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 physiochemical characteristics of soil. Under different climate change scenarios, increased drought frequency and changing precipitation are predicted to impact Mediterranean ecosystems, including in Northern California. Studies based on two major climate models investigated the impact of increased precipitation in parts of California where the additional precipitation occurred in winter or spring months. It was found that changing precipitation seasonality has significant impacts on plant community dynamics, microbial and fungal dynamics, and abiotic processes in soil. Subsoils are large carbon reservoirs. However, most studies investigating precipitation effects on soil organic matter (SOM) primarily focus on near-surface soils. Recent studies indicated different responses to environmental perturbation in surface (<30 cm) versus deep soils (>30 cm) 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 ambient control vs. 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 carbon stock throughout the depth profile. However, there was evidence for vertical translocation of carbon to deep soil layers, specifically of plant-derived carbon, with both winter and spring precipitation additions. The precipitation addition in winter also resulted in the highest subsoil carbon stock compared to the control (ambient) and spring treatments. Overall, added winter precipitation led to the best conditions for carbon accumulation since the added precipitation coincides with lower temperatures and improved growing conditions at our field site. This study highlights the importance of timing of precipitation events, especially with regard to deep carbon stocks (>1 m).