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

Monday, 15 December 2014
Rachel Neurath1, Peter S Nico2, Jennifer Pett-Ridge3 and Mary Firestone1, (1)University of California Berkeley, Berkeley, CA, United States, (2)Lawrence Berkeley National Laboratory, Berkeley, CA, United States, (3)Lawrence Livermore National Laboratory, Chemical Sciences Division, Livermore, CA, United States
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.