Leguminous green manure enhances the soil organic nitrogen pool of cropland via disproportionate increase of nitrogen in particulate organic matter fractions

Abstract The nitrogen (N) content of soil organic matter (OM) fractions is a vital indicator of soil health as it limits crop yield and regulates the loss of N from soil to the environment. We studied the impacts of introducing leguminous green manure (LGM) to replace summer fallow on N distribution in coarse particulate OM (cPOM), fine POM (fPOM), intra-microaggregate POM (iPOM) and mineral associated OM (MAOM). Random forest regression and structural equation modelling were coupled to identify the important OM fractions and reveal how they affect soil organic N pool. The field experiment followed a split-plot design with summer fallow – winter wheat (Triticum aestivum L.) (FW) (control) and LGM – winter wheat as the main treatments. The latter included Huai bean (Glycine soja Sieb. et Zucc) – winter wheat (HW), soybean (Glycine max (L.) Merr.) – winter wheat (SW) and mung bean (Vigna radiata (L.) Wilczek) – winter wheat (MW). The sub-treatments included applying 0 or 135 kg ha−1 of synthetic N fertilizer to winter wheat. Both LGM and synthetic N had limited effects on the mass ratio of OM fractions. Compared with FW, HW and MW increased the N content of unprotected OM fractions (cPOM + fPOM) by 38–58% and all LGM treatments increased the N content of protected OM fractions (iPOM + MAOM) by 9–16%. The main driving factors for enhancing the soil organic N pool were the N in cPOM and iPOM. The indirect paths through the cPOM mediated by the iPOM in large and small macroaggregates were the key pathways that regulated the soil organic N pool. In conclusion, LGM expands the soil organic N pool mainly by regulating the N content of cPOM and iPOM, suggesting this practice can propel sustainable crop production via enhanced short- and long-term soil N supplying capacity.