The impact of extreme drought on carbon cycling and bedrock weathering: In-situ observations from a seasonally dry montane forest

Substantial production and consumption of carbon occurs meters beneath the surface in ecosystems from grasslands to forests. This deeper cycling drives bedrock weathering and can be a significant component of carbon budgets on seasonal to millennial timescales. However, it remains uncertain how climatic perturbation influences this deep cycling. In shallow soils, drought is thought to decrease rates of soil respiration due to the relationship between biological activity, soil moisture and temperature. However, in the deeper subsurface, it is unclear if these relationships hold. This leaves open questions about how drought impacts forest carbon cycling and rates of bedrock weathering. Here, we present findings from a five year monitoring study designed to investigate the relationship between climate forcings and deep carbon cycling in a Mediterranean, mixed conifer hardwood forest ecosystem in the Coast Ranges of Northern California. We document subsoil carbon dioxide production, the partitioning of carbon dioxide between gaseous and aqueous phases, local meteorology and plant water use, and the chemical composition of the aqueous phase. To evaluate the mechanisms linking carbon cycling, chemical weathering, and drought, we contrast fluxes of CO2 and O2 (both dissolved and gaseous) between two significantly high precipitation years and two significantly low precipitation years (less than half mean annual precipitation). Partitioning of CO2 into aqueous and gaseous phases differs strongly between wet and dry years, whereby substantially less CO2 is dissolved into the aqueous phase during dry years. This suggests that more reactivity is delivered to the deep critical zone during wet years. During the dry season, the gaseous fluxes from deep bedrock to soil are typically higher during drought years, as evidenced by higher concentration gradients and higher gas-phase diffusivity associated with earlier drying of the vadose zone. We further monitor the implications this has for weathering of minerals in the subsurface and the export of solutes from the vadose zone to groundwater and streams. These results suggest that deep cycling of water and carbon in upland forested systems strongly control the chemical denudation in response to drought.