Dominant mycorrhizal association of trees determines soil nitrogen availability in subtropical forests

Trees and their associated arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi drive carbon (C) and nitrogen (N) cycling patterns. However, the mechanisms underpinning this relationship and the degree to which mycorrhiza mediate the effects of forest tree composition on soil N cycling outside of the temperate zone remain unclear. Here, we conducted field surveys to establish a natural gradient with increasing ECM tree dominance in subtropical forests, and explored the effects of tree mycorrhizal associations on soil N cycling. Across this mycorrhizal gradient, we quantified soil available N, microbial biomass N, net N transformation rates, as well as N-acquiring enzyme activities, litterfall biomass and quality, and soil pH during the one-year growing season. We found that with increasing ECM tree dominance, the dissolved organic N and nitrate N decreased, while the microbial biomass N increased linearly. Soils dominated by AM trees had 1.8-2.3 times larger mineral N contents, net N mineralization rate, and net nitrification rate than ECM-dominated stands, suggesting a rapid N cycling in AM-dominated stands. ECM trees and their associated fungi reduce the net N mineralization rate in topsoil mainly by producing recalcitrant litterfall (i.e., higher C:N) and by secreting beta-N-acetylglucosaminidase to deplete N directly from soil organic matter, which together increased N limitation for free-living decomposers. The low pH and high C:N ratio in ECM-dominated soils inhibit the proliferation of ammonia-oxidizers and thus decrease the net nitrification rate. Our results demonstrate that the increasing ECM tree dominance increase soil N-acquiring enzyme activity and C:N ratio but decrease pH value, all of them critically mediating soil N availability. Consequently, by altering the relative abundances of tree mycorrhizal associations shifts in forest composition under global changes and plantation establishment can be expected to result in altered soil N cycling.