Changes In Soil Organic Matter Quality with Increasing Mineral Carbon Loading.

In addition to a theoretical upper limit to the amount of SOM that can be stabilized in soil, the soil C saturation theory generates corollaries concerning the relative permanence of SOM that have significant importance in relation to SOM responses to disturbance such as global climate change. We designed a laboratory experiment to test the hypothesis that the quality of SOM changes when a soil approaches its C saturation limit. The rationale is that as the C saturation deficit decreases, the stabilization mechanisms that work to decrease decomposability of SOM become weaker. One potential mechanism that may limit the amount of SOM stabilization is the finite soil mineral surface area onto which SOM may sorb. Mineral carbon loading (mg C per m² mineral) is thus used as a proxy for C saturation, as the literature has suggested that C loadings near the monolayer equivalent represent a potential maximum capacity for SOM stabilization.

Batch sorption experiments were performed using several soils to generate organo-mineral complexes with increasing C loadings by mixing low-C subsoil samples with increasing concentrations (~50-500 mg C L^-1) of dissolved organic matter (DOM) generated from leaf litter leachate. The DOM and minerals were allowed to interact for 24h at 20°C while mixed on an orbital shaker. Degradation of the DOM during the experiment was determined using zero-mineral blank samples. The resultant organo-mineral complexes were subsequently tested for variations in SOM quality as a function of C loading using C and N isotopic ratios (δ¹³C and δ¹⁵N), specific respiration (µg CO₂-C per g sample-C) during short-term laboratory incubations, and thermal analysis.

Preliminary results show that 2.1-34.8% (mean = 18.3%) of initial DOC was sorbed, resulting in estimated C-loadings between 0.15-2.45 mg C m^-2. The expected outcome of subsequent analyses is that as a soil approaches its C saturation limit (i.e., maximal C loading), SOM becomes increasingly decomposable.