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 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 (Fe_O) and aluminum (Al_O), decreased in exchangeable calcium (Ca_ex) and magnesium (Mg_ex), 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 Al_O + [1/2]Fe_O with Ca_ex + Mg_ex 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.

了解土壤有机碳 (SOC) 的控制对于管理土壤以减缓气候变化和粮食安全至关重要。气候对 SOC 具有总体影响，既影响土壤的碳 (C) 输入，也影响参与碳保留的土壤理化性质。为了检验我们的假设，即气候、碳输入和土壤特性会对颗粒有机碳 (POC) 和矿物相关有机碳 (MAOC) 产生不同的影响，我们对美国 16 个农业地点（n = 124 个地块）进行了采样，这些地点的气候范围包括气候（平均年降水量 (MAP) - 潜在蒸散量 (PET; MAP-PET)）、土壤 pH 值 (5.8–7.9) 和土壤质地（淤泥 + 粘土 = 13–96%）。随着 MAP-PET 的增加，土壤中草酸盐可提取的铁 (Fe _O ) 和铝 (Al _{O），交换性钙（Ca ex}）和镁（Mg _ex ）减少，并获得更多的碳输入。土壤理化性质不能强烈预测 POC，这证实了该 SOC 部分与土壤基质的相对独立性。相比之下，通过将 Al2O ₊  [1/2]Fe _O与 Ca _ex  + Mg _{ex相结合，可以很好地预测 MAOC}在“基质容量指数”中，其在所有 pH 水平下的表现均优于单个土壤的理化性质 (r > 0.79)。结构方程模型表明 MAP-PET 对 MAOC 和 POC 具有相似的总效应，这是由总 C 输入和 MAOC 的基质容量指数介导的，但不是 POC。我们的结果强调需要分别概念化对 MAOC 和 POC 的控制，并证明使用统一的土壤基质容量指数来预测土壤 MAOC 储存的合理性。