Foundations of Soil Organic Matter Stabilization: Tracing the Influence of Mineralogy on the Initial Sorption of Root-Derived Carbon

Abstract:

Soils are the largest terrestrial carbon (C) reservoir, storing 2,300 Gt C globally, with the largest C input allocated by plant roots. Many root-derived C inputs are low molecular weight compounds (exudates), although complex C compounds from sloughed off cells and decaying roots also contribute precursors to the soil organic matter (SOM) pool. Root-derived compounds are metabolized by microorganisms, using extracellular enzymes to degrade the more complex C compounds. Thus, products of microbial use of root C may be free in soil solution or occur as microbial cell material. Products of root decomposition are stabilized in soil when C compounds are protected from degradation by (i) chemical recalcitrance, (ii) physical protection by aggregation, or (iii) physical-chemical protection by sorption to mineral surfaces. Previous studies show that sorption of SOM to soil minerals can stabilize C compounds for up to thousands of years. We examined the influence of soil mineralogy on sorption of root-derived C. We hypothesized that differences in specific surface area (SSA) and chemical reactivity of four mineral types: goethite, kaolinite, quartz, as well as native minerals extracted from field soil, are significant controls on the rate, quantity, and composition of mineral-sorbed SOM. Soils were collected at the UC Hopland Research and Extension Center in Hopland, CA and planted in soil microcosms with the common annual grass A. barbata; seeds collected from the field site. Microcosms were incubated in a sealed chamber under ¹³CO₂ (99 atom%) for 8 weeks. Plant photosynthesized-C is allocated to the soil via roots, and with the ¹³C label, allows us to trace the fate of plant-derived C in the soil. Minerals, which were isolated in 18 μm mesh to exclude roots but not microorganisms, were extracted and measured for total C and ¹³C atom% after a 12 week growing season of A. barbata. Preliminary FTIR and ¹³C-NMR analysis show differences in the chemical composition of mineral-associated SOM. Understanding the pathway through which SOM is first stabilized by mineral-association in soils, and the influence of soil mineralogy on that pathway, may improve models of soil C cycling and climate change predictions for soil C pools.