Consequences of rainforest conversion to smallholder rubber and oil palm plantations on soil trace gas fluxes from highly weathered soils in Sumatra, Indonesia

In Sumatra, Indonesia, the establishment of oil palm and rubber plantations is widespread. However, it occurs at the expense of forest area. Since global demand for palm oil and rubber is increasing, forest conversion is expected to continue. Furthermore, studies have shown that forest destruction and the establishment of agricultural land uses influence the soil–atmosphere exchange of the climate-relevant trace gases carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and nitric oxide (NO). Nevertheless, trace gas measurements from oil palm and rubber plantations are scarce. Additionally, researchers have so far not considered oil palm canopy soils as a possible source or sink of trace gases. The present thesis consists of three studies, which assess the impact of forest conversion into smallholder oil palm and rubber plantations on soil CO2 and CH4 fluxes, as well as on soil N2O and NO fluxes, and which investigate the importance of oil palm canopy soil for N2O and CH4 fluxes. We conducted the studies on highly weathered tropical soils in Jambi Province, Sumatra, Indonesia and selected two soil landscapes which mainly differ in texture (clay and loam Acrisol). To examine the impact of land-use change on soil trace gas fluxes we investigated four different land uses per landscape: lowland forest and jungle rubber (rubber trees interspersed in secondary forest), as reference land uses, as well as smallholder rubber (7–17 years old) and oil palm plantations (9–16 years old), as converted land uses. Each land use was replicated four times in both landscapes. The first study investigated changes in soil CO2 and CH4 fluxes with forest conversion to smallholder oil palm and rubber plantations. We determined soil CO2 and CH4 fluxes monthly from December 2012 to December 2013, using static vented chambers. Our findings show that soil CO2 fluxes in oil palm plantations were reduced and that fluxes from the other three land uses were comparable among each other in both landscapes. We attributed this decrease to strongly decomposed soil organic matter, reduced soil carbon (C) stocks as well as to phosphorus fertilization and liming, which led to a lower C allocation to roots. Due to reduced nitrogen (N) availability in the converted land uses CH4 uptake was lower in oil palm and rubber when compared to the reference land uses in both landscapes. Thus, soil fertility appeared to be an important controller of soil CO2 and CH4 fluxes in this tropical landscape. The second study focused on the impact of forest conversion into smallholder oil palm and rubber plantations on soil N2O and NO fluxes. Additionally, we compared soil N2O fluxes from smallholder oil palm plantations with fluxes from a large-scale oil palm plantation. We determined soil N2O fluxes monthly from December 2012 to December 2013 in the two landscapes and weekly to bi-weekly from July 2014 to July 2015 in the large-scale oil palm plantation, using static vented chambers. Using open dynamic chambers, we measured soil NO fluxes four times in all land uses of both landscapes between March and September 2013. Our results show that land use change did not affect soil N2O and NO fluxes because of low initial N availability in the reference land uses, so that N2O and NO fluxes were also low, and any changes due to conversion might have been too small to identify. However, the large-scale oil palm plantation, although not significantly different, showed, because of their higher fertilizer input, on average 3.5 times higher soil N2O fluxes than the smallholder oil palm plantations. The aim of the third study was to quantify N2O and CH4 fluxes from oil palm canopy soils. We measured soil N2O and CH4 from three different stem heights in eight smallholder oil palm plantations across the two landscapes from February 2013 to May 2014, on a bi-weekly to monthly basis, using in-situ incubation. Oil palm canopy soil emitted N2O and CH4 from all stem heights. However, fluxes were low compared to ground soil fluxes. This was due to a low amount of canopy soil on a hectare basis and due to high nitrate contents, which might have suppressed CH4 production. In the synthesis of this dissertation, data on soil trace gas fluxes were embedded into a broader context to gain information on changes of the net biome exchange (NBE) and on partial N budgets with land-use change. Soil CO2 and CH4 fluxes were combined with an ancillary study on net primary production and harvest as well as with estimations on the contribution of heterotrophic soil respiration to total soil respiration. Soil N2O and NO fluxes were combined with ancillary studies on N inputs and outputs via fertilization, bulk precipitation, leaching and harvest. The results revealed that the NBE of oil palm plantations was higher compared to forest. Nevertheless, implications for climate change are negative since forest conversion itself results in a huge C loss, which cannot be compensated over time by oil palm plantations. The lowest partial N budget was detected in oil palm, indicating that N inputs via precipitation and fertilization were smaller than the huge N loss via harvest. Overall, these results illustrate that land-use change has negative effects on the C and N budgets of tropical ecosystems.