Aggregational differentiation of soil-respired CO2 and its δ13C variation across land-use types

Abstract

Land-use type can affect CO2 emission fluxes, but whether it has effects on the δ13C of respired CO2 (δ13CO2), and whether these effects vary with the heterogeneity of the soil microenvironment remain unknown. In this study, long-term soil incubation experiments and soil fractionation were combined to determine the effects of land-use type (upland, paddy and woodland) on soil-respired CO2 and δ13CO2 with different aggregate fractions under three temperature conditions. This study extended the thermal adaptation of soil respiration to the soil microenvironmental scale, i.e., the scale of smaller aggregates than bulk soil. An exponential decay function (R = a × e−bt) was fitted to the temporal soil respiration data, separately for each combination of soil aggregate, land-use type and incubation temperature. At each incubation stage, the woodland exhibited the highest average soil respiration rates. The exponential decay trend of δ13CO2 with time was influenced by the land-use type and incubation temperature; the shift from enriched to depleted δ13CO2 values first appeared in woodland. Interestingly, carbon isotopic discrimination between the substrate and respired CO2 (Δ13C, δ13CO2 – δ13CSOC) increased with aggregate size only in woodland, especially at 30 °C. Overall, aggregational differentiation of soil-respired CO2 and its δ13C was found only in the woodland, and warming intensified these differences, suggesting that it would be difficult to generalize the integrative effects of warming on soil respiration and its isotopic fractionation at the microscale from a single ecosystem study. Our findings highlight that land-use types affect the feedback of soil carbon pool to future global warming at the soil microenvironmental scale (aggregate), and are of great significance for improving carbon emission prediction in terrestrial ecosystems.