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Litter decomposition is the fundamental process in nutrient cycling and soil carbon (C) sequestration in terrestrial ecosystems. The global-wide increase in nitrogen (N) inputs is expected to alter litter decomposition and, ultimately, affect ecosystem C storage and nutrient status. Temperate grassland ecosystems in China are usually N-deficient and particularly sensitive to the changes in exogenous N additions. In this paper, we conducted a 1,200-day in situ experiment in a typical semi-arid temperate steppe in Inner Mongolia to investigate the litter decomposition as well as the dynamics of litter C and N concentrations under three N addition levels (low N with 50 kg N/(hm2·a) (LN), medium N with 100 kg N/(hm2·a) (MN), and high N with 200 kg N/(hm2·a) (HN)) and three N addition forms (ammonium-N-based with 100 kg N/(hm2·a) as ammonium sulfate (AS), nitrate-N-based with 100 kg N/(hm2·a) as sodium nitrate (SN), and mixed-N-based with 100 kg N/(hm2·a) as calcium ammonium nitrate (CAN)) compared to control with no N addition (CK). The results indicated that the litter mass remaining in all N treatments exhibited a similar decomposition pattern: fast decomposition within the initial 120 days, followed by a relatively slow decomposition in the remaining observation period (120–1,200 days). The decomposition pattern in each treatment was fitted well in two split-phase models, namely, a single exponential decay model in phase I ( 398 days). The three N addition levels exerted insignificant effects on litter decomposition in the early stages ( 0.05). However, MN and HN treatments inhibited litter mass loss after 398 and 746 days, respectively (P 0.05). The effects of these two N addition forms differed greatly from those of CAN after 746 and 1,053 days, respectively (P 0.05). Our results indicated that exogenous N additions could exhibit neutral or inhibitory effects on litter decomposition, and the inhibitory effects of N additions on litter decomposition in the final decay stages are not caused by the changes in the chemical qualities of the litter, such as endogenous N and C concentrations. These results will provide an important data basis for the simulation and prediction of C cycle processes in future N-deposition scenarios.

Short-term nitrous oxide (N2O) pulse emissions caused by precipitation account for a considerable portion of the annual N2O emissions and are greatly influenced by soil nitrogen (N) dynamics. However, in Chinese semiarid temperate steppes, the response of N2O emissions to the coupling changes of precipitation and soil N availability is not yet fully understood. In this study, we conducted two 7-day field experiments in a semiarid temperate typical steppe of Inner Mongolia, China, to investigate the N2O emission pulses resulting from artificial precipitation events (approximately equivalent to 10.0 mm rainfall) under four N addition levels (0, 5, 10, and 20 g N/(m(2)center dot a)) using the static opaque chamber technique. The results show that the simulated rainfall during the dry period in 2010 caused greater short-term emission bursts than that during the relatively rainy observation period in 2011 (P 0.05) with single water addition. The peak values of N2O efflux increased with the increasing N input. Only the treatments with water and medium (WN10) or high N addition (WN(2)0) significantly increased the cumulative N2O emissions (P