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Mineralisation of soil organic carbon in two Andisols under oil palm: an incubation study into controlling factors

Iain Goodrick , Paul Nelson

Abstract

Understanding the factors controlling stability against mineralisation of soil organic matter is important for predicting changes in carbon stocks under changed environment or management. Soil carbon dynamics in oil palm plantations are little studied and have some characteristics that are unusual compared to other agricultural soils, such as high management-induced spatial variability and warm moist conditions. The aim of this work was to determine the factors controlling the mineralisability of the intermediate-stability carbon fraction of volcanic ash surface soils (0-5 and 15-20 cm depth) from oil palm plantations in Papua New Guinea. Soils with carbon contents of 2.2 - 35.2 %, from areas with low and high organic matter inputs, were incubated for up to 812 days and soil respiration was measured periodically. Mean carbon turnover rates were 0.18-1.58, 0.07-0.23 and 0.03-0.07 a-1 at days 54, 379 and 812, respectively. Turnover rate was initially (day 54) correlated with pre-incubation total carbon content (r=0.88), the ratio of permanganate-oxidisable-to-total carbon (r=0.62) and the ratio of oxalate-extractable Al- and Fe-to-total carbon (r= -0.51 and -0.54, respectively), but the correlations decreased with time, being insignificant at day 812. In the soils that had changed from C4 grassland 25 years previously, turnover rate was negatively correlated with δ13C, which increased with depth, but δ13C did not change significantly over the course of the incubation. Temperature sensitivity of mineralisation varied little, despite large differences in soil properties and changes in mineralisation rate. This suggested that turnover rates were affected to similar extents by biochemical recalcitrance and physical protection, as these two factors influence temperature sensitivity in opposing directions. Physico-chemicalal protection of organic matter appeared largely related to interaction with poorly crystalline Al and Fe oxides.

SR16089  Accepted 22 July 2017

© CSIRO 2017