Spatial and temporal effects of drought 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 experiment, we compared soil surface CO2 efflux (soil respiration) from three roof plots with three adjacent control plots. Soil respiration rates peaked at intermediate soil moisture conditions and decreased under increasingly dry conditions (drought induced), or increasingly wet conditions (as evidenced in control plots). The roof 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 R-2 = 0.70) (n = 11), others did not react at all (non-responsive) (n = 7). 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. The litter layer contributed approximately 3-4% of the total CO2 efflux during dry periods and up to 40% during wet periods. Within days of roof opening soil CO2 efflux rose to control plot levels. Thereafter, CO2 efflux remained comparable between roof and control plots. The cumulative effect on soil CO2 emissions over the duration of the experiment was not significantly different: the control plots respired 11.1 +/- 0.5 Mg C ha(-1) yr(-1), while roof plots respired 10.5 +/- 0.5 Mg C ha(-1) yr(-1). The relatively mild decrease measured in soil CO2 efflux indicates that this agro-forestry ecosystem is capable of mitigating droughts with only minor stress symptoms.