A soil matrix capacity index to predict mineral-associated but not particulate organic carbon across a range of climate and soil pH

Authors: KING, ALISON - Colorado State University; AMSILI, JOSEPH - Cornell University; CORDOVA, S. CAROLINA - Michigan State University; CULMAN, STEVE - The Ohio State University; FONTE, STEVEN - Colorado State University; KOTCON, JAMES - West Virginia University; Liebig, Mark; MASTERS, MICHAEL - University Of Illinois; MCVAY, KENT - Montana State University; Olk, Daniel - Dan; SCHIPANSKI, MEAGAN - Colorado State University; Schneider, Sharon; Stewart, Catherine; COTRUFO, M. FRANCESCA - Colorado State University

Submitted to: Biogeochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/7/2023
Publication Date: 7/19/2023
Citation: King, A.E., Amsili, J.P., Cordova, S., Culman, S., Fonte, S.J., Kotcon, J., Liebig, M.A., Masters, M.D., Mcvay, K., Olk, D.C., Schipanski, M., Schneider, S.K., Stewart, C.E., Cotrufo, M. 2023. A soil matrix capacity index to predict mineral-associated but not particulate organic carbon across a range of climate and soil pH. Biogeochemistry. 165:1-14. https://doi.org/10.1007/s10533-023-01066-3.
DOI: https://doi.org/10.1007/s10533-023-01066-3

Interpretive Summary: Maintaining or increasing soil organic carbon (SOC) is important to sustainably manage agricultural soils. However, to effectively manage SOC it is important to understand how SOC responds to environmental conditions. Previous studies have investigated how climate, soil, and C inputs affect SOC by looking at effects on different SOC fractions. But these studies have not developed relationships that can be applied across the U.S. To address this knowledge gap, 16 agricultural sites in the U.S. differing in climate, soil pH, and texture were sampled and evaluated for mineral-associated organic carbon (MAOC) and particulate organic carbon (POC). Soil properties did not strongly predict POC, confirming the relative independence of this SOC fraction from soil minerals. In contrast, MAOC was well predicted by combining extractable iron and aluminum with exchangeable calcium and magnesium in a ‘mineral capacity index’, which performed better than individual soil minerals and silt + clay across all soil pH levels. Findings from this study provide a method for better understanding SOC storage by separately looking at the factors affecting the MAOC and POC fractions. The results also show that a unified soil mineral capacity index is useful for understanding soil MAOC storage.

Technical Abstract: 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 (C) inputs to soil and soil minerals participating in C retention. To test our hypothesis that climate, C inputs, and soil minerals 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 (5.3 – 8.2), 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 soil minerals. In contrast, MAOC was well predicted by combining AlO + FeO with Caex + Mgex in a novel ‘mineral capacity index’, which performed better than individual soil minerals and silt + clay across all pH levels (R2 > 0.52). Structural equation modeling suggested a similar total effect of MAP-PET on MAOC and POC, which was mediated by total C inputs and the mineral 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 mineral capacity index for soil MAOC storage.