Conversion of lowland forests to rubber and oil palm plantations changes nutrient leaching and nutrient retention efficiency in highly weathered soils of Sumatra, Indonesia

Abstract

Over the last two decades, Sumatra, Indonesia has experienced rapid expansion of rubber and oil palm plantations through conversion of rainforests. This is evident from the 36% decrease in forest area in this region from 1990-2010. Such rapid land-use change necessitates assessment of its environmental impacts. Forest conversion to rubber and oil palm plantations are expected to increase nutrient leaching losses and decrease nutrient retention efficiency, following the changes in soil cover, litter input, soil nutrient availability and management practices. This thesis presents two studies, which focused on the impact of forest conversion to rubber and oil palm plantations on nutrient leaching and nutrient retention efficiency, and on the difference in nutrient leaching losses between fertilized and frond-stacked areas of oil palm plantations. All studies were conducted in two landscapes of highly weathered soils that mainly differed in texture (loam and clay Acrisol soils), located in the Jambi province, Sumatra, Indonesia. Nutrient leaching losses were measured using suction cup lysimeters installed at 1.5 m soil depth and sampling frequency was bi-weekly to monthly during February to December 2013. In the first study, nutrient leaching losses and nutrient retention efficiency in the soil were measured in four land uses: the reference land uses of lowland forest and jungle rubber (rubber trees interspersed in secondary forest), and the converted land uses of smallholder rubber and oil palm plantations. In each landscape, the first three land uses were represented by four replicate sites and the oil palm by three sites, totaling 30 sites. The results illustrated that for the reference land uses the loam Acrisol soil had higher leaching fluxes of dissolved nitrogen (N) and base cations, and lower retention efficiencies of N and base cations than the clay Acrisol soil. For the converted land uses, management practices such as fertilization and liming in oil palm plantations resulted in higher dissolved N, dissolved organic carbon (DOC), and base cations leaching fluxes, and lower N and base cation retention efficiencies in the soil than the reference land uses. On the other hand, in the unfertilized rubber plantations leaching losses of dissolved N, DOC, and base cations were lower than in the oil palm plantations. Overall, the results showed that clay content and management practices controlled nutrient leaching losses and nutrient retention efficiencies in heavily weathered Acrisol soils of these converted landscapes. In the second study, nutrient leaching losses were measured in fertilized and frond-stacked areas of smallholder oil palm plantations in clay and loam Acrisol soils. The results exhibited higher leaching losses (i.e. N, base cations, total aluminum (Al), total manganese (Mn), total sulfur (S), and chloride (Cl)) in the fertilized area than the frond-stacked area due to pulse rates of applications of mineral fertilizers and lime. At the landscape scale, higher soil nutrient stocks and lower nutrient leaching losses in the clay Acrisol soil compared to the loam Acrisol soil both in the fertilized and frond stack areas were caused by the higher nutrient retention as a result of higher clay content. Combining nutrient leaching losses and nutrient input (i.e. bulk precipitation and fertilizers) with ancillary studies on nutrient output through harvest export provides more comprehensive information about the changes in partial nutrient budgets of N, phosphorus (P), and base cations due to forest conversion to oil palm and rubber plantations. Fertilized oil palm plantations had the lowest annual partial budget of N, calcium (Ca) and magnesium (Mg) due to the high annual leaching losses and harvest export. However, the high negative partial budgets of N, Ca and Mg in oil palm plantations did not significantly decrease those stocks at 1-m soil depth compared to all the other land uses, except for exchangeable Mg in the loam Acrisol landscape. Even though unfertilized rubber plantations have lower leaching losses (e.g. P) than forest, harvest export caused the lower annual partial budget of P. Overall, these results from the two studies suggests for improved management practices on these highly weathered soils through synchronizing rate of application of fertilizer with plant uptake and frequency of fertilizer application.