King, A. E., J. P. Amsili, S. C. Córdova, S. Culman, S. J. Fonte, J. Kotcon, M. Liebig, M. D. Masters, K. McVay, D. C. Olk, M. Schipanski, S. K. Schneider, C. E. Stewart, and M. F. Cotrufo. 2023. A soil matrix capacity index to predict mineral-associated but not particulate organic carbon across a range of climate and soil pH. Biogeochemistry doi: 10.1007/s10533-023-01066-3
Understanding controls on soil organic carbon (SOC) will be crucial to managing soils for climate change mitigation and food security. Climate exerts an overarching influence on SOC, affecting both carbon © inputs to soil and soil physicochemical properties participating in C retention. To test our hypothesis that climate, C inputs, and soil properties would differently affect particulate organic carbon (POC) and mineral-associated organic carbon (MAOC), we sampled 16 agricultural sites (n = 124 plots) in the United States, ranging in climate (mean annual precipitation (MAP)—potential evapotranspiration (PET; MAP-PET)), soil pH (5.8–7.9), and soil texture (silt + clay = 13–96%). As MAP-PET increased, soils increased in oxalate-extractable iron (FeO) and aluminum (AlO), decreased in exchangeable calcium (Caex) and magnesium (Mgex), and received greater C inputs. Soil physicochemical properties did not strongly predict POC, confirming the relative independence of this SOC fraction from the soil matrix. In contrast, MAOC was well predicted by combining AlO + [1/2]FeO with Caex + Mgex in a ‘matrix capacity index’, which performed better than individual soil physicochemical properties across all pH levels (r > 0.79). Structural equation modeling indicated a similar total effect of MAP-PET on MAOC and POC, which was mediated by total C inputs and the matrix capacity index for MAOC but not POC. Our results emphasize the need to separately conceptualize controls on MAOC and POC and justify the use of a unified soil matrix capacity index for predicting soil MAOC storage.
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