Search

Free‐living N2 fixation (FNF) is an important source of bioavailable nitrogen for forests. Though increasing plant species diversity (PSD) benefits soil nitrogen accumulation, how it impacts FNF rate has not been explored.Forty‐five plots covering a PSD gradient were selected in a subtropical karst forest, southwest China. FNF rates in leaf litter and soil were measured using acetylene reduction assay calibrated with a 15N2 fixation method. Diazotrophic community, litter and soil properties were determined as well.The nifH gene abundance was not significantly altered in litter but was significantly decreased in soil by higher PSD. Diazotrophic community's Shannon diversity was significantly increased by higher PSD in both litter and soil. Increasing PSD inhibited litter FNF rate via aggravating phosphorus limitation and decreasing the relative abundance of Betaproteobacteria and Deltaproteobacteria. For the mineral soil horizon, increasing PSD suppressed FNF rate via decreasing nifH gene abundance caused by elevated N availability and lowered phosphorus and iron availability or through decreasing the relative abundance of Deltaproteobacteria but increasing that of Betaproteobacteria.Our findings, for the first time, reveal the microbial and abiotic controls on FNF in litter and soil in response to PSD, and hence benefit the prediction of nitrogen input via biological N2 fixation under changes in PSD. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

Biological N2 fixation (BNF) is a major pathway of external nitrogen input to terrestrial ecosystems. Nevertheless, the relative effects of diazotrophic community and soil properties on symbiotic (SNF) or free-living (FNF) N2 fixation remain poorly understood. Here, 20 sites of kudzu (Pueraria lobata) communities, which were at the early succession stage following agricultural abandonment, were selected across a karst area in southwest China. BNF rates were determined using the acetylene reduction assay calibrated with the 15N2 fixation method. The average rate was 17.81 ± 2.17 mg N g−1 day−1 nodule for SNF and 45.36 ± 5.15 ng N g−1 day−1 soil for FNF. Diazotrophic communities were dominated by Bradyrhizobium at the genus level in both nodule and soil. According to structural equation modeling, the relative abundances of two diazotrophic genera (Desulfovibrio and Geobacter) were the strongest explanatory variable for the variation of SNF rates with the second strongest variable being nifH gene abundance. Soil water content and available phosphorus indirectly affected SNF via their effects on diazotrophic community composition. In contrast, soil FNF rates were most pronouncedly affected by the availability of vanadium and iron, followed by soil water content, available phosphorus, and the relative abundances of two diazotrophic genera, i.e., Burkholderia and Cupriavidus. Our findings therefore help to improve the understanding of the relationship between BNF rates and their associated diazotrophic communities, and hence benefit a better prediction of BNF under global change. [ABSTRACT FROM AUTHOR]

Protein, derived from plant and microbial residues, constitutes a significant portion of soil nitrogen or organic carbon pool and plays a crucial role in determining soil nitrogen and carbon sequestration. Though diversifying plant species has been regarded as a promising strategy for promoting soil nitrogen or organic carbon accumulation during vegetation restoration or afforestation, how increasing plant species diversity (PSD) would influence soil protein pool remains unknown. We selected 45 plots covering a natural gradient of PSD, which was reflected by the Shannon index and varied from 0.15 to 3.57, in a subtropical karst forest. The major objective was to investigate the impact of increasing PSD on soil protein abundance, with soil protein depolymerization rate and multiple biotic and abiotic variables being measured to explore the underlying mechanism. The relative abundance of soil protein, that is proportion in soil organic matter varied between 24.1 and 34.0% with an average of 29.2 ± 2.3% across the 45 plots. Soil protein abundance increased on average by 1.56% with each unit of PSD increase. Nevertheless, soil protease activity, protein depolymerization rate and microbial amino acid uptake rate all significantly (p < 0.05) decreased as PSD increased. The mechanism underlying the increase of soil protein abundance in response to higher PSD included three aspects, that is stimulating microbial biomass and subsequent microbial residue input to the soil, promoting mineral protection of soil protein via elevating levels of soil exchangeable calcium and magnesium, and decreasing soil protease activity and protein depolymerization due to aggravated soil microbial phosphorus limitation. Synthesis and application: Our study suggests that increasing PSD is an efficient way to promote soil protein accumulation. Given that protein is a major component of soil nitrogen or organic carbon pool, diversifying plant species during vegetation restoration or afforestation would hence benefit soil nitrogen and organic carbon sequestration. [ABSTRACT FROM AUTHOR]