Effects of long-term green manure application on organic carbon fractions and clay minerals and their interactions in paddy soil aggregates

Background and aims Green manure is commonly used to improve soil organic carbon (SOC) stock, which is mainly combined with soil minerals and stored in aggregates. The effects of three green manure varieties on mineral associated organic carbon (mSOC) and clay minerals in macroaggregates and microaggregates and SOC sequestration mechanism were explored in paddy soils in southern China.Methods A 36-year (1982-2018) field experiment was conducted on a rice-rice-green manure crop rotation system, including rice-rice-winter fallow (WF), rice-rice-Chinese milk vetch (MV), rice-rice-rape (RP), and rice-rice-ryegrass (RG). Fourier-transform infrared, X-ray diffraction, field emission scanning electron microscopy and high resolution transmission electron microscopy were used to analyze mSOC, revealing composition characteristics and interactions between SOC and minerals.Results Green manure application increased SOC contents in the bulk soil by 8.4% (MV, p < 0.05), 1.2% (RP) and 4.8% (RG) compared to WF, and mainly altered SOC composition in mSOC of macroaggregates. Dominant functional groups in mSOC were polysaccharide C and alkene C. Phyllosilicate minerals of mSOC were mainly illite (51.0-61.9%) and kaolinite (29.2-37.3%). MV application caused significant changes in illite, with fractured lattice stripes within mSOC in macroaggregates and bent lattice stripes within mSOC in microaggregates. Futhermore, lattice fringe spacing under MV contracted from 1.0 to 0.7 nm within mSOC in microaggregates, suggesting that MV favored the transformation of illite into kaolinite. The SOC content of mSOC was positively correlated with the amount of illite but negatively with kaolinite.Conclusions Green manure returning (especially MV and RG) altered the SOC content of mSOC by converting 2:1 minerals into 1:1 minerals, and emerged as an effective strategy for SOC accumulation in paddy soils, which was further governed by aggregate size.