Leguminous green manure promotes N accrual in labile and persistent soil organic matter pools

Soil organic nitrogen (SON) provides an important source of crop N uptake. Our study aimed to better understand N incorporation in soils and the importance of various labile and persistent SOM pools in supplying plant-available N.

In this 80-day greenhouse study, we used ¹⁵N isotope to trace the fate of N from green manure (Austrian winter pea) vs. ammonium sulfate among uptake by wheat plants, accrual to various soil pools, and losses. We compared new N retention and changes in existing SON reserves between two soils with long-term histories of receiving leguminous green manure or solely inorganic fertilizer (F_i) inputs.

The green manure had a greater capacity to build SON pools, with 51% of the green manure N retained in soil compared to 31% of the ammonium-N recovered from soil. Soil with historical green manure inputs showed a 19% greater SON content and an 18% greater plant N uptake than soil with a history of solely F_i input. Microbial assimilation of the newly added N was positively correlated with N incorporation in the mineral-associated organic matter (MAOM) and the occluded particulate organic matter (oPOM) pools, which were negatively associated with N loss. Moreover, plant N uptake was positively correlated with initial oPOM-N content and the corresponding decline in oPOM-N as plants grew.

Enhancing microbial N assimilation and its subsequent incorporation into soils, particularly the oPOM pool, can be an effective strategy to reduce N loss and to replenish SON for long-term soil fertility.