COS 39-5
Response of ecosystem carbon exchange to warming and nitrogen addition in a desert steppe in Eurasian steppe region
lobal warming and nitrogen deposition impact ecosystem functions, especially carbon exchange and primary productivity; however, it is unclear whether elevated temperature and nitrogen concentration would increase or decrease ecosystem carbon fluxes. We hypothesized that: (1) warming and nitrogen addition lead to a higher increase in gross ecosystem productivity than in ecosystem respiration, and (2) nitrogen addition affects ecosystem carbon fluxes by shifting the relative abundances of plant functional groups. To test the hypotheses, we started an experiment with warming and nitrogen addition in a desert steppe in Inner Mongolia, China in 2006. The experimental design was a paired, spilt-plot with warming as the main plot factor and nitrogen addition as the subplot factor. There were six pairs of 3 × 4 m plots. In each pair, one plot was randomly assigned for a warming treatment and the other non-warming treatment. Each plot was divided into two 2 × 3 m subplots, with random assignments for with and without nitrogen addition. Ecosystem carbon fluxes, precipitation, soil temperature and moisture, and vegetation cover by species were measured seasonally.
Results/Conclusions
During the first five years warming led to increase in only ecosystem respiration, whereas nitrogen addition led to increases in gross ecosystem productivity, ecosystem respiration, and net ecosystem productivity. The magnitude of net ecosystem productivity increase under nitrogen addition was highly correlated to precipitation and temperature. We also found that seasonal mean gross ecosystem productivity, ecosystem respiration, and net ecosystem productivity were linearly and positively correlated to both mean soil moisture and vegetation cover, nitrogen addition significantly increased the cover of the dominant species Stipa breviflora, and S. breviflora cover was positively correlated to seasonal precipitation. Therefore we concluded that nitrogen addition increased the strength of ecosystem carbon sink via enhancing primary productivity, and that in a short-to-medium term warming led to loss of soil organic carbon. Our results teased apart the effects of warming vs. nitrogen addition on carbon exchange.