Precipitation overrides warming in mediating soil nitrogen pools in an alpine grassland ecosystem on the Tibetan Plateau

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.

Figure 1

Soil temperature (°C) (a) and soil moisture (v/v %) (b) at depths of 5 cm, 10 cm and 20 cm under warming and altered precipitation regimes during the growing season of 2013. C, control; H, heated; D, dry; HD, heated and dry; W, wet; HW, heated and wet. Different letters mean significant differences between treatments at P < 0.05 level for each soil depth. Mean ± SE is shown in the figure.

Figure 2

Effects of warming on NO₃⁻-N (a) and microbial biomass N (b) stocks in the topsoil, the subsoil and the entire soil profile. C, control; H, heated. Different letters mean significant differences between factors at P < 0.05 for each soil depth. Mean ± SE is shown in the figure.

Figure 3

Effects of altered precipitation on NO₃⁻-N (a), dissolved organic N (2 M KCl extracts) (b), amino acid N (c) and microbial biomass N (d) stocks in the topsoil, the subsoil and the entire soil profile. A, ambient; D, dry; W, wet. Different letters mean significant differences between factors at P < 0.05 for each soil depth. Mean ± SE is shown in the figure.

Figure 4. Biplot of principal components analysis (PCA) of six soil N stocks in the topsoil under warming and altered precipitation regimes.

C, control; H, heated; D, dry; HD, heated and dry; W, wet; HW, heated and wet. STN, soil total nitrogen; NH₄⁺-N, ammonium-N; NO₃⁻-N, nitrate-N; DON, dissolved organic N (2 M KCl extracts); AAN, amino acid N; MBN, microbial biomass N.

Figure 5. Biplot of redundancy analysis (RDA) of the relationships between six soil N stocks (black arrows) and other key variables (red arrows) in the topsoil under warming and altered precipitation regimes.

Soil N pools: STN, soil total N; NO₃⁻-N, nitrate-N; DON, dissolved organic N; AAN, amino acid N; MBN, microbial biomass N. Explanatory variables: TPN, total plant N content; SM, soil moisture; ST, soil temperature; Clay, soil clay content; pH, soil pH.