Increased water availability accelerates C cycling in a dry forest ecosystem

Climate change is intensifying the frequency and severity of droughts, with profound implications for carbon (C) cycling in forest ecosystems. While progress has been made in understanding how drought alters plant and microbial ecophysiology, it remains unclear how these changes affect the overall magnitude and pace of C cycling within the plant-soil continuum. In this study, we examined the effects of 22 years of experimental irrigation in a naturally drought-prone Scots pine forest. We integrated long-term measurements of C inputs (i.e., litterfall) and outputs (i.e., soil respiration) with radiocarbon (¹⁴C) analysis of soil organic carbon, fine roots, and CO₂ from in situ soil respiration and its autotrophic and heterotrophic components. Our study demonstrates that long-term shifts in water availability enhance both C inputs and outputs, reshaping C cycling within the plant-soil system. Radiocarbon ( ¹⁴ C) analysis revealed that irrigation accelerated C cycling within plants and the organic layer, reducing the time that assimilated C remained in the ecosystem before being respired back to the atmosphere. The observed increase in soil respiration under irrigation was largely driven by enhanced autotrophic activity, associated with greater fine root biomass. Concurrently, the decomposition of labile, young organic matter intensified under irrigation, potentially contributing to a net C loss from the organic layer. Despite this increased respiration under irrigation, ¹⁴C contents in bulk SOC indicated greater inputs of young C to the mineral soil and enhanced downward translocation of C from the organic layer to the mineral phase, possibly through rhizodeposition and soil faunal activity. The enhanced input to the mineral soil under irrigation results in net C gains and may promote C stabilization through organo-mineral interactions and aggregate formation. Our findings also indicate that drought conditions limit both the magnitude and rate of C cycling within the plant-soil continuum and potentially reduce long-term C sequestration in mineral soils.