Nitrogen (N) deposition may have a substantial impact on structure and function of N-limited grassland ecosystem as well as the ecophysiological processes across the systems. Carbon (C) cycle is one of the important ecological processes for ecosystem. N addition can play a key role in regulating C cycle and C balance in grassland. The semiarid temperate grassland in Inner Mongolia is a significant component of China's grassland. However, there is very limited information on the responses of C cycle and C balance to multi-level N addition in the N-deficient grassland ecosystem. We conducted a field N addition experiment in the temperate grassland in Inner Mongolia in 2014 and 2015. Six N-addition treatments were established as: the control, N1 (0 g N m−2 yr−1), N2 (2 g N m−2 yr−1), N5 (5 g N m−2 yr−1), N10 (10 g N m−2 yr−1), and N25 (25 g N m−2 yr−1). We measured net ecosystem CO2 exchange (NEE), net ecosystem production (NEP), and C sequestration rates in treatments with N addition. The main objective of the study was to evaluate the responses of grassland C balance to different levels of N addition at the ecosystem scale.
Vegetation, soil and whole-ecosystem C sequestration rates tended to increase with N addition. The N25 and N10 significantly increased the whole-ecosystem C sequestration rate compared to the control. The values ranged from 106.8 g C m−2 (N1) to 258.8 g C m−2 (N25). Cumulative NEE during the growing season decreased significantly at the N5, N10 and N25, ranging from –91.52 g C m−2 (N25) to 5.73 g C m−2 (N5). The N25 slightly increased cumulative heterotrophic soil respiration compared to the control, but the effect was only significant at the highest N level in 2014 growing season. N addition significantly increased cumulative NEP in both years, with values ranging from ‒5.8 g C m−2 (control) to 128.3 g C m−2 (N10). The C input from litter decomposition was significant, and must be quantified to estimate NPP. Measuring C sequestration rates and NEP together may allow tracking of the effect of N addition on grassland C budgets. Adding 25 or 10 g N m−2 yr−1 increased C sequestration and improved the C balance of the ecosystem, and increased grassland C sequestration. Overall, our results demonstrate that N addition is an important mechanism for maintaining C balance within the temperate grassland.