In ecosystems with a Mediterranean climate, low soil water potentials during the dry season limit microbial activity. Despite low summertime biogeochemical rates, studies in annual grasslands have shown the mobility and decomposability of soil organic carbon to increase over sequential dry-season measurements. The sources and chemistry of this carbon are not well understood, nor are the controls on its accumulation or implications for transport and stabilization in deeper soil horizons. In an evergreen mixed forest with a Mediterranean climate, we studied dry-season changes in the most mobile and bioavailable soil carbon fractions. We coupled laboratory extractions, soil incubations, and liquid-state 1H-NMR with field measurements of soil respiration and soil water potential. To our knowledge, no similar studies have been conducted in forest soils, where perennial root systems may contribute water and carbon during dry periods. Across sequential dry-season measurements, we observed changes in potential carbon mineralization and the quantity and chemistry of water-extractable organic carbon (WEOC). Importantly, the timing of these changes and associations between them varied among organic, shallow mineral, and deep mineral soil layers. In organic soils, we observed large dry-season increases in soil carbon availability, with a four-fold increase in WEOC and a 50% increase in potential mineralization between March and August. In mineral soils, in contrast, soil carbon availability had smaller seasonal changes and a greater dependence on vegetation metrics, likely due to variations in root-derived carbon. 1H-NMR spectra supported these observations, indicating differences among soil layers in the temporal evolution of WEOC chemistry. At all depths, we saw positive correlations between soil moisture and the relative transformation of WEOC due to decay/degradation. In the organic layer, this relationship was due to a large dry-season increase in oxygen-containing carbon (e.g., carbohydrates), whereas in deep mineral soils this relationship was due to a decrease in aliphatic carbon. Overall, we found that over the summer dry season, stocks of mobile and bioavailable soil organic carbon increased in depth-specific ways.
Vaughn, L.S., Santos, F., Wahab, L.M., Berhe, A.A., Dawson, T.E. (2019): Dry season changes in soil carbon composition: tracking decomposability, mobility, and chemistry over a Mediterranean-type summer. American Geophysical Union 2019 Fall Meeting, San Francisco, CA, 9-13 December 2019 Abstract #B33C-08.
This Paper/Book acknowledges NSF CZO grant support.