Soil carbon dynamics under oil palm plantations

Abstract

Oil palm crops are expanding rapidly in the tropics, with implications for the global carbon cycle. Understanding the carbon dynamics of oil palm is important for maximising organic matter content of soils and minimising greenhouse gas emissions of plantations. In a series of laboratory and field experiments the nature of the oil palm carbon cycle is described for a group of plantations in Papua New Guinea. In this work, oil palm soil organic carbon (SOC) stocks were found to be highly stable and SOC stocks and input/output fluxes were highly spatially variable. Where oil palm was planted on former grassland, unplanted areas of grassland and oil palm had average SOC stocks of 10.7 and 12.0 kg m⁻² respectively to a depth of 1.5 m. In the 0–0.05 m depth interval, 0.79 kg m⁻² of SOC was gained from oil palm inputs over 25 years and approximately the same amount of the original grass-derived SOC was lost. For the whole soil profile (0–1.5 m), 3.4 kg m⁻² of SOC was gained from oil palm inputs, with no significant losses of grass-derived SOC. The grass-derived SOC stocks were more resistant to mineralisation than reported in other studies. Black carbon produced in grassfires could partially but not fully account for the persistence of the original SOC stocks. Oil palm-derived SOC accumulated more slowly where soil nitrogen contents were high. Forest soils in the same region had smaller carbon stocks than the grasslands. In the majority of cases, conversion of grassland to oil palm plantations in this region resulted in net sequestration of soil organic carbon. Tree-scale spatial distribution of soil carbon inputs and outputs in a mature oil palm plantation were spatially correlated at the tree scale (r2 0.605), with a slope of 1:1. However, outputs were higher than inputs at all locations, with a mean overall output of 7.25 and input of 3.01 μmol m⁻² s⁻¹. Frond-, root- and groundcover-related inputs constituted 60, 36 and 4% of estimated inputs, respectively so frond inputs mainly controlled the spatial variability. The spatial correlation of carbon inputs and outputs suggests that mineralisation rate is controlled by the amount rather than nature or input depth of the additions. A spatially uniform net carbon loss was attributed to errors in root-related input estimates. The laboratory study confirmed that carbon turnover for oil palm soils was slow, 6% of the soil carbon was mineralised over the first year of incubation. Fourier transform infrared spectroscopy revealed that fractions described in literature as resistant to decay contained more lignin or other aromatic carbon forms than labile fractions. Biochemical recalcitrance and physico-chemical protection controlled turnover rates of intermediate stability organic carbon and protection appeared to be related to interactions between organic matter and poorly crystalline Al and Fe oxides. These findings help in understanding of the pathways and rates of carbon cycling in oil palm systems.