Drought effects on soil CO2 efflux in a cacao agroforestry system in Sulawesi, Indonesia.

Climate change induced droughts pose a serious threat to ecosystems across the tropics and sub-tropics, particularly to those areas not adapted to natural dry periods. In order to study the vulnerability of cacao (Theobroma cacao) -- Gliricidia sepium agroforestry plantations to droughts a large scale throughfall displacement roof was built in Central Sulawesi, Indonesia. In this 19-month replicated experiment, we measured soil surface CO₂ efflux (soil respiration) in three simulated drought plots compared with three adjacent control plots. Soil respiration rates peaked at intermediate soil moisture and decreased under increasingly dry conditions (drought induced), but also decreased when soils became water saturated, as evidenced in control plots. The simulated drought plots exhibited a slight decrease in soil respiration compared to the control plots (average 13% decrease). The strength of the drought effect was spatially variable -- while some measurement chamber sites reacted strongly ("responsive") to the decrease in soil water content (up to R²=0.70) (n=11), others did not react at all ("non-responsive") (n=7). The degree of soil CO₂ respiration drought response was highest around cacao tree stems and decreased with distance from the stem (R²=0.22). A significant correlation was measured between "responsive" soil respiration chamber sites and sap flux density ratios of cacao (R=0.61) and Gliricidia (R=0.65). Leaf litter CO₂ respiration decreased as conditions became drier. During dry periods the litter layer contributed approximately 3-4% of the total CO₂ efflux and up to 40% during wet periods. A CO₂ flush was recorded during the rewetting phase that lasted for approximately two weeks, during which time accumulated labile carbon stocks mineralized. The net effect on soil CO₂ emissions over the duration of the experiment was neutral, control plots respired 11.1±0.5MgCha^-1 yr^-1, while roof plots respired 10.5±0.5MgCha^-1 yr^-1. [ABSTRACT FROM AUTHOR]