Decomposing litter and the C and N dynamics as affected by N additions in a semi-arid temperate steppe, Inner Mongolia of China

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) and a linear decay function in phase II (>398 days). The three N addition levels exerted insignificant effects on litter decomposition in the early stages (<398 days, phase I; P>0.05). However, MN and HN treatments inhibited litter mass loss after 398 and 746 days, respectively (P<0.05). AS and SN treatments exerted similar effects on litter mass remaining during the entire decomposition period (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). During the decomposition period, N concentrations in the decomposing litter increased whereas C concentrations decreased, which also led to an exponential decrease in litter C:N ratios in all treatments. No significant effects were induced by N addition levels and forms on litter C and N concentrations (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.