Tree diversity, growth status, and spatial distribution affected soil N availability and NO efflux: Interaction with soil physiochemical properties.

Soil nitrogen (N) is an essential nutrient for tree growth, and excessive N is a source of pollution. This paper aims to define the effects of plant diversity and forest structure on various aspects of soil N cycling. Herein, we collected soils from 720 plots to measure total N content (TN), alkali-hydrolyzed N (AN), nitrate N (NO-N), ammonium N (NH-N) in a 7.2 ha experimental forest in northeast China. Four plant diversity indices, seven structural metrics, four soil properties, and in situ NO efflux were also measured. We found that: 1) high tree diversity had 1.3-1.4-fold NO-N, 1.1-fold NH-N, and 1.5-1.8-fold NO efflux (p < 0.05). 2) Tree growth decreased soil TN, AN, and NO-N by more than 13%, and tree mixing and un-uniform distribution increased TN, AN, and NH-N by 11-22%. 3) Soil organic carbon (SOC) explained 34.3% of the N variations, followed by soil water content (1.5%), tree diameter (1.5%) and pH (1%), and soil bulk density (0.5%). SOC had the most robust linear relations to TN (R = 0.59) and AN (R = 0.5). 4) The partial least squares path model revealed that the tree diversity directly increased NO-N, NH-N, and NO efflux, and they were strengthened indirectly from soil properties by 1%-4%. The effects of tree size-density (-0.24) and spatial structure (0.16) were mainly achieved via their soil interaction and thus indirectly decreased NH-N, AN, and TN. Overall, high tree diversity forests improved soil N availability and NO efflux, and un-uniform spatial tree assemblages could partially balance the soil N consumed by tree growth. Our data support soil N management in high northern hemisphere temperate forests from tree diversity and forest structural regulations.