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

The trenching method of root exclusion is generally used to estimate heterotrophic (microbial decomposition) (Fh) and autotrophic (root and associated rhizosphere respiration) (Fa) components of soil respiration (F0), 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 CO2 production and transport in soil, we show that: (a) CO2 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 CO2 flux from below the root exclusion depth to the measured efflux at the surface of a root exclusion plot (F0re) is increased because of the higher concentration gradient at the bottom of the root exclusion layer due to the decreased rate of CO2 production above this depth. Consequently, Fa, calculated as F0c measured in control (non-disturbed) plots minus F0re measured in root exclusion plots, is underestimated. We describe an analytical model, derived from the soil CO2 production and diffusion equation, to obtain correct estimates of Fa measured using small-area root exclusion plots. The analytical model requires knowledge of depth distribution of soil CO2 diffusivity and source strength as inputs.