Analytical estimation of the vertical distribution of CO2 production within soil: application to a Japanese temperate forest

We present methods, based on an analytical solution to a diffusion equation, for estimating the vertical distribution of the rate of CO2 production within soil. The first method assumes an a priori equation that represents the profile of CO2 production. The second method divides the soil into four layers, from the ground surface to a depth of 80 cm; within each layer, constant CO2 production is assumed. These methods were applied to a secondary deciduous temperate forest in Japan. Soil respiration and CO2 concentrations within the soil pores were measured in the forest from March to December of 1998. The value of Q10, the factor by which metabolic processes increase for each 10 °C increase in temperature, was estimated to be 2.55, using the soil temperature at a depth of 10 cm. The annual soil respiration was estimated to be 642 ± 91 g C m−2 y−1. The CO2 concentration within soil pores increased with soil depth in every season, and showed temperature dependency at all depths, which was similar to the relationship between soil respiration and soil temperature. By applying the proposed methods, the profiles of the actual CO2 concentrations within soil pores were well reproduced. Within soil, the amplitude of the seasonal variation in the CO2 production rate was larger in the shallow soil layers than in the deep layers. The CO2 production rate in the shallow soil layers was highly correlated with soil temperature, and its response to soil temperature was more sensitive than that estimated from the CO2 efflux from the soil surface. It was found that the temperature sensitivity of the CO2 production rate differed between the shallow and deep soil layers; this would affect the estimated value of Q10. Adding to the necessity of distinguishing heterotrophic and autotrophic respiration, it is suggested that the way in which they are distributed vertically within soil is critical for predicting soil respiration, and should be included in the parameterization of soil respiration.