from a mechanistic model. Biogeosciences 2021, 18, 1917–1939. [Google Scholar] [CrossRef]
  8. Bharath, S.; Borer, E.T.; Biederman, L.A.; Blumenthal, D.M.; Fay, P.A.; Gherardi, L.A.; Knops, J.M.H.; Leakey, A.D.B.; Yahdjian, L.; Seabloom, E.W. Nutrient addition increases grassland sensitivity to droughts. Ecology 2020, 101, e02981. [Google Scholar] [CrossRef] [Green Version]
  9. Smith, V.H.; Schindler, D.W. Eutrophication science: Where do we go from here? Trends Ecol. Evol. 2009, 24, 201–207. [Google Scholar] [CrossRef]
  10. Meng, B.; Li, J.; Maurer, G.E.; Zhong, S.; Yao, Y.; Yang, X.; Collins, S.L.; Sun, W. Nitrogen addition amplifies the nonlinear drought response of grassland productivity to extended growing-season droughts. Ecology 2021, 102, e03483. [Google Scholar] [CrossRef]
  11. Suding, K.N.; Collins, S.L.; Gough, L.; Clark, C.; Cleland, E.E.; Gross, K.L.; Milchunas, D.G.; Pennings, S. Functional-and abundance-based mechanisms explain diversity loss due to N fertilization. Proc. Natl. Acad. Sci. USA 2005, 102, 4387–4392. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  12. Socher, S.A.; Prati, D.; Boch, S.; Müller, J.; Klaus, V.H.; Hölzel, N.; Fischer, M. Direct and productivity-mediated indirect effects of fertilization, mowing and grazing on grassland species richness. J. Ecol. 2012, 100, 1391–1399. [Google Scholar] [CrossRef]
  13. Harpole, W.S.; Sullivan, L.L.; Lind, E.M.; Firn, J.; Adler, P.B.; Borer, E.T.; Chase, J.; Fay, P.A.; Hautier, Y.; Hillebrand, H.; et al. Addition of multiple limiting resources reduces grassland diversity. Nature 2016, 537, 93–96. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  14. Hautier, Y.; Zhang, P.F.; Loreau, M.; Wilcox, K.R.; Seabloom, E.W.; Borer, E.T.; Wang, S. General destabilizing effects of eutrophication on grassland productivity at multiple spatial scales. Nat. Commun. 2020, 11, 5375. [Google Scholar] [CrossRef]
  15. Liu, Z.P.; Shao, M.A.; Wang, Y.Q. Spatial patterns of soil total nitrogen and soil total phosphorus across the entire Loess Plateau region of China. Geoderma 2013, 197, 67–68. [Google Scholar] [CrossRef]
  16. Zhang, L.; Wei, X.; Hao, M.; Zhang, M. Changes in aggregate-associated organic carbon and nitrogen after 27 years of fertilization in a dryland alfalfa grassland on the loess plateau of china. J. Arid Land 2015, 7, 429–437. [Google Scholar] [CrossRef]
  17. Chen, Z.F.; Xiong, P.F.; Zhou, J.J.; Lai, S.B.; Jian, C.X.; Wang, Z.; Xu, B.C. Photosynthesis and nutrient-use efficiency in response to N and P addition in three dominant grassland species on the semiarid Loess Plateau. Photosynthetica 2020, 58, 1028–1039. [Google Scholar] [CrossRef]
  18. Qu, Q.; Wang, M.; Xu, H.; Yan, Z.; Liu, G.; Xue, S. Role of soil biotic and abiotic properties in plant community composition in response to nitrogen addition. Land Degrad. Dev. 2022, 33, 904–915. [Google Scholar] [CrossRef]
  19. Senock, R.S.; Devine, D.L.; Sisson, W.B.; Donart, G.B. Ecophysiology of three C4 perennial grasses in the northern Chihuahuan Desert. Southwest. Nat. 1994, 39, 122–127. [Google Scholar] [CrossRef]
