JGR-BNF of Pueraria lobata communities

Biological N2 fixation plays crucial roles in determining the pattern and pace of ecological succession and carbon sequestration. Nevertheless, the rates and controls of symbiotic (SNF) and free-living (FNF) N2 fixation have rarely been investigated simultaneously. Here we measured SNF and soil FNF rates for eight early succession ecosystems dominated by kudzu (Pueraria lobata (Willd.) Ohwi) in a subtropical region of China in growing and non-growing seasons. N2 fixation rates were measured using acetylene reduction assay against a 15N2 uptake method. Both SNF and soil FNF rates were significantly (P < 0.05) higher in the growing season than in the non- growing season. The two forms of N2 fixation contributed equally to total N2 fixation rate across the eight sites with soil FNF being higher than SNF in half of the sites. Total N2 fixation rates ranged from 7.12 to 61.93 kg N ha-1 yr-1 with an average of 22.93 ± 6.52 kg N ha-1 yr-1. N2 fixation contributed significantly to soil nitrogen and carbon accumulation. Both SNF and FNF were significantly related to soil moisture. Additionally, SNF and soil FNF were strongly correlated with phosphorus availability and vanadium/iron availability, respectively, mostly likely implying that SNF was limited by phosphorus, and soil FNF was limited by vanadium or iron. Our findings suggest that SNF and soil FNF may be limited by different sets of nutrients within an ecosystem.