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

Precipitation is a major driver of ecosystem change and the
physicochemical 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 examined the impact
of increased precipitation in parts of California, where the additional
precipitation occurred during winter or spring months. It was found that
altering precipitation seasonality has significant impacts on plant
community dynamics, microbial and fungal dynamics, and abiotic soil
processes. 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 physicochemical
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 the
timing of precipitation events, especially with regard to deep carbon
stocks (>1 m).