  20. Nippert, J.B.; Fay, P.A.; Knapp, A.K. Photosynthetic traits in C3 and C4 grassland species in mesocosm and field environments. Environ. Exp. Bot. 2007, 60, 412–420. [Google Scholar] [CrossRef]
  21. Huxman, T.E.; Smith, S.D. Photosynthesis in an invasive grass and native forb at elevated CO2 during an El Nino year in the Mojave Desert. Oecologia 2001, 128, 193–201. [Google Scholar] [CrossRef] [PubMed]
  22. Adam, N.R.; Owensby, C.E.; Ham, J.M. The effect of CO2 enrichment on leaf photosynthetic rates and instantaneous water use efficiency of Andropogon gerardii in the tallgrass prairie. Photosynth. Res. 2000, 65, 121–129. [Google Scholar] [CrossRef] [PubMed]
  23. Nijs, I.; Impens, I.; Van Hecke, P. Diurnal changes in the response of canopy photosynthetic rate to elevated CO2 in a coupled temperature-light environment. Photosynth. Res. 1992, 32, 121–130. [Google Scholar] [CrossRef] [PubMed]
  24. Haase, P.; Pugnaire, F.I.; Clark, S.C.; Incoll, L.D. Environmental control of canopy dynamics and photosynthetic rate in the evergreen tussock grass Stipa tenacissima. Plant Ecol. 1999, 145, 327–339. [Google Scholar] [CrossRef]
  25. Evans, J.R.; Terashima, I. Effects of nitrogen nutrition on electron transport components and photosynthesis in spinach. Funct. Plant Biol. 1987, 14, 59–68. [Google Scholar] [CrossRef]
  26. Sinha, D.; Tandon, P.K. An Overview of Nitrogen, Phosphorus and Potassium: Key Players of Nutrition Process in Plants. Sustain. Solut. Elem. Defic. Excess Crop Plants 2020, 5, 85–117. [Google Scholar] [CrossRef]
  27. Wang, D.; Ling, T.Q.; Wang, P.P.; Fan, J.Z.; Wang, H.; Zhang, Y.Q. Effects of 8-year nitrogen and phosphorus treatments on the ecophysiological traits of two key species on tibetan plateau. Front. Plant Sci. 2018, 9, 1290. [Google Scholar] [CrossRef]
  28. Lai, S.B.; Xu, S.; Jian, C.X.; Chen, Z.F.; Zhou, J.J.; Yang, Q.; Wang, Z.; Xu, B.C. Leaf photosynthetic responses to nitrogen and phosphorus additions of dominant species in farm-withdrawn grassland in the loess hilly-gully region. Acta Ecol. Sin. 2021, 41, 5454–5464. (In Chinese) [Google Scholar] [CrossRef]
  29. Xing, H.; Zhou, W.; Wang, C.; Li, L.; Li, X.; Cui, N.; Hao, W.; Liu, F.; Wang, Y. Excessive nitrogen application under moderate soil water deficit decreases photosynthesis, respiration, carbon gain and water use efficiency of maize. Plant Physiol. Biochem. 2021, 166, 1065–1075. [Google Scholar] [CrossRef]
  30. Holford, I.C.R. Soil phosphorus: Its measurement, and its uptake by plants. Aust. J. Soil Res. 1997, 35, 227–240. [Google Scholar] [CrossRef]
  31. Carstensen, A.; Herdean, A.; Schmidt, S.B.; Sharma, A.; Spetea, C.; Pribil, M.; Husted, S. The impacts of phosphorus deficiency on the photosynthetic electron transport chain. Plant Physiol. 2018, 177, 271–284. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  32. Chaudhary, M.I.; Adu-Gyamfi, J.J.; Saneoka, H.; Nguyen, N.T.; Suwa, R.; Kanai, S.; El-Shemy, H.A.; Lightfoot, D.A.; Fujita, K. The effect of phosphorus deficiency on nutrient uptake, nitrogen fixation and photosynthetic rate in mashbean, mungbean and soybean. Acta Physio. Plant 2008, 30, 537–544. [Google Scholar] [CrossRef]
