Soil physical properties and aggregate-associated C, N, and P distributions in organic and conventional cropping systems

Organic farming, which is growing in popularity, has been proposed as a sustainable alternative to conventional farming practices. However, it is not known how organic farming systems affect soil erosion risk and sediment-bound nutrient transport. Our objectives were to compare soil erosion risk and sediment bound nutrient transport potential for grain-based conventional and organic cropping systems by determining selected soil physical properties and distributions of carbon (C), nitrogen (N), and phosphorus (P) in soil aggregates of the 0-5 cm depth of a Christiana-Matapeake-Keyport soil association. We measured soil bulk density, aggregate stability, aggregate size distributions, and total C, N, and P associated with five soil aggregate size classes in no-till (NT) and chisel till (CT) systems and in an organic system (ORG). No-till soils had lesser bulk density and greater aggregate stability than did CT and ORG soils. Carbon, N, and P concentrations were greater in large (>2.00 mm) and small macroaggregates (0.21 to 2.00 mm) than in microaggregates (<0.21 mm) regardless of cropping system. When nutrient concentrations were combined with aggregate distribution data, the quantity of aggregate associated nutrients was greatest in microaggregates in ORG and CT soils but greatest in macroaggregates in NT soils. These results indicate an increased risk of sediment associated nutrient transport from ORG and CT soils compared with NT soils, since microaggregates in these soils are preferentially lost through sediment transport. The NT cropping system promoted macroaggregate formation and reduced the risk of particulate nutrient transport in this warm, humid region soil.