Converting rice paddy to upland fields decreased plant lignin but increased the contribution of microbial residue to SOC

• Conversion of paddy to upland lowered SOC , microbial biomass and nutrient availability. • Contribution of plant lignin phenols to SOC decreased in upland soils . • Microbial residues declined but their relative contribution to SOC increased in upland soils. • Soil moisture determined plant lignin phenols and microbial residues accumulation. Most land-use changes profoundly alter the composition of soil organic carbon (SOC) via changes in plant- and microbial-derived C in soils. Whether this equally holds true for soils converted from paddy fields to upland fields is hitherto unknown. Herein, we investigate and report on plant-derived C (lignin phenols as biomarkers), microbial-derived C (amino sugars as biomarkers), Phospholipid fatty acids (PLFAs), and soil properties following the conversion of rice paddy to upland (vegetable and jasmine) fields in a subtropical farmland ecosystem. Lower moisture, pH, and available nitrogen (AN) but higher temperatures were observed in upland soils compared to paddy soils. Moreover, fungal- and bacterial-PLFAs were 41–63% and 58–69% lower, respectively, in upland soils than in paddy soils. Compared to paddy soils, concentrations of SOC, total lignin phenols (VSC) and amino sugars (TAS) declined in upland soils by 18–46%, 32–70% and 6–31%, respectively. Converting rice paddy fields to vegetable/jasmine cropping fields also altered the composition of SOC; i.e., the contribution of total lignin phenols to SOC declined by 25–48% in the upland soils, while total amino sugars input towards SOC increased by 22–28%. Our findings highlight the ecological risk of carbon loss caused by land-use changes, namely, the conversion of paddy fields to upland field systems.