Changes in the biological N2-fixation rates and diazotrophic community as vegetation recovers on abandoned farmland in a karst region of China

Abstract

The successful regeneration of karst landscapes that have been severely degraded by intensive agriculture relies on the recovery of biological nitrogen fixation (BNF) by diazotrophic communities in soils after nitrogen (N) derived from synthetic fertilizers dissipates through the soil. It is important to understand the mechanisms that drive BNF because there must be enough bioavailable N in soils to support the recovery of native vegetation communities that are best adapted to erosion-prone karst landscapes. In this study, we quantified the response of the diazotrophic population in topsoil (0–10 cm deep) along a vegetation restoration gradient (sloping cropland > abandoned sloping cropland > secondary forest > primary forest) in the Puding Karst Critical Zone Observatory, Guizhou Province, southwest China from the absolute and specific BNF rates, determined using the acetylene reduction method and qPCR and MiSeq sequencing of nifH genes. The nifH phylogenetic diversities increased as the vegetation increased in maturity and were positively correlated with soil moisture and the N to phosphorus ratio (p < 0.05). The diazotrophic community structure changed with the vegetation recovery time (p < 0.05). Rhizobiales accounted for between 38% and 45% of the total nifH gene sequences and were the most abundant order in the vegetation that had been recovering for the longest time. We found that the soil bulk density was a strong control on the diazotroph community structure and was negatively correlated with Rhizobiales. The absolute and specific BNF rates were the highest in primary forest and abandoned sloping cropland, respectively (p < 0.05). The results from stepwise multiple regression showed that 50% of the variation in the absolute BNF rates was explained by SWC, and 55% of the variation in the nifH specific rates was explained by the available N contents and the diazotrophic community structure. We suggest that the recovery of degraded karst could be accelerated by managing the soil physical structure so that the environment is suitable for diazotrophic communities, which would then improve the BNF efficiency.