A First Look at the Turnover Dynamics of Low Molecular Weight Organic Carbon in Shallow and Deep Soils of Coastal Prairie Grassland Ecosystem

Abstract:

The functional importance of low molecular weight organic compounds (LMWOC) is in disproportion to their abundance within soil organic carbon (SOC) pools. They are critical in driving microbial metabolism, though microbial utilization of LMWOC is likely dependent on C chemistry. Studies of C turnover in soils tend to focus in shallower horizons despite that for many ecosystems a substantial fraction of SOC resides below 1 m. In this study, we examined the fate of two important components of soluble SOC, sugars and carboxylic acids, through a soil profile extending to 150 cm. Our objective was to evaluate the turnover of LMWOC under varying physical, biological, and chemical conditions through the soil column. Our study area is part of a soil chronosequence near Santa Cruz, CA. From the side wall of a soil pit we installed intact soil collars and injected ¹³C-labeled glucose (GLU), ¹³C-labeled oxalic acid (OA), or deionized water (control) into the A, B (argillic) , and B/C (mottled) horizons at depths of 25, 75, and 125 cm, respectively. We sampled soil gas for ¹³CO₂ intensively at graduated sampling intervals (6 hours to 2 weeks). The entire experiment was also replicated in the laboratory. We measured dissolved organic C (DOC) and microbial biomass C (MBC), and calculated total recovery of ¹³C in atmospheric and soil pools. Measures of DOC indicated a significant priming effect in the deepest (mottled) horizon and an increase in MBC in the argillic and mottled horizons. In all instances residence time was significantly lower for GLU than OA and increased with depth for both substrates. Mass balance calculations from the laboratory component indicated stronger retention for GLU than OA for the upper soils; however, this trend reversed below the argillic horizon. We hypothesize the greater retention of OA in the deepest (mottled) soil horizon may result from enhanced organo-metal complexation (e.g., between OA and dissolved Fe or Al). This hypothesis is consistent with modeled depth profiles of dissolved Al³⁺, which forms strong aqueous complexes with OA, and may limit the availability of OA at depth compared to shallower soils. Similarly, OA could act to displace native C from mineral surfaces in deep vs. shallow soils, a process, which evidence suggests, could result in a greater priming effect from the turnover of native C.