A FIRST LOOK AT THE TURNOVER DYNAMICS OF LOW MOLECULAR WEIGHT ORGANIC CARBON IN SHALLOW AND DEEP SOILS OF COASTAL PRAIRIE GRASSLAND ECOSYSTEM

The functional importance of low molecular weight organic compounds (LMWOC) is in disproportion to their abundance within soil organic carbon (SOC) pools. LMWOC are critical in driving microbial metabolism, though microbial utilization is likely dependent on C chemistry. In this study, we examined the fate of two important, often plant-derived, components of soluble SOC: sugars and carboxylic acids, through an extended soil profile. Our objective was to evaluate the short-term (9 d) and long-term (16 mo) turnover dynamics of LMWOC under varying physical, biological, and chemical conditions through the soil column. Our study area is a coastal prairie near Santa Cruz, CA; soils are classified as fine-loamy, mixed thermic typic Argixerolls. The long-term component of this study was conducted in situ and replicated in the laboratory to quantify the short-term dynamics. We injected ¹³C-labeled glucose (GLU), ¹³C-labeled oxalic acid (OA), or deionized water 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 in the lab and at increasingly graduated intervals in the field. At the conclusion of each experiment, soils were harvested to assess total recovery of ¹³C in soil pools. Measures of extractable C indicated a significant priming effect in the A horizon, and an increase in microbial biomass C (MBC) in the argillic horizon in response to GLU and OA. In all instances, residence time was lower for GLU than OA and increased with depth for both substrates. In the short-term OA- C was mineralized to a greater degree than GLU- C in the A and argillic horizons, but in the deepest (mottled) soil horizon recovery of OA- C was substantially higher, and likely resulted from enhanced organo-metal complexation. Dissolved Al³⁺, which forms strong aqueous complexes with OA,  may limit the availability of OA at depth compared to shallower soils. Long-term recovery favored GLU- C, which increased with depth. GLU contains no significant functional groups for adsorption to the mineral phase, but immobilization within MBC was comparatively high, which suggests long-term retention of GLU-C is linked to mineral interaction with microbial residues following turnover.

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