  33. Liu, C.; Wang, Y.; Pan, K. Effects of phosphorus application on photosynthetic carbon and nitrogen metabolism, water use efficiency and growth of dwarf bamboo (Fargesia rufa) subjected to water deficit. Plant Physiol. Bioch. 2015, 96, 20–28. [Google Scholar] [CrossRef] [PubMed]
  34. Suriyagoda, L.; Lambers, H.; Ryan, M.H.; Renton, M. Effects of leaf development and phosphorus supply on the photosynthetic characteristics of perennial legume species with pasture potential: Modelling photosynthesis with leaf development. Funct. Plant Biol. 2010, 37, 713–725. [Google Scholar] [CrossRef]
  35. Walker, A.P.; Beckerman, A.P.; Gu, L.H.; Kattge, J.; Cernusak, L.A.; Domingues, T.F.; Scales, J.C.; Wohlfahrt, G.; Wullschlrger, S.D.; Woodward, F.L. The relationship of leaf photosynthetic traits–Vcmax and Jmax–to leaf nitrogen, leaf phosphorus, and specific leaf area: A meta-analysis and modeling study. Ecol. Evol. 2015, 4, 3218–3235. [Google Scholar] [CrossRef] [Green Version]
  36. Wright, I.J.; Reich, P.B.; Westoby, M.; Ackerly, D.D.; Baruch, Z.; Bongers, F.; Cavender-Bares, J.; Chapin, T.; Cornelissen, J.H.C.; Diemer, M.; et al. The worldwide leaf economics spectrum. Nature 2004, 428, 821–827. [Google Scholar] [CrossRef]
  37. Reich, P.B. The world-wide ‘fast-slow’ plant economics spectrum: A traits manifesto. J. Ecol. 2014, 102, 275–301. [Google Scholar] [CrossRef]
  38. Wright, J.P.; Sutton-Grier, A. Does the leaf economic spectrum hold within local species pools across varying environmental conditions? Funct. Ecol. 2012, 26, 1390–1398. [Google Scholar] [CrossRef]
  39. Mao, W.; Li, Y.L.; Zhao, X.Y.; Zhang, T.H.; Liu, X.P. Variations of leaf economic spectrum of eight dominant plant species in two successional stages under contrasting nutrient supply. Pol. J. Ecol. 2016, 64, 14–24. [Google Scholar] [CrossRef]
  40. Sasaki, T.; Lauenroth, W.K. Dominant species, rather than diversity, regulates temporal stability of plant communities. Oecologia 2011, 166, 761–768. [Google Scholar] [CrossRef]
  41. Avolio, M.L.; Forrestel, E.J.; Chang, C.C.; La Pierre, K.J.; Burghardt, K.T.; Smith, M.D. Demystifying dominant species. New Phytol. 2019, 223, 1106–1126. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  42. Avolio, M.L.; Koerner, S.E.; La Pierre, K.J.; Wilcox, K.R.; Wilson, G.W.T.; Smith, M.D.; Collins, S.L. Changes in plant community composition, not diversity, during a decade of nitrogen and phosphorus additions drive above-ground productivity in a tallgrass prairie. J. Ecol. 2014, 102, 649–1660. [Google Scholar] [CrossRef] [Green Version]
  43. Yang, Y.; Dou, Y.; An, S.S. Environmental driving factors affecting plant biomass in natural grassland in the Loess Plateau, China. Ecol. Indic. 2017, 82, 250–259. [Google Scholar] [CrossRef]
  44. Chen, Z.F.; Xiong, P.F.; Zhou, J.J.; Yang, Q.; Wang, Z.; Xu, B.C. Grassland productivity and diversity changes in responses to N and P addition depend primarily on tall clonal and annual species in semiarid Loess Plateau. Ecol. Eng. 2020, 145, 105727. [Google Scholar] [CrossRef]
