RAPID CHANGE IN SOIL CARBONATES: DISCERNING ECOHYDROLOGICAL AND ANTHROPOGENIC DRIVERS USING GEOCHEMICAL AND ISOTOPIC TECHNIQUES

Soil carbonates are recognized as an important factor in global carbon (C) storage and reconstruction of paleoclimatic conditions. They account for roughly 39% of the terrestrial C pool (~950 Pg C) and in drylands can be >10 × organic C pools. Carbonates are often used to infer terrestrial precipitation regime and dominant vegetation types over timescales of 10³-10⁵ yr. Although carbonate formation and storage are often described on millennial timescales, several studies provide evidence of dynamic changes (i.e. yearly to decadal), with both increases and decreases in storage. These rapid gains or losses in soil carbonates due to natural and human-induced environmental changes may alter our accounting of terrestrial C budgets and provide both insights and challenges for paleoclimatic research.

In this study, we aim to characterize the isotopic signature of carbonates known to have formed during a long-term field experiment (Huber et al. 2019) as a means of assessing their sensitivity to anthropogenic activities, seasonality, and fast (i.e. decadal) environmental change. The ~20-year ecohydrological experiment manipulated water availability and thus carbonate formation in three ways: 1) plant community; 2) seasonality of water availability; and 3) maximum soil depth (i.e. control volume). The first two factors are both projected to change in the northwest region of the US.

Preliminary data shows significant changes in soil carbonate pools with change in vegetation and seasonal water availability did not result in clear shifts in δ¹³C and δ¹⁸O values. Profile carbonate δ¹³C and δ¹⁸O values were both enriched near the surface and depleted at depth relative to ambient controls, indicative of distinct atmospheric vs. respired CO₂ sources. Although δ¹³C profile carbonates failed to differentiate, δ¹⁸O values showed distinct patterns between small vs. large control volume treatments, as well as underplant vs. interplant soil patch spaces. Noteworthy is the lack of differentiation in clumped isotope values (Δ₄₇) between dormant vs. growing season irrigation treatments, indicative of similar environmental conditions during carbonate formation. This may denote key temporal dependence in ecohydrological processes during formation or result from insensitivity due to high background carbonate concentrations.