Topographic variation in heterotrophic and autotrophic soil respiration in a tropical seasonal forest in Thailand

Soil respiration is a carbon flux that is indispensable for determining carbon balance despite variations over time and space in forest ecosystems. In Kanchanaburi, western Thailand, we measured the soil respiration rates at different slope positions-ridge (plot R), upper slope (plot U), and lower slope (plot L) on a hill in a seasonal tropical forest [mixed deciduous forest (MDF)] to determine the seasonal and spatial variations in soil respiration on the slope. The heterotrophic (organic layer and soil) and autotrophic (root) respiration was differentiated by trenching. Soil respiration rates showed clear seasonal patterns: high and low rates in rainy and dry seasons respectively, at all plots, and tended to decrease up the slope. Soil respiration rates responded significantly to soil water content in the 0-30 cm layer, but the response patterns differed between the lower slope (plot L) and the upper slope (plots R and U): a linear model could be applied to the lower slope but exponential quadratic models to the upper slope. The annual carbon dioxide (CO2) efflux from the forest floor was also associated with the slope position and ranged from 1908 gC m(-2) year(-1) in plot L to 1199 gC M-2 year(-1) in plot R. With ascending position from plot L to R, the contribution of autotrophic respiration increased from 19.4 to 36.6% of total soil respiration, while that of the organic layer decreased from 26.2 to 9.4%. Mineral soil contributed to 46.3 to 54.4% of the total soil respiration. Soil water content was the key factor in controlling the soil respiration rate and the contribution of the respiration sources. However, the variable responses of soil respiration to soil water content create a complex distribution of soil respiration at the watershed scale.