Changes in Aggregate-Associated Carbon Pools and Chemical Composition of Topsoil Organic Matter Following Crop Residue Amendment in Forms of Straw, Manure and Biochar in a Paddy Soil

In agricultural ecosystems, incorporation of crop residues has been practiced as a recycling approach for sustaining soil organic matter (SOM) and soil fertility. However, how crop residue amendments in different forms (direct straw return, converted as manure and pyrolyzed as biochar) affect soil organic carbon (SOC) pools and SOM composition is not well known. In this study, a short-term (2015–2019) field experiment in paddy soil was conducted with a one-time maize residue amendment, consistently in a single doses of 10 Mg ha−1 organic carbon (OC) equivalent, in three forms: air-dried straw (CS), cattle manure (CM) and biochar (CB). No residue amendment (CK) was used as control. Topsoil organic matter changes were analysed using 13C isotopic tracing, biomarker analysis, and solid-state 13C nuclear magnetic resonance spectroscopy in combination with soil aggregate density/size fractionation. After four cropping cycles following amendment, SOC content was unchanged under CS and CM but increased by 24% under CB; However, the OC pool ratio of particulate organic matter (POM) to mineral-associated organic matter (MAOM) significantly increased under all amendments compared to CK. The δ13C values indicated that maize-derived OC was preserved the most in the POM within the macroaggregates, particularly under CB. Regarding the molecular composition, all residue amendments increased the abundance ratio of plant- to microbe-derived lipids. Plant-derived lipids were primarily concentrated in macroaggregates, whereas microbial lipids were more prevalent in the silt–clay fractions. Lignin phenols were significantly enriched only in microaggregates under CS relative to CK. Overall, SOM changes in the paddy topsoil following crop residue amendment in different forms were depicted by OC pool redistribution and molecular composition alteration. The study highlighted that biochar amendment, instead of straw or manure, greatly enhanced SOC accumulation by promoting macro-aggregation, which in turn preserved plant-derived carbon through the direct input of the persistent char in a rice paddy.