Labile fractions of soil organic C are considered important indicators of soil quality as these can respond rapidly to land-use changes and agricultural management. We studied the impact of three different land-use systems viz. poplar-based agroforestry involving wheat-legume rotation, rice-wheat and maize-wheat agroecosystems, on dynamics of total organic C (TOC), oxidisable soil organic C (SOC), very labile, labile, less labile, and recalcitrant C fractions, water extractable organic carbon (WEOC), hot water soluble C (HWC), microbial biomass C (MBC), and mineralizable C in the semi-arid subtropical India. The maize-wheat and agroforestry systems had 65-88% higher SOC stocks than the rice-wheat system and were characterized by predominantly labile C. About 56-60% of the total organic C in maize-wheat and agroforestry systems occurred as labile and very labile C compared to 37% under rice-wheat rotation. Contrarily, the majority of organic C (63%) in rice-wheat soils was stabilized in less labile and recalcitrant forms. The HWC and MBC were also higher in maize-wheat and agroforestry systems as opposed to the rice-wheat system. In the discriminant function analysis, a composite of indicators involving TOC, recalcitrant C and total N correctly distinguished the soils under the three systems. The results suggested that in agroforestry and maize-wheat systems the organic C in soils was less stable and thus could be lost following the land-use change.
