Soil carbon dynamics during a hysteretic drying-rewetting event

Soil moisture is an important driver of soil carbon (C) fluxes, and drying-rewetting events influence C stabilization in soils. Drought events are increasing in intensity and frequency, and it is therefore crucial to understand soil C fate and transport as a soil dries and is subsequently rewet. Soils exhibit hysteresis during drying-rewetting events because of differential water retention in heterogeneous pores, and this phenomenon is not factored into current soil biogeochemical models. Here, we study the hysteretic nature of the water retention characteristic to inform our molecular understanding of soil organic C dynamics occurring during a drying and rewetting cycle. Our objectives are (a) to track the changing complexity and bioavailability of the SOC pool as the soil dries and is rewet; and (b) to determine the influence of antecedent moisture content on SOC bioavailability. We subjected soils of varying textures to a drying or wetting treatment; for the drying treatment, we saturated the soils and then dry to -1, -10, -33, or -100 kPa; for the wetting treatment, we dried the soils to -100 kPa and then wet to -33, -10, -1, or 0 kPa (saturation). We measured CO ₂ and CH ₄ flux during the wetting/drying treatments and used FT-ICR-MS to identify the organic molecules in pore water and in the water-extractable C pool. We measured aggregate size distribution in the soils to determine aggregate stability, an important mechanism of soil C protection. This study moves away from the assumption of linearity often used in soil C models and attempts a realistic mechanistic approach to how the soil C pool responds to drying and rewetting events.