Fertilization Effects on Symbiotic and Free-Living Biological Nitrogen Fixations: Similar Effects But Different Mechanisms

Biological nitrogen fixation (BNF) by N-fixing microorganisms (NFMs) is one of the critical nitrogen (N) input pathways in terrestrial ecosystems, and is largely affected by human activity during fertilization. However, the responses and underlying mechanisms of symbiotic and free-living BNFs to fertilization remain unclear. This study conducted a global meta-analysis to reveal the responses and mechanisms of symbiotic and free-living BNFs to N, phosphorus (P), potassium (K), and organic (O) fertilizers in agroecosystems. The results showed that symbiotic and free-living BNFs had similar responses to different fertilizers. Specifically, N fertilization significantly decreased the symbiotic BNF (absolute N fixation rate: -46.43%) and free-living BNF (absolute N fixation rate: -48.94%), whereas P, K, and O fertilizers had opposite effects. However, the regulatory mechanisms underlying similar responses were different. The negative effect of N fertilization on symbiotic BNF was due to the decrease in weight and number of nodules, whereas the negative effect of N fertilization on free-living BNF was due to the free-living NFMs’ preference for soil N, decreased in soil pH and C/N ratio, and increased in NH4+ concentration. The P, K, and O fertilizers promoted symbiotic BNF by directly or indirectly stimulating root nodule growth, whereas P, K, and O fertilizers increased free-living BNF by promoting the abundance of free-living NFMs and mitigating the decrease in soil pH and C/N ratio caused by N fertilization. These results indicate that inappropriate fertilization can lead to soil inactivation by suppressing NFMs. Optimizing fertilization strategies is critical to achieving high yields by exploiting BNFs and maintaining sustainable soil health.