COS 298-6 - The persistence of metabolically protected vs. mineral-associated SOC in the presence of organic inputs

Abstract: Soil organic carbon (SOC) stability is controlled by factors that limit the microbial degradation of SOC, including metabolic limitations (labile C and nutrient limitations) and the adsorption of SOC to clay organo-mineral complexes. Microbial metabolic limitation is not considered a long-term mechanism of SOC stability given the ability for new labile C inputs to “prime” recalcitrant organic matter. While mineral protection of SOC is the most persistent form of SOC stabilization, certain root inputs to the soil may displace SOC from mineral surfaces, causing SOC losses that mimic metabolic priming effects.

We sought to understand how metabolic limitation and organo-mineral complexes interact to influence SOC stability with a three-year laboratory incubation of synthetic soils. To test how these SOC stabilization mechanisms interact, synthetic soils were created with a composite of canopy soils from Costa Rica. Canopy soils form in tree canopies and are ideal for this application given that they are entirely devoid of clay minerals. A portion of the air-dried canopy soil was pre-decomposed to deplete labile C and inorganic nutrients. After one year, another portion of dried canopy soil was wet-up and each batch of canopy soil was either incubated as-is (organic soils) or mixed with montmorillonite clay and sand to form synthetic mineral soils. To evaluate the relative stability of metabolically protected vs. mineral-associated SOC in the presence of organic inputs, microcosms were amended with either ¹³C-oxalic acid (an organic compound that can displace C from mineral surfaces) or ¹³C-glucose (an organic compound easily incorporated into microbial biomass) in a fully factorial experiment.

We found that after one year of pre-incubation, the pre-decomposed soils were significantly depleted in inorganic nitrogen & phosphorus. Metabolic limitation and organo-mineral complexes interacted to influence soil C losses: in non-depleted soils, ¹²C-CO₂ production per unit organic matter was significantly lower in mineral soils than in the organic soils. ¹²C-CO₂ production in depleted soils was lower than in non-depleted soils but similar in both mineral and organic soils. Thus, in soils with abundant labile C and nutrients, the presence of clay minerals stabilizes C, while in depleted soils, metabolic limitation appears more important to SOC stabilization than organo-mineral complexes. We will present pending results of ¹³C-CO₂ production, density fractionations, microbial biomass, and dissolved organic C across treatments that will demonstrate how metabolic limitation and organo-mineral complexes interact to influence SOC stability in the presence of organic inputs.