Rhizosphere nitrogen cycle

Plants have evolved a range of nutrient mining mechanisms that are influenced by the environment. To optimize crop N uptake and reduce fertilizer N inputs, the feedback effects of plants on the nitrogen (N) cycle in the rhizosphere need to be determined. So far, isolated studies have prevented our general understanding of the rhizosphere N cycle. Here, we assembled a paired dataset containing 698 N cycling fluxes to analyze plant feedback on the rhizosphere N cycle. We found that plants significantly stimulated heterotrophic nitrification of organic N (P < 0.001), a long-neglected N mineralization process, and microbial nitrate immobilization (P < 0.001), which were associated with the stimulation of dissolved organic carbon (C) (+12%), bacterial (+29%) and fungal (+19%) abundance. Moreover, plants significantly inhibited autotrophic nitrification (P = 0.003) and tended to inhibit ammonification (P = 0.070). Structural equation modeling revealed that increased C-to-N ratio weakened plant regulation of ammonification and heterotrophic nitrification, thereby impairing N uptake. Since meta-analyses do not address causality, we carried out specific experiments using 15N tracing combined with numerical analysis methods and microbial isolation culture to investigate the interaction between heterotrophic nitrification in soil and plants. We confirmed that heterotrophic nitrification is usually triggered by plant growth and rapidly disappears after plant removal, implying a high dependence of microbial-mediated heterotrophic nitrification of organic N on plant activity, a potential N supply mechanism that has largely been neglected. Our findings indicated that optimizing the rhizosphere N cycle can reduce exogenous N supply and thus provide a nature-based solution towards sustainable cropping systems.