Relationships Among Water Stable Aggregates and Organic Matter Fractions Under Conservation Management

Abstract

Soil organic carbon (SOC) plays an important role in soil aggregation, yet the exact nature of this relationship is not well understood. The objectives of this study were to (i) evaluate relationships among field‐moist and air‐dried macroaggregate stability (WSAFM and WSAAD, respectively), cold water‐extractable organic C from whole soil and the particulate, adsorbed and occluded (PAO) fraction, and associated soil properties, (ii) compare properties of the PAO fraction to those of whole soil, and (iii) evaluate the effects of conservation management practices, including grass vegetative filter strips (VFS), agroforestry VFS, and no‐till, plus landscape positions on these variables. Soil samples were collected in 2007 from an agroecosystem established on claypan soils located in Missouri. This study found that cold water‐extractable organic C and aggregate‐associated organic C were more highly correlated with WSAAD (r ≥ 0.74) than SOC, suggesting that these organic C fractions may play an important role in aggregate stability. In addition, less degraded water‐extractable organic C (as indicated by a greater C/N ratio) was preferentially concentrated in the PAO fraction (C/N = 10.6) relative to whole soil (C/N = 4.2). Further, macroaggregation and accumulation of less degraded organic C (in solid and water‐extracts) was enhanced under grass VFS and agroforestry VFS relative to no‐till. Overall, this study found that macroaggregate stability, cold water‐extractable organic C, and the PAO fraction served as effective early indicators of soil C change in the soil studied.