  45. Geiger, D.R.; Servaites, J.C. Diurnal regulation of photosynthetic carbon metabolism in C3 plants. Annu. Rev. Plant Biol. 1994, 45, 235–256. [Google Scholar] [CrossRef]
  46. Du, Y.C.; Nose, A.; Kondo, A. Diurnal changes in photosynthesis in sugarcane leaves. I. carbon dioxide exchange rate, photosynthesic enzyme activities and metabolite levels relating to the C4 pathway and the calvin cycle. Plant Prod. Sci. 2008, 3, 3–8. [Google Scholar] [CrossRef] [Green Version]
  47. Muraoka, H.; Tang, Y.; Terashima, I.; Koizumi, H.; Washitani, I. Contributions of diffusional limitation, photoinhibition and photorespiration to midday depression of photosynthesis in Arisaema heterophyllum in natural high light. Plant Cell Environ. 2000, 23, 235–250. [Google Scholar] [CrossRef]
  48. Farquhar, G.D.; Sharkey, T.D. Stomatal conductance and photosynthesis. Annu. Rev. Plant Physiol. 1982, 33, 317–345. [Google Scholar] [CrossRef]
  49. Wu, F.Z.; Bao, W.K.; Li, F.L.; Wu, N. Effects of water stress and nitrogen supply on leaf gas exchange and fluorescence parameters of Sophora davidii seedlings. Photosynthetica 2008, 46, 40–48. [Google Scholar] [CrossRef]
  50. Peng, Y.; Li, C.; Fritschi, F.B. Diurnal dynamics of maize leaf photosynthesis and carbohydrate concentrations in response to differential N availability. Environ. Exp. Bot. 2014, 99, 18–27. [Google Scholar] [CrossRef]
  51. Zhao, H.B.; Qi, L.; Liu, Y.G. Effects of combined application of nitrogen and phosphorus on diurnal variation of photosynthesis at grain-filling stage and grain yield of super high-yielding wheat. Chin. J. Appl. Ecol. 2010, 21, 2545–2550. (In Chinese) [Google Scholar]
  52. Xu, B.C.; Xu, W.Z.; Wang, Z.; Chen, Z.F.; Palta, J.A.; Chen, Y.L. Accumulation of N and P in the legume Lespedeza davurica in controlled mixtures with the grass Bothriochloa ischaemum under varying water and fertilization conditions. Front. Plant Sci. 2018, 9, 165. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  53. Tian, Z.W.; Liu, X.X.; Gu, S.L.; Yu, J.H.; Zhang, L.; Zhang, W.W.; Jiang, D.; Cao, W.X.; Dai, T.B. Postponed and reduced basal nitrogen application improves nitrogen use efficiency and plant growth of winter wheat. J. Integr. Agric. 2018, 17, 2648–2661. [Google Scholar] [CrossRef] [Green Version]
  54. Ackerman, D.; Millet, D.B.; Chen, X. Global estimates of inorganic nitrogen deposition across four decades. Glob. Biogeochem. Cycles 2019, 33, 100–107. [Google Scholar] [CrossRef] [Green Version]
  55. Li, Y.; Niu, S.; Yu, G. Aggravated phosphorus limitation on biomass production under increasing nitrogen loading: A meta-analysis. Global Change Biol. 2016, 22, 934–943. [Google Scholar] [CrossRef]
  56. Güsewell, S. N: P ratios in terrestrial plants: Variation and functional significance. New Phytol. 2004, 164, 243–266. [Google Scholar] [CrossRef]
  57. Naeem, M.; Khan, M.M.A.; Moinuddin; Idrees, M.; Aftab, T. Phosphorus ameliorates crop productivity, photosynthetic efficiency, nitrogen-fixation, activities of the enzymes and content of nutraceuticals of Lablab purpureus L. Sci. Hortic. 2010, 126, 205–214. [Google Scholar] [CrossRef]
