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

• Using LGM is an effective practice to enhance soil organic nitrogen (SON) pool. • The N content of physically protected iPOM is directly linked to SON. • Unprotected cPOM is responsive to LGM and it controls the N content of iPOM. • Higher N content of cPOM under LGM increases SON via both direct/ indirect paths. • The responses of N in cPOM to management practices could be crucial to predict SON. 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. [ABSTRACT FROM AUTHOR]