Factors controlling labile soil organic matter vulnerability to loss following disturbance as assessed by measurement of soil‐respired δ¹³CO₂

Abstract

Summary Labile soil organic matter (SOM) is often stabilized in soil, but is vulnerable to loss after soil disturbance. We used measurements of the 13carbon (13C) signature of soil‐respired CO2 (δ13CO2) immediately after the disturbance caused by coring and sampling the soil, to assess labile SOM availability and potential vulnerability to loss. We incubated a range of pasture soils over 300 minutes and periodically measured δ13CO2. Strong temporal trends in δ13CO2 suggest that labile SOM became vulnerable to loss within minutes of soil disturbance. Equilibrium soil‐respired δ13CO2 values were a function of the amount of labile SOM (measured as hot‐water extractable C, HWEC), total soil C and soil protection capacity (measured as specific soil surface area, SSA). An independent experimental approach that immobilized labile SOM on to allophane (a clay mineral with a large active surface area) was used to assess the effect of SSA on equilibrium soil‐respired δ13CO2. In this experiment δ13CO2 shifted progressively to more enriched values during the first six days of soil incubations after allophane addition, suggesting that labile SOM became less available after stabilization by allophane. At the same time there was a large reduction in HWEC when compared with the control treatments, also indicating limited availability of labile SOM. Further studies coupling the isotopic measurements with CO2 evolution rates are needed to test directly whether equilibrium δ13CO2 can reflect labile SOM vulnerability to loss.

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