Different strategies for regulating free-living N2 fixation in nutrient-amended subtropical and temperate forest soils

Abstract

Biological nitrogen (N2) fixation by free-living diazotrophs is an important source of bioavailable N in forest ecosystems. However, little is known about how these free-living diazotrophs are affected by exogenous nitrogen (N) and phosphorus (P) additions. Set in the context of increases in both N deposition and P deficits in forest soils, we examined how the abundance, composition, and activity of free-living microbes from the diazotrophic functional group changed in response to N and/or P additions, and identified the main factors that drove free-living N2 fixation in nutrient-amended subtropical and temperate forest soils. We found that the free-living N2 fixation was higher in the temperate soils than in the subtropical soils, which was mainly explained by the soil pH. The compositions and richness of the free-living diazotrophs were stable, but different strategies were used to regulate the N2 fixation in these two sites. While the free-living N2 fixation did not change much when P was added, it decreased when N was added to the temperate soils, reflecting a decrease in the abundance of the nifH gene. In the subtropical soils, the free-living N2 fixation was mainly stimulated at the physiological level by P additions and did not change when N was added. The nifH gene abundances and soil available P concentrations were the main predictors on N2 fixation by free-living diazotrophs in nutrient-amended temperate and subtropical forest soils, respectively, and so should be considered when modelling N cycling in these ecosystems under scenarios of increased N deposition and P deficiency.