Context-dependent effects of grassland degradation on soil nitrogen cycling processes

Grassland degradation, as a result of climate change and anthropogenic activities, is reported to impair soil nitrogen cycling processes. However, there is little evidence on the spatial patterns of the degradation effects across broad environmental gradients. Here we measured soil available nitrogen contents and related nitrogen transformation processes from 45 grassland degradation sequences on the Tibetan Plateau. On average across all degradation sequences, the nitrogen-cycling parameters tended to decline from non-degraded to heavily degraded stages in most cases. However, a cross-sequence analysis showed that grassland degradation decreased soil nitrogen-cycling parameters when their corresponding background status (i.e., nitrogen-cycling parameters of non-degraded plots) was high but increased them when the respective background status was low, regardless of degradation stages. The above findings demonstrate that grassland degradation may not always, as previously assumed, reduce soil nitrogen cycling processes. Considering background nitrogen status is a key for grassland restoration when applying the nutrient-based strategies. Grassland degradation reduces soil nitrogen cycling in grasslands with high background nitrogen cycling rates, but increases it in those with low background rates, suggested by standardized field sampling experiments along grassland degradation sequences on the Tibetan Plateau.