Carbon dynamics in teak planted laterite soils of Kerala

Abstract

Global warming due to increased concentration of greenhouse gases (GHGs) is one of the important concerns of mankind today. The global soils store about 2500 Gt of carbon which is approximately 3.3 times the atmospheric carbon pool (760 Gt) and 4.5 times than that of the biotic pool (560 Gt) . Kerala has a rich forest cover of which 10 per cent is occupied by forest plantations. Among the plantation species, teak occupies the most prominent position both in acceptance and coverage. However, continuous teak rotation affects the quantity and stability. Hence the present study was taken up to assess the changes in carbon fractions and carbon stability under continuous teak rotations in midland laterites of Kerala. Soils were collected to a depth of 1 m with a depth interval of 20 cm in a chronosequence from 16 plantations at Nilambur, Kerala. Basic soil parameters such as pH, electrical conductivity, bulk density, mean weight diameter, texture, available nitrogen, potassium, phosphorus, iron, calcium, copper, manganese, magnesium, zinc and boron content were analyzed using standard protocols. The carbon fractions were assessed with respect to active, slow, passive and total carbon in soil as well as macro and micro aggregates. The study also assessed the rate kinetics, thermal stability and cumulative CO2 efflux of soil carbon decomposition in these soils by batch incubation experiments at different temperatures. The results showed a reduction of soil basic characters below critical levels with continuous teak rotations. In general, the carbon content was found to decrease depth wise with rotation. However, in plantations without felling organic carbon was found distributed equally in all the layers which may be due to the unhindered transportation and translocation of humic materials with time in these plantations. The carbon stocks in the plantations do not have significant difference between the age groups in their carbon storage capacity. This indicated that carbon losses from the teak plantation are readily replenished and the changes may be expected only in the quality of the stored carbon. Macroaggregates were found to store more organic carbon than the microaggregates. The correlation between carbon and aggregate stability was found to decrease with rotation. Plantation that was grown continuously showed a relatively lower proportion of active carbon than slow and passive carbon fractions. This indicated that even though there was a total carbon increase in plantations under continuous growth, the entire carbon may not be in a labile form to support ecosystem services. Thermal stability studies showed that microaggregates provided better protection to organic carbon by a greater complex mechanism compared to macroaggregates. With temperature, there was an increased conversion of active carbon to passive forms and this conversion could lead to higher carbondioxide evolution once the threshold energy levels were attained. Carbon dioxide efflux studies confirmed these results as higher cumulative CO2 evolution was obtained at 40°C than 25°C in all soils. Further, cumulative CO2 evolution from continuous plantation without felling didn‘t get affected with temperature indicating a dynamic equilibrium with atmosphere. The present study concluded that continuous teak rotation destabilizes carbon in soil and shows the potential to revert to a carbon source than sink if not managed sustainably.