Hydro-ecological controls on pedogenesis and soil organic carbon: A climo-chronosequence study

Background/Question/Methods

Millennial scale- or long-age gradient- soil chronosequences provide a powerful framework for evaluating the influence of time (i.e., soil age) on soil organic carbon (SOC) cycling. By linking multiple chronosequences formed in similar geomorphological environments (e.g. soils developed from paleo-marine terraces) but divergent climatic conditions, we have established a “climo-chronosequence” gradient to study the interaction of climate, organisms, and soil development. Specifically, we have intensively sampled two soil chronosequences in northern California, the Santa Cruz Marine Terraces (Mean Annual Precipitation = 50 cm) and Mattole River Marine Terraces (MAP=100 cm), in order to address the question: How do difference in soil water availability influence the interactions of soil development and SOC cycling? To quantify differences in SOC storage and stability across the climo-chronosequence, we sampled to depths up to 2m and collected between 150 and 200 soil samples from each of 8 terraces. We mapped C and N distributions of soil profiles and compared them with bulk elemental chemistry from x-ray fluorescence. We also measured carbon isotopes (δ¹³C, Δ¹⁴C), mineralogy (by x-ray diffraction), soil surface area and other parameters on a subset of the samples. Finally, we collected and measured CO₂concentrations and C isotopic composition of soil gas from several depths in each soil profile. 

Results/Conclusions

We found significantly greater SOC accumulation in the wetter Mattole soils in both shallow and deep soil environments but the Δ¹⁴C of bulk soil suggests SOC stored in the Santa Cruz soils is older. Variations in SOC between chronosequences were larger than variations within each chronosequence, indicating the dominant influence of water availability. Across all soils studied, hydro-ecological controls on the formation of secondary weathering products, including organo-metal complexes and short-range order minerals, appeared to regulate C storage and stability. However, our climo-chronosequence approach for evaluating controls on SOC cycling is complicated by limited natural replication of soil environments and by difficulty constraining ecological variations, including difference in the composition of plant and microbial communities within and between soils. To address these problems, we are developing a process-based reactive transport model based on these datasets that can be more broadly tested and we have initiated experimental manipulations designed to determine the role of microbiology in the coupling of soil development and C cycling. Overall, our work suggests climate or specifically in this case, soil water, is an important modulator of the interactions between soil development and SOC cycling.