Impact of land use changes on the storage of soil organic carbon in active and recalcitrant pools in a humid tropical region of India

Quantifying soil organic carbon (SOC) dynamics is important in understanding changes in soil properties and carbon (C) fluxes. However, SOC measures all C fractions and it is not adequate to distinguish between the active C (AC) and recalcitrant or passive C (PC) fractions. It has been suggested that PC pools are the main drivers of long term soil C sink management. Therefore, the present study was undertaken with the objective of determining whether or not SOC fractions vary with land use changes under a humid tropical climate in the North East India. A chronosequence study was established consisting of natural forest, Imperata cylindrica grassland and 6, 15, 27 and 34 yr old rubber (Hevea brasiliensis) plantations to determine changes in the different fractions of SOC and total SOC stock. SOC stocks significantly varied with soil depth in each land use practice. SOC stocks increased from 106 Mg ha(-1) under 6 yr to 130 Mg ha(-1) under 34 yr old rubber plantations. The SOC stocks under 34 yr old plantations were 20% higher than that under I. cylindrica grassland, but 34% lower than SOC stocks recorded under natural forest soil. The proportion of AC pools decreased with increase in plantation age, AC pools being 59% of SOC stock in 6 yr old stands and 33% of SOC stocks in 34 yr old plantations. In contrast, the proportion of PC pools increased from 41% of SOC stock in 6 yr old plantation to 67% of SOC in 34 yr old plantation. In the 50-100 cm soil depth, the PC pool under 27-34 yr old plantations was comparable with that under natural forest but much higher than in I. cylindrica grassland. Therefore, it is concluded that old rubber plantations can play a significant role in long term soil C sink management. (C) 2017 Elsevier B.V. All rights reserved.