Factors controlling labile soil organic matter vulnerability to loss following disturbance as assessed by measurement of soil‐respired δ 13 CO 2

Summary Labile soil organic matter ( SOM ) is often stabilized in soil, but is vulnerable to loss after soil disturbance. We used measurements of the 13 carbon ( 13 C ) signature of soil‐respired CO 2 ( δ 13 CO 2 ) 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 δ 13 CO 2 . Strong temporal trends in δ 13 CO 2 suggest that labile SOM became vulnerable to loss within minutes of soil disturbance. Equilibrium soil‐respired δ 13 CO 2 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 δ 13 CO 2 . In this experiment δ 13 CO 2 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 CO 2 evolution rates are needed to test directly whether equilibrium δ 13 CO 2 can reflect labile SOM vulnerability to loss.