Short-term responses of ecosystem respiration to warming and nitrogen addition in an alpine swamp meadow
Introduction
According to the fifth IPCC report, the global mean air temperature increased by 0.85 °C from 1880 to 2012, and an additional increase of 1.5–4.8 °C is expected by the end of the 21st century [1]. Rising temperatures can substantially influence plant productivity, photosynthesis, and the decomposition of litter and soil organic matter, consequently affecting ecosystem CO2 emission and fixation (ecosystem respiration, soil respiration and net ecosystem CO2 exchange) [2,3]. This will provide both positive and negative feedbacks to future climate change [4]. Meanwhile, atmospheric nitrogen (N) deposition has continued to increase in the last century around the world [5], and increasing rates of atmospheric N deposition due to fossil fuel combustion and N fertilizer application were also found on the Qinghai-Tibetan Plateau (QTP) [6]. The increase in N deposition could strongly affect many ecological processes, such as soil microbial activities, the allocation pattern of plant photosynthetic assimilates, and the composition of the soil microbial and plant communities [[7], [8], [9]], resulting in corresponding changes in carbon cycling and storage. Therefore, understanding the effects of the combination of climate warming and increasing N deposition on CO2 emissions is essential for an accurate estimation of the ecosystem carbon balance.
Ecosystem respiration (ER) contributes to one of the largest terrestrial carbon fluxes, and its dynamics play critical roles in regulating the feedback of terrestrial ecosystems to climate changes [10]. Most previous studies in arctic and alpine ecosystems have generally suggested that warming had significantly positive effects on ER [[11], [12], [13]]. However, some studies have also found that warming had negative or no effects on ER [14,15]. Such different responses of ER might be related to warming-induced variations in biotic and abiotic factors such as soil temperature and moisture, plant biomass, and soil nutrient condition [13,16]. Meanwhile, recent studies in the same site suggested that N addition will stimulate ER [8]. Other researchers have found that N addition may have a negative or null effect on ER [9,17,18]. Thus, the effects of increasing N deposition on ER are still unclear in alpine grassland ecosystems.
The QTP, which is regarded as the highest unique terrestrial ecosystem in the world [19], is sensitive to climate change, and it also plays a vital role in moderating such changes at the global level. The surface air temperature on the QTP has increased at approximately twice the rate of global warming over the past 50 years [20], and N deposition is very dramatic in this region, ranging from 8.7 to 13.8 kg N ha−1 year−1 [6]. The alpine swamp meadow is one of the most widespread grassland ecosystems on the QTP, which covers an area of approximately 50,000 km2 [21]. Compared to the alpine meadow, more CO2 may be released in alpine swamp meadow due to the greater live biomass, more abundant soil nutrients, and higher soil water content [22]. Although many studies have investigated the effects of climate change on carbon cycle processes in alpine grasslands on the QTP, most of them focused on a single climate change factor, such as warming or N addition [9,11,23,24]. A few studies on the interactive effects of warming and N addition on CO2 emissions in this region were highly contrasting. Results from an alpine steppe showed that the significant interaction between warming and N addition strongly inhibited CO2 emission [15]. In contrast, Chen et al. [18] reported that the interaction between warming and N addition on ER was not found in alpine meadow due to the contrary effect of warming and N fertilization on soil C and N turnover. In any case, there remains a lack of information about the underlying mechanism responsible for the interactive effects of warming and N addition on ecosystem CO2 emissions.
To provide more evidence to accurately predict the effect of future climate change on ER, we conducted manipulative warming and N addition experiments in an alpine swamp meadow on the QTP. The objectives of this study were to investigate: (1) the effects of warming, N addition, and their interaction on ER; (2) the relationships of ER with biotic (plant production and allocation pattern, soil microbial biomass and activity) and abiotic (air temperature, soil temperature and soil moisture) factors under warming and N addition. We hypothesized that (1) warming combined with N addition would have significant positive effects on ER throughout the growing season. (2) ER would be controlled by different factors as warming and N addition might have different effects on biotic and abiotic factors.
Section snippets
Study site
This study was conducted on Fenghuo Mountain in the hinterland of the Qinghai-Tibetan Plateau, China (34°43′57.5″N, 92°53′22.5″E; 4600–4800 m a.s.l.). The region is located in the plateau-continental climatic zone, with a cold and dry climate. The mean annual temperature is −5.3 °C. The extreme maximum temperature is 24.7 °C, and the extreme minimum temperature is −38.5 °C. The mean annual precipitation is 310.7 mm, 80% of which falls during the growing season (from May to September). The mean
Microclimate
Air temperature and precipitation during the study period (from 1 May to 10 November) are shown in Fig. S1. The total precipitation was 378.4 mm, and the daily precipitation peaked at 18.1 mm in early August. The mean value of the air temperature was 3.3 °C, and the daily air temperature peaked at 12.8 °C on 1 August. Warming significantly increased the mean values of the air temperature by 4.7 °C.
The soil temperature was raised by 2.9 °C and 0.9 °C at 5 cm and 20 cm depths, respectively (Fig. 1
The interactive effects of warming and N addition on ER
Interaction between two global change factors (e.g., warming, N addition, elevated CO2 and irrigation) can be identified as synergistic or antagonistic when their combined effect is greater or weaker than the sum of the two individual effects, while it is considered as additive when the combined effect has no significant difference from the sum [16]. There have been numerous warming experiments regarding the CO2 effluxes of different ecosystems in recent decades. Many of them suggested that
Conclusions
Warming, N addition and their interaction significantly enhanced the average ER by 47.1%, 23.5% and 88.2%, respectively. Because of sufficient water conditions and the gas transport capacities of vascular plants, warming-induced drought stress or the limitation of overabundant soil moisture was not found in our experiment. Thus, air temperature and soil temperature were dominant factors controlling ecosystem CO2 emissions in all treatments. Warming and N addition significantly enhanced plant
Acknowledgments
This research was funded by the National Natural Science Foundation of China (41563005, 41271224) and the Young Scholars Science Foundation of Lanzhou Jiaotong University (2015013). The time and effort of the editors and anonymous reviewers are appreciated.
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