A comparison of throughfall rate and nutrient fluxes in rainforest and cacao plantation in Central Sulawesi, Indonesia

Abstract

This study investigates throughfall rates and nutrient fluxes in two cacao plots and one natural forest plot in Central Sulawesi, Indonesia. Throughfall was collected over a 12 month period on each plot with bulk precipitation samplers and compared with open area precipitation of nearby reference sites. Samples of both throughfall and gross precipitation were analysed for chemical composition and nutrient enrichment. Throughfall rates (based on total sums for the whole measurement period) differed strongly between cacao and rainforest sites: throughfall was 89 - 91% for the cacao plots and 81% in the natural forest. In-plot variance was high both between samplers and between sampling dates. Our findings imply an increase of yearly water input by 10% with the conversion of natural forests to cacao plantations as a result of reduced interception. Additional increases through reduced transpiration are documented in the literature. Higher runoff, probably mainly in low flow situations, is to be expected. For the volume-weighted average concentrations of sodium (Na), calcium (Ca), magnesium (Mg), silicon (Si) and potassium (K) a moderate (Na: 0.9 – 1.5 times) to very high (K: 5.1 – 18.9 times) enrichment in throughfall compared with open area precipitation was determined. Volume-weighted average concentrations were similar on both cacao plots. As expected, enrichment factors in the natural forest were much higher than on the cacao plots. A reduction of nutrient input with rainfall into the soil will undoubtedly affect soil chemistry and biology.