  58. Reich, P.B.; Oleksyn, J.; Wright, I.J. Leaf phosphorus influences the photosynthesis–nitrogen relation: A cross-biome analysis of 314 species. Oecologia 2009, 160, 207–212. [Google Scholar] [CrossRef]
  59. Schleuss, P.M.; Widdig, M.; Heintz-Buschart, A.; Kirkman, K.; Spohn, M. Interactions of nitrogen and phosphorus cycling promote P acquisition and explain synergistic plant-growth responses. Ecology 2020, 101, e03003. [Google Scholar] [CrossRef] [Green Version]
  60. Elser, J.J.; Bracken, M.E.; Cleland, E.E.; Gruner, D.S.; Harpole, W.S.; Hillebrand, H.; Ngai, J.T.; Seabloom, E.W.; Shurin, B.J.; Smith, J.E. Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine and terrestrial ecosystems. Ecol. Lett. 2007, 10, 1135–1142. [Google Scholar] [CrossRef] [Green Version]
  61. Mariotte, P.; Cresswell, T.; Johansen, M.P.; Harrison, J.J.; Keitel, C.; Dijkstra, F.A. Plant uptake of nitrogen and phosphorus among grassland species affected by drought along a soil available phosphorus gradient. Plant Soil 2020, 448, 121–132. [Google Scholar] [CrossRef]
  62. Kinugasa, T.; Tsunekawa, A.; Shinoda, M. Increasing nitrogen deposition enhances post-drought recovery of grassland productivity in the Mongolian steppe. Oecologia 2012, 170, 857–865. [Google Scholar] [CrossRef] [PubMed]
  63. Zhong, S.Z.; Xu, Y.; Meng, B.; Loik, M.E.; Ma, J.Y.; Sun, W. Nitrogen addition increases the sensitivity of photosynthesis to drought and re-watering differentially in C3 versus C4 grass species. Front. Plant Sci. 2019, 10, 815. [Google Scholar] [CrossRef] [Green Version]
  64. Niu, F.R.; Duan, D.P.; Chen, J.; Xiong, P.F.; Zhang, H.; Wang, Z.; Xu, B.C. Eco-physiological responses of dominant species to watering in a natural grassland community on the semi-arid Loess Plateau of China. Front. Plant Sci. 2016, 7, 663. [Google Scholar] [CrossRef] [Green Version]
  65. Sun, C.X.; Huang, G.H.; Fan, Y.; Zhou, X.; Lu, C.; Wang, X.Q. Drought occurring with hot extremes: Changes under future climate change on Loess Plateau, China. Earth’s Future 2019, 7, 587–604. [Google Scholar] [CrossRef] [Green Version]
  66. Wan, H.W.; Yang, Y.; Bai, S.Q.; Xu, Y.H.; Bai, Y.F. Variations in leaf functional traits of six species along a nitrogen addition gradient in Leymus chinensis steppe in Inner Mongolia. J. Plant Ecol. 2008, 32, 611–621. [Google Scholar] [CrossRef]
  67. Zhao, W.; Zhang, R.; Huang, C.Q.; Wang, B.Q.; Cao, H.; Koopal, L.K.; Tan, W.F. Effect of different vegetation cover on the vertical distribution of soil organic and inorganic carbon in the Zhifanggou watershed on the Loess Plateau. Catena 2016, 139, 191–198. [Google Scholar] [CrossRef]
  68. Liang, T.; Tong, Y.A.; Xu, W.; Wei, Y.; Lin, W.; Pang, Y.; Liu, F.; Liu, X.J. Atmospheric nitrogen deposition in the Loess area of China. Atmos. Pollut. Res. 2016, 7, 447–453. [Google Scholar] [CrossRef]
Plants 11 02921 g001 550
Figure 1. Diurnal changes of photosynthetically active radiation (PAR), air relative humidity (RH), and air temperature (Ta) during the measurement period (20–22 July 2019).
Figure 1. Diurnal changes of photosynthetically active radiation (PAR), air relative humidity (RH), and air temperature (Ta) during the measurement period (20–22 July 2019).
