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 (${\mathrm{NH}}_4^{+}$-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 ${\mathrm{NH}}_4^{+}$-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 ${\mathrm{NH}}_4^{+}$-N than the effects of diazotrophic abundance in predicting the changes in BNF rate in response to mowing management in semiarid grassland.