Stabilization mechanisms of isotope-labeled carbon substrates in soil under moisture pulses and conservation agricultural management

Understanding the mechanisms of carbon (C) stabilization in soil under the influence of moisture pulses is essential for predicting the stability of terrestrial C in the face of climate change. Conservation agricultural management is widely recognized to mitigate climate change through soil C stabilization or retention. We conducted a 24-day mesocosm incubation using an agricultural soil under 36 years of conservation agricultural management including winter cover cropping and no-tillage farming. We added ¹³C-labeled simple (glucose) or complex (corn litter) C substrates to mesocosms, applied different frequencies of moisture pulses, and traced the fate of the added glucose- and corn-C in labile and stable soil C pools. Structural equation modelling was used to quantify the effect sizes of the short-term moisture pulses and the long-term conservation management on the stabilization of the added glucose- and corn-C in soil. Our results show that the added corn-C, which is more biochemically complex, had a lower recovery in microbial biomass than glucose-C after 24 days (0.15% vs. 46.98%). Compared to rewetting the soil, extended drought increased accumulation of glucose-C (46.39% vs. 77.45%) but decreased corn-C (0.22% vs. 0.06%) in microbial biomass. The extended drought also increased extractable organic C (0.22% vs. 0.91%) and H₂O₂-resistant C (1.48% vs. 3.14%) compared to rewetting the soil. Vetch, a nitrogen-fixing cover crop, appeared to stabilize the added C via facilitating incorporation into the H₂O₂-resistant fraction of soil C compared to no cover crop (3.37% vs. 1.95%). Moisture pulses disrupted macroaggregates and induced release of microaggregates, which may lead to destabilization of the added C. Structural equation models show that the multiple moisture pulses destabilized the added glucose-C and corn-C in soil, while the conservation management could mitigate some of these effects. Our results suggest that conservation agriculture can help improve the resilience of agroecosystem function in relation to soil C storage and stability in the face of climate change.