Carbon and nitrogen dynamics in subsoils after 20 years of added precipitation in a Mediterranean grassland

Abstract

Precipitation is a major driver of ecosystem change and the physiochemical characteristics of soil. Under different climate change scenarios, increased drought frequency and changing precipitation are predicted to impact Mediterranean ecosystems, including those in Northern California. Subsoils are large carbon (C) reservoirs; however, most studies that have investigated precipitation effects on soil organic matter (SOM) have primarily focused on near-surface soils. Recent studies have indicated different responses to environmental perturbation in surface (< 30 cm) versus deep (> 30 cm) soils due to important differences in physiochemical characteristics. Here, we present soil data at depth (∼ 300 cm) from a 20-year precipitation manipulation experiment. We determined changes in total elemental concentration and stable isotope composition of soil C, N, δ13C, and δ15N for an ambient control versus additional precipitation in the winter and spring months. The addition of winter precipitation resulted in the largest cumulative C stock (0-300 cm); however, there were no statistically significant changes in C stock throughout the depth profile. Nevertheless, there was evidence of the vertical translocation of C to deep soil layers, specifically of plant-derived C, with both winter and spring precipitation additions. The precipitation addition in winter also resulted in the highest subsoil C stock compared to the control (ambient) and spring treatments. Overall, added winter precipitation led to the best conditions for C accumulation, as the added precipitation coincided with lower temperatures and improved growing conditions at our field site. This study highlights the importance of the timing of precipitation events, especially with regard to deep C stocks (> 1 m).