Plants 11 02921 g001
Plants 11 02921 g002 550
Figure 2. Diurnal changes of net photosynthetic rate (Pn), transpiration rate (Tr), instantaneous water use efficiency (WUE), and stomatal limitation value (Ls) of the three species under different N and P addition treatments. Vertical bars indicate LSD values.
Figure 2. Diurnal changes of net photosynthetic rate (Pn), transpiration rate (Tr), instantaneous water use efficiency (WUE), and stomatal limitation value (Ls) of the three species under different N and P addition treatments. Vertical bars indicate LSD values.
Plants 11 02921 g002
Plants 11 02921 g003 550
Figure 3. Leaf maximum net photosynthetic rate (PNmax) of the three species under different N and P addition treatments. Values are mean ± SD. Different capital letters above the column indicate significant differences among P additions under each N addition rate, while different small letters indicate significant differences among N additions under each P addition rate. Numbers in parentheses are F-values, while ‘**’, and ‘***’ indicate p ≤ 0.01, and p ≤ 0.001, respectively. ‘n.s.’ indicates no significant difference.
Figure 3. Leaf maximum net photosynthetic rate (PNmax) of the three species under different N and P addition treatments. Values are mean ± SD. Different capital letters above the column indicate significant differences among P additions under each N addition rate, while different small letters indicate significant differences among N additions under each P addition rate. Numbers in parentheses are F-values, while ‘**’, and ‘***’ indicate p ≤ 0.01, and p ≤ 0.001, respectively. ‘n.s.’ indicates no significant difference.
Plants 11 02921 g003
Plants 11 02921 g004 550
Figure 4. Leaf SPAD values of the three species under different N and P addition treatments. Different capital letters above the column indicate significant differences among P additions under each N addition rate, while different small letters indicate significant differences among N additions under each P addition rate. Numbers in parentheses are F-values, while ‘*’ and ‘***’ indicate p ≤ 0.05 and p ≤ 0.001, respectively. ‘n.s.’ indicates no significant difference.
Figure 4. Leaf SPAD values of the three species under different N and P addition treatments. Different capital letters above the column indicate significant differences among P additions under each N addition rate, while different small letters indicate significant differences among N additions under each P addition rate. Numbers in parentheses are F-values, while ‘*’ and ‘***’ indicate p ≤ 0.05 and p ≤ 0.001, respectively. ‘n.s.’ indicates no significant difference.
Plants 11 02921 g004
Plants 11 02921 g005 550
Figure 5. Leaf nitrogen (N) and phosphorus (P) content (Nmass and Pmass), as well as the Nmass/Pmass ratio, of the three species under different N and P addition treatments. Values are mean ± SD. Different capital letters above the column indicate significant differences among P additions under each N addition rate, while different small letters indicate significant differences among N additions under each P addition rate. Numbers in parentheses are F-values, while ‘*’, ‘**’, and ‘***’ indicate p ≤ 0.05, p ≤ 0.01, and p≤ 0.001, respectively. ‘n.s.’ indicates no significant difference.
Figure 5. Leaf nitrogen (N) and phosphorus (P) content (Nmass and Pmass), as well as the Nmass/Pmass ratio, of the three species under different N and P addition treatments. Values are mean ± SD. Different capital letters above the column indicate significant differences among P additions under each N addition rate, while different small letters indicate significant differences among N additions under each P addition rate. Numbers in parentheses are F-values, while ‘*’, ‘**’, and ‘***’ indicate p ≤ 0.05, p ≤ 0.01, and p≤ 0.001, respectively. ‘n.s.’ indicates no significant difference.
Plants 11 02921 g005
Plants 11 02921 g006 550
Figure 6. Specific leaf area (SLA) and leaf dry matter content (LDMC) of the three species under different N and P addition treatments. Different capital letters above the column indicate significant differences among P additions under each N addition rate, while different small letters indicate significant differences among N additions under each P addition rate. Numbers in parentheses are F-values, while ‘*’, ‘**’, and ‘***’ indicate p ≤ 0.05, p ≤ 0.01, and p ≤ 0.001, respectively. ‘n.s.’ indicates no significant difference.
