Abstract: Vegetation types play an important role in controlling soil nitrogen (N) leaching that is a key indicator of soil N loss and water eutrophication in a watershed. In this study, we measured soil NH₄⁺-N and NO₃^--N leaching using ionexchange resin bags method in Acer mono-Quercus mongolica, Larix gmelinii and Populus davidiana water conservation forests in the Dahuofang watershed. Our results showed that soil NH₄⁺-N and NO₃^--N concentrations were significantly different under different vegetations during the precipitation abundant season from July to September. Soil NH₄⁺-N accounted for 73.40%-93.30% of soil total inorganic N. In July, soil NO₃^--N concentration was the highest in A. mono-Q. mongolica and P. davidiana forests, while soil NO₃^--N content in L. gmelinii plantation was the highest in August. NO₃^--N is the main form of soil N under leaching loss, and soil N loss was higher in L. gmelinii plantation compared to A. mono-Q. mongolica and P. davidiana forests. Soil microbial biomass carbon (MBC) and nitrogen (MBN) decreased gradually with increasing soil depth, and soil MBC and MBN was significantly higher in A. mono-Q. mongolica and P. davidiana forests than that in L. gmelinii plantation. There was a negative relationship between soil N loss and MBN, suggesting a reduced soil N leaching loss by soil microbial retention. Our results implied that more attention should be paid to the protection of natural secondary forests and the promotion of the artificial water conservation plantations into natural secondary forests in the Dahuofang watershed in order to reduce soil N loss.

Key words: association dynamic, forest landscape, spatial pattern dynamic, spatial point pattern