Dynamics of soil N cycling and its response to vegetation presence in an eroding watershed of the Chinese Loess Plateau

Soil nitrogen (N) mineralization is an important ecological process influencing N availability and ecosystem functionality, and is sensitive to vegetation presence and soil erosion. However, how vegetation presence affects N mineralization and its response to erosion has not been directly examined, hindering our ability to predict N cycling in eroding environments. Herein, we compared the effect of vegetation presence on N mineralization and mineral N dynamics in two growing seasons between erosion vs. deposition sites in three sub-watersheds in the transition zone between water erosion and wind erosion of North China. We aim to examine how soil N cycling responds to erosion and vegetation presence at temporal and spatial scales. The mineral N concentration in the soil solution and mineralization rates between May 2017 and October 2018 in plots with and without vegetation were measured to assess the effects of vegetation presence. We showed that N mineralization rates did not significantly vary among the three sub-watersheds or between erosion and deposition sites. The concentrations of mineral N in the soil solution were higher in relatively larger sub-watersheds for the erosion sites and in smaller sub-watersheds for the deposition sites. Vegetation decreased soil NO3--N and total mineral N and increased NH4+-N concentrations in the early and middle periods of the growing season. The effects of vegetation presence on net N mineralization rates varied with size of sub-watersheds, with increased rates of net nitrification and mineralization at both deposition and erosion sites in relatively larger sub-watershed, but reduced rates of net nitrification and mineralization in smaller sub-watersheds. Moreover, the presence of vegetation shifted the seasonal pattern of N mineralization. Therefore, vegetation presence had important ecological regulation on soil N cycling at spatial and temporal scales in the transition zone between water erosion and wind erosion.