Sand is the unifying textural component influencing surface-soil carbon and nitrogen fractions across undisturbed land uses in North Carolina

Soil organic matter concentrations are associated with soil texture in some but not in all studies. Why there are variable responses to soil texture can have logical reasons, the most obvious of which are inconsistent historical land uses, interactions with climatic and landscape settings, and management variations within a land use. In an evaluation of surface soils (0- to 10-cm depth) under consistently undisturbed land use from 648 sites across relatively narrow climatic variations in North Carolina, large soil texture variations were assembled into structured populations (n = 27) of sand and clay categories (n = 24). Sand concentration varied from 220 to 881 g kg⁻¹, silt concentration varied from 67 to 517 g kg⁻¹, and clay concentration varied from 47 to 360 g kg⁻¹ (5%–95% limits). Overwhelmingly, total, particulate, and non-particulate organic C and N fractions were more statistically associated with sand concentration than with clay concentration alone. Sand concentration is the inverse of clay + silt summation and is a necessary feature when determining particulate organic C and N. Soil bulk density and sieved soil density were also more closely associated with sand concentration than with clay concentration alone. This study confirmed there was no saturation limit for the accumulation of non-particulate organic C and N (sometimes labeled mineral-associated organic matter). Therefore, sand concentration should be considered the best indicator of soil textural influence on soil organic matter properties and a key contextual feature necessary for soil health assessments.