University of California - Merced, California, United States
Abstract: Purpose
Precipitation is a major driver of ecosystem change and physiochemical characteristics of soil. California is predicted to have changes in precipitation seasonality in coming decades that could significantly affect organic carbon (C) and nitrogen (N) dynamics. Greater precipitation during winter months leads to soil C losses that could be mediated by changes in the vertical distribution of reactive soil minerals that play key roles in storage and stabilization of soil organic matter (SOM). Most studies on the effects of precipitation on SOM have focused on near surface soils; here we present data on soils at depth (~300cm) from a 20-year precipitation manipulation experiment.
Methods
The Angelo Precipitation Experiment, which has been ongoing for 20 years, presents a unique study system to delve into these questions of how changing seasonality of precipitation will affect plant community structure as well as the belowground C and N response. For this experiment, thirty-six large circular plots (70-m2) were regularly spaced across 2.7 ha meadows. Plots were set up in random block design for three separate treatments: (1) an ambient rainfall control; (2) a winter-addition of precipitation; (3) a spring addition of precipitation. This water addition results in a 20% increase over the mean annual precipitation. Soils were collected at the 20-year mark of this experiment in October 2021, and through a Geoprobe soils were collected up to 300cm depth for each treatment (4 per treatment, 12 total cores). We then determined changes in total elemental and stable isotopic concentrations of soil C, N, δ13C, and δ15N for the different experimental precipitation treatments (over the entire soil profile). We performed a free light fraction extraction and sequential iron/aluminum extraction to understand changing chemistry of inputs and mineral associations in surface soils (to 50 cm).
Results
We found greater C stocks with spring addition of precipitation. We also found weakened associations with iron and aluminum in the winter treatment, suggesting a mechanism for this loss and gain of C stock in this study system. We also found unique distributions of soil isotope values to 300cm depending on treatment, especially for δ15N values. This suggests unique N cycling impacts with spring addition of precipitation.
Conclusion
This study aimed to understand SOM dynamics at depth at a precipitation experiment, after 20 years of precipitation addition. The conclusions of this study suggest that over decadal time scales, changing precipitation seasonality could affect SOM stabilization in grassland ecosystems.
