Graduate Student University of California - Merced Catheys Valley, California, United States
The lack of data on how organic compounds in soil respond to varying climatic conditions, such as additional precipitation and drought, has limited our ability to accurately determine how soil organic matter (SOM) dynamics are likely to respond to changes in environmental conditions. To further our understanding of the mechanisms behind changing SOM dynamics under altered precipitation regimes, it is essential to determine how SOM-mineral interactions and potential decoupling of soil C and N occurs with changes in soil moisture regimes. This project will address a key question: how do changes in the amount of precipitation affect the distribution and composition of bulk SOM?
To answer these questions, we focused the study on SOM composition across a precipitation gradient at three UC reserve sites: Sedgwick National Reserve, Hopland Research and Extension Center, and the Angelo Reserve in California. This precipitation gradient was sampled in Summer 2020, with samples collected up to 1m at each site. We then determined changes in total elemental and stable isotopic concentrations of soil C, N, δ13C, and δ15N across the precipitation gradients (over the entire soil profile). We then used Fourier-Transformed Infrared Spectroscopy (FTIR) to broadly characterize OM composition in the bulk soil samples.
In terms of elemental analysis, we observed consistent relationships with depth for C and N concentrations. All sites had similar C concentrations at the surface (~2% C), but diverged at depth, with the wettest site having the lowest C of all sites from 50-100cm. The dry site and intermediate site contained similar amounts of C throughout the profile. N concentrations were similarly low across all sites, even in surface soils (~0.25% N). In terms of isotopic measurements, δ13C values in surface soils were consistent with C3 vegetation present at all sites (-28‰), and similarly increased with depth for all sites. This increase with depth is consistent with previous work and is indicative of 12C being preferentially incorporated at depth. Interestingly, there was not a similar relationship with δ15N values, though we expected these to also increase with depth. δ15N values did not vary consistently with depth or across the precipitation gradient. This suggests that factors other than depth and moisture content could be driving δ15N values. FTIR data suggested that sources of SOM could be changing across the gradient, with key differences emerging in proportions of simple plant derived, complex plant derived, and microbial biomass.