Estimating heterotrophic and autotrophic soil respiration using small-area trenched plot technique: Theory and practice

Abstract

The trenching method of root exclusion is generally used to estimate heterotrophic (microbial decomposition) ( F h) and autotrophic (root and associated rhizosphere respiration) ( F a) components of soil respiration ( F 0), particularly in forest ecosystems. However, some uncertainties exist on the accuracy and interpretation of the results from such experiments using small-area root exclusion plots. Using field and laboratory measurements as well as simulations using a process-based model of CO 2 production and transport in soil, we show that: (a) CO 2 concentrations at or immediately below the depth of root exclusion in small-area root exclusion plots are similar to those at the same depth in nearby undisturbed soil and (b) the contribution of soil CO 2 flux from below the root exclusion depth to the measured efflux at the surface of a root exclusion plot ( F 0re) is increased because of the higher concentration gradient at the bottom of the root exclusion layer due to the decreased rate of CO 2 production above this depth. Consequently, F a, calculated as F 0c measured in control (non-disturbed) plots minus F 0re measured in root exclusion plots, is underestimated. We describe an analytical model, derived from the soil CO 2 production and diffusion equation, to obtain correct estimates of F a measured using small-area root exclusion plots. The analytical model requires knowledge of depth distribution of soil CO 2 diffusivity and source strength as inputs.