N balance and cycling of Inner Mongolia typical steppe: a comprehensive case study of grazing effects

Increasing grazing pressure and climate change affect nitrogen (N) dynamics of grassland ecosystems in the Eurasian steppe belt with unclear consequences for future delivery of essential services such as forage production, C sequestration, and diversity conservation. The identification of key processes responsive to grazing is crucial to optimize grassland management. In this comprehensive case study of a Chinese typical steppe, we present an in-depth analysis of grazing effects on N dynamics, including the balance of N gains and losses, and N cycling. N pools and fluxes were simultaneously quantified on three grassland sites of different long-term grazing intensities. Dust deposition, wind erosion, and wet deposition were the predominant but most variable processes contributing to N losses and gains. Heavy grazing increased the risk of N losses by wind erosion. Hay-making and sheep excrement export to folds during nighttime keeping were important pathways of N losses from grassland sites. Compared to these fluxes, gaseous N losses (N2O, NO, N-2,N- and NH3) and N losses via export of sheep live mass and wool were of minor relevance. Our N balance calculation indicated mean annual net N losses of 0.9 +/- 0.8 g N/m(2) (mean +/- SD) at the heavily grazed site, whereas the long-term ungrazed site was an N sink receiving mean annual inputs of 1.8 +/- 1.1 g N/m(2), mainly due to dust deposition. Heavy grazing reduced pool sizes of topsoil organic N, above- and belowground biomass, and N fluxes with regard to plant N uptake, decomposition, gross microbial N turnover, and immobilization. Most N-related processes were more intensive in seasons of higher water availability, indicating complex interactions between land use intensity and climate variability. The projected increase of atmospheric N depositions and changes in rainfall pattern imposed by land use change will likely affect N sink-source pathways and N flux dynamics, indicating high potential impact on grassland ecosystem functions. Land use practices will be increasingly important for the management of N dynamics in Chinese typical steppe and, therefore, must be considered as key component to maintain, restore or optimize ecosystem services.