Indirect effects of soil moisture reverse soil C sequestration responses of a spring wheat agroecosystem to elevated CO₂

Increased plant productivity under elevated atmospheric CO₂ concentrations might increase soil carbon (C) inputs and storage, which would constitute an important negative feedback on the ongoing atmospheric CO₂ rise. However, elevated CO₂ often also leads to increased soil moisture, which could accelerate the decomposition of soil organic matter, thus counteracting the positive effects via C cycling. We investigated soil C sequestration responses to 5 years of elevated CO₂ treatment in a temperate spring wheat agroecosystem. The application of ¹³C-depleted CO₂ to the elevated CO₂ plots enabled us to partition soil C into recently fixed C (Cnew) and pre-experimental C (Cold) by ¹³C/¹²C mass balance. Gross C inputs to soils associated with Cnew accumulation and the decomposition of Cold were then simulated using the Rothamsted C model 'RothC.' We also ran simulations with a modified RothC version that was driven directly by measured soil moisture and temperature data instead of the original water balance equation that required potential evaporation and precipitation as input. The model accurately reproduced the measured Cnew in bulk soil and microbial biomass C. Assuming equal soil moisture in both ambient and elevated CO₂, simulation results indicated that elevated CO₂ soils accumulated an extra ~40-50 g C m⁻² relative to ambient CO₂ soils over the 5 year treatment period. However, when accounting for the increased soil moisture under elevated CO₂ that we observed, a faster decomposition of Cold resulted; this extra C loss under elevated CO₂ resulted in a negative net effect on total soil C of ~30 g C m⁻² relative to ambient conditions. The present study therefore demonstrates that positive effects of elevated CO₂ on soil C due to extra soil C inputs can be more than compensated by negative effects of elevated CO₂ via the hydrological cycle.