Impact of temperature and moisture on heterotrophic soil respiration along a moist tropical forest gradient in Australia

Tropical forests represent the largest store of terrestrial carbon (C) and are potentially vulnerable to climatic variations and human impact. However, the combined influence of temperature and precipitation on aboveground and belowground C cycling in tropical ecosystems is not well understood. To simulate the impact of climate (temperature and rainfall) on soil C heterotrophic respiration rates of moist tropical forests, we translocated soil cores among three elevations (100, 700 and 1540 m a.s.l.) representing a range in mean annual temperature of 10.9°C and in rainfall of 6840 mm. Initial soil C stocks in the top 30 cm along the gradient increased linearly with elevation from 6.13 kg C m⁻² at 100 m a.s.l. to 10.66 kg C m⁻² at 1540 m a.s.l. Respiration rates of translocated soil cores were measured every 3 weeks for 1 year and were fitted to different model functions taking into account soil temperature, soil moisture, mean annual temperature and total annual rainfall. Measured data could be best fitted to the model equation based on temperature alone. Furthermore, Akaike's information criteria revealed that model functions taking into account the temperature range of the entire translocation gradient led to better estimates of respiration rates than functions solely based on the site-specific temperature range. Soil cores from the highest elevation revealed the largest temperature sensitivity (Q₁₀ = 2.63), whereas these values decreased with decreasing elevation (Q₁₀ = 2.00 at 100 m a.s.l.) or soil C stocks. We therefore conclude that increased temperatures will have the greatest impact on soil C stocks at higher elevations, and that best projections for future soil respiration rates of moist tropical forest soils can be achieved based on temperature alone and large soil cores exposed to temperatures above site-specific temperature regimes