Atmospheric nitrogen deposition is an important contributor to global and regional nitrogen cycles, and atmospheric nitrogen could be a critical limit nutrient for remote areas. In this study, nitrogen species compositions, deposition fluxes, and historical records in the mountains of Western China, including the Tibetan Plateau, were determined from snowpit and ice core samples collected from mountain glaciers. The mean concentration of total dissolved nitrogen (TDN) in the snowpit samples was 12.6 mu mol L-1 (8.0-17.8 mu mol L-1) and comprised 59% ammonium nitrogen, 35% nitrate nitrogen, and similar to 6% dissolved organic nitrogen. The deposition of nitrogen species, except organic nitrogen (likely due to its low concentrations and/or different origination), varied seasonally in a similar way based on the records of the snowpit profile. Based on monthly surface sample collection in one of the glaciers, the mass concentration and composition of nitrogen species varied monthly, mainly because of melting processes. During melting, the inorganic nitrogen content could be lost significantly, whereas the dissolved organic nitrogen concentration could be enriched because of microbial activity. For the historical records, the nitrogen deposition in mountain areas of Western China after 1960s was increased by about one time of that during 1900-1950 and was dominated by ammonium-N. From the snowpit data, we estimated the total dissolved nitrogen deposition flux at 0.56-1.3 (mean 0.88) kg ha(-1) a(-1) in the mountain area of Western China. These results could improve our understanding of nitrogen deposition and cycle in the mountain areas of Western China.

In the Tibetan Plateau grassland ecosystems, nitrogen (N) availability is rising dramatically; however, the influence of higher N on the arbuscular mycorrhizal fungi (AMF) might impact on plant competitive interactions. Therefore, understanding the part played by AMF in the competition between Vicia faba and Brassica napus and its dependence on the N-addition status is necessary. To address this, a glasshouse experiment was conducted to examine whether the grassland AMF community's inocula (AMF and NAMF) and N-addition levels (N-0 and N-15) alter plant competition between V. faba and B. napus. Two harvests took day 45 (1(st) harvest) and day 90 (2(nd) harvest), respectively. The findings showed that compared to B. napus, AMF inoculation significantly improved the competitive potential of the V. faba. In the occurrence of AMF, V. faba was the strongest competitor being facilitated by B. napus in both harvests. While under N-15, AMF significantly enhanced tissue N:P ratio in B. napus mixed-culture at 1(st) harvest, the opposite trend was observed in 2(nd) harvest. The mycorrhizal growth dependency slightly negatively affected mixed-culture compared to monoculture under both N-addition treatments. The aggressivity index of AMF plants was higher than NAMF plants with both N-addition and harvests. Our observation highlights that mycorrhizal associations might facilitate host plant species in mixed-culture with non-host plant species. Additionally, interacting with N-addition, AMF could impact the competitive ability of the host plant not only directly but also indirectly, thereby changing the growth and nutrient uptake of competing plant species.

The contributions of long-lived nitrous oxide (N2O) to global climate and environment have received increasing attention. Especially, atmospheric nitrogen (N) deposition has substantially increased in recent decades due to the extensive use of fossil fuels in industry, which strongly stimulates the N2O emissions of terrestrial ecosystem. Several models have been developed to simulate the impacts of environmental factors on N2O emission from soil, but there are still large differences in the simulations of N2O emission and their responses to atmospheric deposition over global or regional scales. Using observations from N addition experiments in a subtropical forest, this study compared five widely-used N2O modules or algorithms (i.e. the N2O modules of DayCENT, PnET-NDNDC and DyN, and the algorithm of NOE and NGAS) to investigate their performances for reproducing N2O emission, and especially the impacts of two forms of N additions (i.e. NH4+-N and NO3--N, respectively) of two levels (low and high) on N2O emission. In general, the five modules reproduced the seasonal variations of N2O emission. Under the high levels of N addition compared to low ones for both NH4+-N and NO3--N, however, not all modules can reproduce larger N2O emission. Relatively larger N2O emissions in measurements due to NH4+N compared to NO3--N additions were not indicated neither in all the modules. Moreover, there were substantial differences in simulating the ratios of N2O emission from nitrification and denitrification processes due to disagreements in the structure of these modules or algorithms. The comparison highlights the need to improve the representation of N2O production and diffusion processes. At the same time, it also highlights the application of WFPS in the model methodology as a key scheme that mediates the two microbial processes, i.e. nitrification and denitrification, could probably improve the performances of N2O models in future research.