Conversion of tropical forests to smallholder rubber and oil palm plantations impacts nutrient leaching losses and nutrient retention efficiency in highly weathered soils

摘要

Conversion of forest to rubberand oil palm plantations is widespread in Sumatra, Indonesia, and it islargely unknown how such land-use conversion affects nutrient leachinglosses. Our study aimed to quantify nutrient leaching and nutrient retentionefficiency in the soil after land-use conversion to smallholder rubber andoil palm plantations. In Jambi province, Indonesia, we selected twolandscapes on highly weathered Acrisol soils that mainly differed in texture:loam and clay. Within each soil type, we compared two reference land uses,lowland forest and jungle rubber (defined as rubber trees interspersed insecondary forest), with two converted land uses: smallholder rubber and oilpalm plantations. Within each soil type, the first three land uses wererepresented by 4 replicate sites and the oil palm by three sites, totaling30 sites. We measured leaching losses using suction cup lysimeterssampled biweekly to monthly from February to December 2013. Forests andjungle rubber had low solute concentrations in drainage water, suggesting lowinternal inputs of rock-derived nutrients and efficient internal cycling ofnutrients. These reference land uses on the clay Acrisol soils had lowerleaching of dissolved N and base cations (P =  0.01–0.06) and higher N andbase cation retention efficiency (P < 0.01–0.07) than those onthe loam Acrisols. In the converted land uses, particularly on the loamAcrisol, the fertilized area of oil palm plantations showed higher leachingof dissolved N, organic C, and base cations (P < 0.01–0.08) andlower N and base cation retention efficiency compared to all the other landuses (P < 0.01–0.06). The unfertilized rubber plantations,particularly on the loam Acrisol, showed lower leaching of dissolved P (P = 0.08) and organic C (P < 0.01) compared to forest or junglerubber, reflecting decreases in soil P stocks and C inputs to the soil. Ourresults suggest that land-use conversion to rubber and oil palm causesdisruption of initially efficient nutrient cycling, which decreases nutrientavailability. Over time, smallholders will likely be increasingly reliant onfertilization, with the risk of diminishing water quality due to increasednutrient leaching. Thus, there is a need to develop management practices tominimize leaching while sustaining productivity.