Relations of mineral-soil C and N to climate and texture: regional differences within the conterminous USA

Soil is a prominent component of terrestrial C and N budgets. Soil C and N pools are influenced by, and may reciprocally influence, many environmental factors. Our objective was to determine the quantitative relations of surface mineral-soil organic C, N, and C/N ratios to climate and soil texture across seven ecological regions that make up the conterminous USA. Up to 608 soil profiles per region and their corresponding climates were evaluated with regression analysis. The organic C pool (kg C m⁻²) in the upper 20 cm of mineral soil was positively related to mean annual precipitation, evapotranspiration and clay content in all regions. It was negatively related to a temperature/precipitation index in all regions and negatively related to mean annual temperature, except in the northwest temperate forest region. Soil C/N ratios were negatively related to clay or silt content in all regions. These relations are consistent with concepts of moisture and temperature controls on detrital production, differential effects of temperature on detrital production and decomposition, and stabilization of organic matter by clay and silt. Differences in quantitative relations among regions may be related to vegetation-composition effects on soil organic matter processes, clay mineralogy, and faunal mixing of surface organic horizons with mineral soil. Regional differences also occurred in the importance of climate vs. soil texture in explaining the variability in soil C. The regional differences indicate the importance of using region-specific, rather than generalized, equations for projecting long-term soil responses to climate change and for conducting ecosystem-model calibration or validation.