Response of soil nutrients and stoichiometry to elevated nitrogen deposition in alpine grassland on the Qinghai-Tibetan Plateau
Introduction
Human activities such as deforestation, expansion of agriculture, and increased combustion of fossil fuels have affected the nitrogen (N) cycle (Tian et al., 2006). Some scholars reported that global N deposition in recent years was 3–5 times higher than in the last century (Janssens et al., 2010). In the past decades, total N deposition in China has increased significantly, with a mean value higher than that in the United States and Europe. Grasslands are more sensitive to N deposition (Man, 2011). Qinghai-Tibetan Plateau (QTP), the most important eco-region of alpine grassland biome in the world, is experiencing a significant N deposition of 7.55 kg N ha−1 yr−1 on average, with a minimum of 1.08 kg N ha−1 yr−1 and a maximum of 17.81 kg N ha−1 yr−1 (Lv and Tian, 2007). Because of its high sensitivity to environmental disturbance, the alpine grasslands in the QTP have pronounced responses to climate change and human activities (Li et al., 2015). Too much N is considered as one of the main drivers for biodiversity loss across the world (Sala et al., 2000). Long-term, chronic N deposition has significantly reduced plant species richness (Stevens et al., 2004). Many sensitive species have declined in areas of high N deposition, and it takes many years to recover (Dise et al., 2011). Growing evidence has proven that atmospheric N deposition is a threat to biodiversity and ecosystem function in acid grasslands in Europe (Stevens et al., 2004). However, little is known about the divergent responses of critical loads of soil element concentration to elevated N deposition.
Numerous studies have documented the effects of N deposition on soil fertility in the temperate grasslands of China. Zhang et al. (2010) reported that the available N, but not total N, total carbon (C) and phosphorus (P), was remarkably improved by N addition. Gao et al., 2015a, Gao et al., 2015b found that the N addition increased the total N and available N in the soil but decreased the total C, total P and available P. In addition, Cao et al. (2015) stated that both total nutrients and available nutrients increased with N deposition. Huang et al. (2016) found that N addition increased N availability in a desert steppe. Z.Z. Zhao et al. (2017) and H. Zhao et al. (2017) found that total N and available P in fertile islands increased with the increase in N deposition, and N and P contents in surface soil increased significantly with increased N deposition. Although there are some differences in the results obtained from the various studies, the increase in soil available N with N addition was commonly found in most studies.
Stoichiometric studies have mainly dealt with C, P, and N (Tang and Dam, 1999). Usually, the N is considered the primary limiting nutrient in alpine grassland ecosystems (Ka Zhuo et al., 2015). It is imperative to clarify the feedbacks between the elemental pools in the environment and the stoichiometry of individual organisms and ecosystems. There is complicated feedback relationship between stoichiometry of organism and environment. Once the stoichiometric ratio between them does not match, there would be changes in population behavior and the evolution of organisms (Schimel, 2003) and influences on organism growth and development processes and morphology (Méndez and Karlsson, 2005). Stoichiometry can provide a new tool to link separate parts as a whole. It has become a new trend to re-unify ecological knowledge by using biological stoichiometry (Elser et al., 2000). The effects of N deposition on soil stoichiometry may vary across different sites (Waldrop et al., 2004).
Despite the results from numerous studies have demonstrated there were significant effects of N deposition on grassland soils in other eco-regions, knowledge regarding the response of soil ecological stoichiometry and soil fertility to N deposition remains limited in the alpine regions. In this study, we conducted manipulation experiments with six N application levels in three key types of alpine grasslands on the QTP, alpine meadow, alpine steppe and cultivated grassland to determine the response threshold of soil element concentration to N addition and examine the influences of N deposition on soil fertility and stoichiometry along different soil depths. This study was designed to test two hypotheses: 1) N deposition can alter the nutrient compositions and stoichiometry in alpine grassland soils; 2) there are different response thresholds of soil element concentration to N addition among different types of alpine grasslands.
Section snippets
Study site
The study sites Xihai Town of Haiyan County and Tiebujia Town of Gonghe County of the Qinghai Province are the representative distribution areas of three typical types of alpine grasslands on the QTP, alpine meadow, alpine steppe and cultivated grassland. Xihai Town of Haiyan County is located in northeastern Qinghai Province (100°23′–101°20′E and 36°44′–37°39′N) at an altitude above 3000 m. There is a changeable climate with hypoxia, long sunshine time, strong radiation, and low ground
Changes of soil total C, N, and P
Soil total C, N and P varied significantly among different types of grassland in both years (Table 2). Soil total C and N was the highest in alpine meadow and the lowest in cultivated grassland, while total P was the highest in the alpine meadow and lowest in the alpine steppe (Fig. 2, Table 3). Soil total P was significantly higher in the surface soil (0–10 cm) than that in the deep soil (10–20 cm). There is no significant difference in soil total C between the two years under N addition. The
Discussion
The results of the first year showed that low N deposition could improve the soil total N, while high N deposition would decrease the soil total N. This is consistent with the results of the studies conducted by previous scholars; e.g., Liu (2014) noted that low N addition would benefit the accumulation of soil total N. Rainer and Joergensen (1999) reported that high N addition restrained the activity of soil microbes and decreased the soil total N. The reduction in microbial biomass under high
Conclusion
The alpine grasslands responded differently to N addition and nutrient distribution across grassland types, meaning that close attention to nutrient background and environmental conditions should be paid to the responses of alpine grassland ecosystems to climate change. Soil C and P were relatively stable under N deposition. Soil total N increased under low N addition rate and decreased under higher rates. Soil available N increased under N deposition in surface soil while it decreased in the
Acknowledgements
This work was supported by the grants from the China National Key R&D program (2016YFC0501906) and China State Key Laboratory of Environment Simulation and Pollution Control (17L03ESPC). The authors wish to express the gratitude to the reviewers and editors for their time and effort.
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