Figure 6. Specific leaf area (SLA) and leaf dry matter content (LDMC) of the three species under different N and P addition treatments. Different capital letters above the column indicate significant differences among P additions under each N addition rate, while different small letters indicate significant differences among N additions under each P addition rate. Numbers in parentheses are F-values, while ‘*’, ‘**’, and ‘***’ indicate p ≤ 0.05, p ≤ 0.01, and p ≤ 0.001, respectively. ‘n.s.’ indicates no significant difference.
Plants 11 02921 g006
Plants 11 02921 g007 550
Figure 7. Heatmap of Pearson correlation testing relationships between PNmax, WUE, and leaf functional traits (Nmass, Pmass, SLA, and LDMC) in the three species. Numbers in circles indicate the Pearson correlation coefficients.
Figure 7. Heatmap of Pearson correlation testing relationships between PNmax, WUE, and leaf functional traits (Nmass, Pmass, SLA, and LDMC) in the three species. Numbers in circles indicate the Pearson correlation coefficients.
Plants 11 02921 g007
Plants 11 02921 g008 550
Figure 8. Principal component analysis (PCA) of photosynthetic characteristics (PNmax and WUE) and leaf functional traits (SPAD, Nmass, Pmass, SLA, and LDMC) in the three species under N and P additions. (A) Loadings for each leaf trait; (B) Factor scores for each species.
Figure 8. Principal component analysis (PCA) of photosynthetic characteristics (PNmax and WUE) and leaf functional traits (SPAD, Nmass, Pmass, SLA, and LDMC) in the three species under N and P additions. (A) Loadings for each leaf trait; (B) Factor scores for each species.
Plants 11 02921 g008
Plants 11 02921 g009 550
Figure 9. Score plots of PCA analysis of photosynthetic characteristics (PNmax and WUE) and leaf economic traits (SPAD, Nmass, Pmass, SLA, and LDMC) in the three species under N and P additions. (A) N addition alone; (B) P addition alone; (C) N and P combined addition.
Figure 9. Score plots of PCA analysis of photosynthetic characteristics (PNmax and WUE) and leaf economic traits (SPAD, Nmass, Pmass, SLA, and LDMC) in the three species under N and P additions. (A) N addition alone; (B) P addition alone; (C) N and P combined addition.
Plants 11 02921 g009
Table
Table 1. Instantaneous water use efficiency (WUE) and stomatal limitation value (Ls) of the three species under different N and P additions (mean ± SD, n = 3).
Table 1. Instantaneous water use efficiency (WUE) and stomatal limitation value (Ls) of the three species under different N and P additions (mean ± SD, n = 3).
SpeciesTreatmentWUE (μmol mmol–1)Ls
P0P40P80P0P40P80
B. ischaemumN00.95 ± 0.02 B,b0.97 ± 0.02 A,b0.81 ± 0.01 C,b0.44 ± 0.00 B,c0.45 ± 0.01 B,b0.48 ± 0.00 A,a
N501.15 ± 0.02 A,a1.17 ± 0.01 A,a1.10 ± 0.01 B,a0.50 ± 0.01 A,a0.46 ± 0.01 B,b0.48 ± 0.01 B,a
N1001.10 ± 0.05 A,a1.21 ± 0.02 A,a1.11 ± 0.04 A,a0.42 ± 0.00 B,b0.50 ± 0.01 A,a0.48 ± 0.01 A,a
S. bungeanaN00.69 ± 0.01 B,b0.78 ± 0.01 A,b0.59 ± 0.01 C,b0.68 ± 0.00 A,a0.35 ± 0.01 B,a0.33 ± 0.01 B,b
N501.05 ± 0.01 A,a0.97 ± 0.02 B,a0.97 ± 0.01 B,a0.36 ± 0.02 A,c0.35 ± 0.01 A,a0.32 ± 0.01 B,b
