Plant diurnal cycle drives the variation in soil respiration in a C.sub.4-dominated tropical managed grassland exposed to high CO.sub.2 and warming

Aims To identify factors driving soil respiration (R.sub.soil) in a tropical C.sub.4-dominated perennial managed grassland ecosystem exposed to elevated carbon (C) dioxide concentration ([CO.sub.2]) and temperature. Methods The perennial grass Panicum maximum was grown at 600 [mu]mol CO.sub.2 mol.sup.-1 and + 2 °C above ambient temperatures for one full growing cycle (from grazing to regrowth for about ~30-45 days) using a free-air CO.sub.2 and infrared warming system. Plant growth and CO.sub.2 fluxes were measured during the growing cycle. Results Both high [CO.sub.2] and warming increased canopy photosynthesis but warming alone increased biomass by 53% and R.sub.soil by 26%. There was a strong diel effect on R.sub.soil, which was 16% greater at noon than at 18:00 h. R.sub.soil had low sensitivity to soil temperature (Q.sub.10 ~ 1) regardless of the CO.sub.2 treatment. Conclusions In this tropical managed pasture, diel variation in photosynthesis strongly affected R.sub.soil, suggesting that R.sub.soil may be more limited by substrate availability than abiotic factors such as temperature. Predicted changes in climate for the region will likely affect the C dynamics of C.sub.4-dominated tropical pastures. Although, the short-term experiment may limit the extrapolations of our findings, the highly controlled settings of the experiment highlighted the role of canopy photosynthesis on R.sub.soil respiration in tropical C.sub.4-pastures.