Abstract
Soils in the alpine grassland store a large amount of nitrogen (N) due to slow decomposition. However, the decomposition could be affected by climate change, which has profound impacts on soil N cycling. We investigated the changes of soil total N and five labile N stocks in the topsoil, the subsoil and the entire soil profile in response to three years of experimental warming and altered precipitation in a Tibetan alpine grassland. We found that warming significantly increased soil nitrate N stock and decreased microbial biomass N (MBN) stock. Increased precipitation reduced nitrate N, dissolved organic N and amino acid N stocks, but increased MBN stock in the topsoil. No change in soil total N was detected under warming and altered precipitation regimes. Redundancy analysis further revealed that soil moisture (26.3%) overrode soil temperature (10.4%) in explaining the variations of soil N stocks across the treatments. Our results suggest that precipitation exerted stronger influence than warming on soil N pools in this mesic and high-elevation grassland ecosystem. This indicates that the projected rise in future precipitation may lead to a significant loss of dissolved soil N pools by stimulating the biogeochemical processes in this alpine grassland.
Highlights
Ecosystems, warming showed a significantly positive effect on inorganic N pools[26]
A few studies on how altered precipitation affects soil N pools have been conducted in the grassland ecosystems, which showed that precipitation is a key factor in shaping the pattern of soil N pools[28]
We observed significant changes in most soil N stocks in the topsoil, but no detectable changes in all measured soil N stocks in the subsoil and across the entire profile after 3 years of treatments. This suggested that soil N pools in the subsoil were not affected in the short period, which is partially inconsistent with our hypothesis (1)
Summary
Ecosystems, warming showed a significantly positive effect on inorganic N pools[26]. Secondly, warming could accelerate N2O emission by stimulating microbial denitrification and gas transportation, which may decrease soil NO3−-N concentration in soils[20]. Water addition significantly increased soil NO3−-N and soil inorganic N (SIN) in a semi-arid temperate steppe of northern China[16,29,30]. This could result from enhanced microbial activities in response to higher water availability[28]. Considering the previous results that found temperature as the factor controlling nutrients cycling and functions of the alpine ecosystem[12,13,27], we hypothesized that (1) soil labile N pools responded to warming and its interaction with altered precipitation in the short period, and (2) temperature is the main driver to the changes. Our results will lead us to generate new insights into effects of climate change on soil N pools in alpine grassland ecosystems and to predict trends of soil N pools in the future
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