Soil Organic Carbon Stocks Under Long-Term Rice-Wheat and Rice-Fallow Systems

Abstract

Soil organic carbon under rice-wheat and rice-fallow systems is often discussed as if a single number could describe it, yet the figures that circulate in the literature rest on choices that are rarely stated: the depth sampled, the way bulk density is handled, the baseline against which change is judged, and the number of samples behind each mean. This narrative review examines what is known about organic carbon stocks in long-term rice-based systems of South Asia and why estimates from different experiments are so hard to compare. It draws on published work on paddy soil biogeochemistry, on long-term fertiliser and manure experiments, and on the methods used to convert concentrations into stocks, and it reports no new data. The alternation of flooded rice with a drained wheat phase, or with a fallow, changes how residues decompose and how carbon is stabilised, so the same practice can behave differently in each phase. Change in stock is slow relative to sampling noise, and equal-depth comparisons can mislead when management alters bulk density. The review concludes that apparent gains and losses should be read with their method attached, and it sets out a reporting standard that would make future long-term records comparable.