Soil Carbon Fractions from an Alluvial Soil Texture Gradient in North Carolina.

Soil texture is known to affect soil organic C (SOC) concentration and microbial activity, but these relationships are not always straightforward. We characterized total, mineral-associated and mineralizable C fractions along a gradient of soil texture within a flood plain field in the Coastal Plain region of North Carolina. Soil was collected from 0- to 5-, 5-to 15- and 15- to 30-cm depth intervals at 204 locations within a 7-ha area. Samples were analyzed for soil particle size distribution, specific surface area (SSA), oxalate-extractable Al and Fe to estimate short-range-ordered (i.e., poorly crystalline) oxyhydroxides and soil C fractions. Overall, relationships among soil C fractions, textural classes and depths were complex. Both SOC (0.4-13.9 g kg^-1 soil) and mineral-associated organic C (0-12 g kg^-1 soil) increased as soil clay concentration increased (73-430 g kg^-1 soil), but having two distinct slopes in each relationship with an inflection point of ∼150 g clay kg^-1 soil at 0 to 5 and 5 to 15 cm and an inflection point of ∼250 g clay kg^-1 soil at 15 to 30 cm. As clay concentration increased, SSA (12-76 m² g^-1 soil) and oxalateextractable Fe (0.45-5.9 g kg^-1 soil) also increased. A weaker relationship was observed between oxalate-extractable Al (0.38-1.5 g kg^-1 soil) and either SSA or mineral-associated organic C. Mineralizable C increased with increasing clay concentration up until 143 ± 3, 152 ± 5, and 161 ± 11 g kg^-1 (0-5, 5-15 and 15 -30 cm, respectively), but decreased (0-5 and 5-15 cm) or stayed constant (15-30 cm) at higher clay concentrations. On the basis of untested observations, we surmise that binding of C to oxalate-extractable Fe contributed to the accumulation of SOC and suppression of mineralizable C as the clay concentration increased. These results suggest that complex soil texture- physicochemical interactions underlie the inherent fertility of floodplain soils. [ABSTRACT FROM AUTHOR]