Converting leguminous green manure into biochar: changes in chemical composition and C and N mineralization

•Conversion of leguminous green manure into biochar was conducted.•NMR and NEXAFS were applied for determining C and N structures.•Both C and N mineralization rate in the biochar were reduced.•C sequestration potential of green manure can be achieved by charring.•Charring may limit green manure to provide N nutrients.

Leguminous green manure is an important source of nitrogen (N) and carbon (C) in cropping systems. The fast turnover of leguminous green manure enables it to release N quickly, but limits its effectiveness in maintaining soil organic C content. Converting leguminous green manure into biochar facilitates its use as a soil amendment. In this study, we assessed how the conversion of leguminous green manure (Sesbania roxburghii) into biochar altered its chemical composition and subsequent C and N mineralization. Biomass was charred along a temperature gradient from 200 to 500 °C. Using nuclear magnetic resonance and near-edge X-ray adsorption fine structure spectroscopy, we found that both C and N became enriched in aromatic and heterocyclic aromatic structures in biochar, and this structural change led to a reduction in C and N mineralization rates. The mineralized C decreased from 32.7% of the added C of raw biomass to < 0.5% of that of biochar at charring temperatures above 400 °C. N release shifted from N mineralization in raw biomass to N immobilization at charring temperatures at 500 °C. As such, soil amended with biochar produced at charring temperatures exceeding 400 °C demonstrated a 25% decrease in dry shoot biomass compared with unamended soil. The results indicated that the C stability of leguminous green manure can be achieved by converting raw material into biochar, but that the charring process may limit it to providing N.