N1000.91 ± 0.02 A,a1.09 ± 0.02 A,a0.84 ± 0.02 B,a0.57 ± 0.03 A,b0.36 ± 0.00 B,a0.37 ± 0.00 B,a
L. davuricaN00.93 ± 0.01 C,b1.36 ± 0.03 A,a1.18 ± 0.01 B,b0.45 ± 0.01 B,b0.52 ± 0.01 A,a0.51 ± 0.01 A,b
N501.24 ± 0.00 C,a1.33 ± 0.02 B,b1.47 ± 0.06 A,a0.48 ± 0.01 B,a0.50 ± 0.00 A,a0.52 ± 0.01 A,b
N1001.22 ± 0.02 B,a1.36 ± 0.01 A,a1.43 ± 0.03 A,a0.51 ± 0.00 B,a0.51 ± 0.02 B,a0.55 ± 0.01 A,a
B. ischaemum N ***(269); P ***(47.9); N × P ***(8.4)N ***(23.3); P ***(36.6); N × P ***(63.2)
S. bungeana N ***(1084); P ***(223); N × P ***(75.5)N ***(66.6); P **(8.62); N × P ***(14.1)
L. davurica N ***(156); P ***(223); N × P ***(46.8)N ***(26.4); P ***(45.8); N × P ***(8.93)
Data with different capital letters indicate significant differences among P additions under each N addition rate, while different small letters indicate significant differences among N additions under each P addition rate. Numbers in parentheses are F-values, while ‘**’ and ‘***’ indicate p ≤ 0.01, and p ≤ 0.001, respectively.
Table
Table 2. Regression slopes (SE) derived from the multiple linear regression analysis between photosynthetic characteristics (PNmax and WUE) and N addition, P addition, and N and P addition interaction in the three species.
Table 2. Regression slopes (SE) derived from the multiple linear regression analysis between photosynthetic characteristics (PNmax and WUE) and N addition, P addition, and N and P addition interaction in the three species.
SpeciesVariablePNmaxWUE
N0.0894 (0.0161) ***0.0060 (0.0010) ***
B. ischaemumP0.0182 (0.0201) n.s.0.0012 (0.0013) n.s.
N × P−0.0001 (0.0003) n.s.0.0000 (0.0000) n.s.
N0.0640 (0.0131) ***0.0049 (0.0009) ***
S. bungeanaP−0.0250 (0.0164) n.s.−0.0029 (0.0012) *
N × P−0.0002 (0.0003) n.s.−0.0001 (0.0001) n.s.
N0.0238 (0.0205) n.s.0.0022 (0.0019) n.s.
L. davuricaP0.0782 (0.0257) **0.0071 (0.0024) **
N × P−0.0002 (0.0004) n.s.0.0000 (0.0000) n.s.
‘n.s.’, ‘*’, ‘**’, and ‘***’ indicate p > 0.05, p ≤ 0.05, p ≤ 0.01, and p ≤ 0.001, respectively. Significant slopes are in bold.
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Share and Cite

MDPI and ACS Style

Jin, Y.; Lai, S.; Chen, Z.; Jian, C.; Zhou, J.; Niu, F.; Xu, B. Leaf Photosynthetic and Functional Traits of Grassland Dominant Species in Response to Nutrient Addition on the Chinese Loess Plateau. Plants 2022, 11, 2921. https://doi.org/10.3390/plants11212921

AMA Style

Jin Y, Lai S, Chen Z, Jian C, Zhou J, Niu F, Xu B. Leaf Photosynthetic and Functional Traits of Grassland Dominant Species in Response to Nutrient Addition on the Chinese Loess Plateau. Plants. 2022; 11(21):2921. https://doi.org/10.3390/plants11212921

Chicago/Turabian Style

Jin, Yuan, Shuaibin Lai, Zhifei Chen, Chunxia Jian, Junjie Zhou, Furong Niu, and Bingcheng Xu. 2022. "Leaf Photosynthetic and Functional Traits of Grassland Dominant Species in Response to Nutrient Addition on the Chinese Loess Plateau" Plants 11, no. 21: 2921. https://doi.org/10.3390/plants11212921

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop