Soil biological nitrogen fixation is closely associated with soil ammonium nitrogen content in a mowing semiarid natural grassland

Soil biological nitrogen fixation (BNF) plays a significant role in N input in terrestrial ecosystems, and can be largely altered by management. In semi-arid region of Northern China, natural grasslands rely on soil BNF to maintain nitrogen input under long-mowing, but the effects of mowing on BNF remain unclear. Here we assessed the soil BNF rate and the nifH gene abundance in soil (0-10 cm) of the grasslands subject to long-term (19 years) annual mowing (MO) versus no mowing (NM) in a semiarid natural steppe grassland. Our results indicated that mowing significantly increased the BNF rate (P < 0.01) from 11.48 g N ha(-1) d(-1) (NM) to 25.16 g N ha(-1) d(-1) (MO); mowing also significantly increased average N fixation activity per nifH gene (P < 0.05), while reduced the nifH gene abundance (P < 0.05). The nifH gene abundance was not significantly correlated with the BNF rate (P > 0.05), suggesting that the nifH gene abundance based on DNA analysis was not indicative for BNF rate; while soil ammonium nitrogen (NH4+-N) content was identified by stepwise multiple regression the only variable that can significantly explain the variation in BNF rate. Our results suggest that soil NH4+-N content is the most efficient predictor of BNF rate instead of nifH gene abundance, and it is more crucial to quantify the impacts of soil NH4+-N than the effects of diazotrophic abundance in predicting the changes in BNF rate in response to mowing management in semiarid grassland.