Runoff and soil loss from midwestern and souteastern US silt loam soils as affected by tillage practice and soil organic matter content

No-till practices generally reduce runoff (RO) and soil loss (SL) by contributing to accumulations of soil organic matter (SOM) in the near-surface zone. This research was conducted to determine the effects of SOM contents on RO and SL from two highly erodible soils using crops that produce a wide range of residue, in the context of long-term tillage studies in widely separated climatic regions. Rainfall simulator plots, measuring 6.1 mx0.9 m, were imposed on a 9-year-old corn (Zea mays L.) and cotton (Gossypium hirsutum L.) conservation tillage study at Senatobia, Mississippi, and on a similar 34-year-old corn study at Coshocton, Ohio. All RO was collected from two replications of conventional (CT) and no-till (NT) treatments following application of simulated rainfall at an intensity of 50 mm h-1 for 1 h. Soil samples collected in depth increments of 0-1, 1-3, 3-7.6 and 7.6-15.2 cm were characterized for SOM content, aggregate stability (AS), water dispersible clay (WDC) and particle size distribution. Bulk density (BD) samples were collected in increments of 0-3.8, 3.8-7.6, 7.6-15.2 and 15.2-30.5 cm. Overall, RO from the CT and NT treatments averaged 27.8 and 16.5 mm, respectively. SL loss from the CT treatments averaged 3.9 Mg ha-1 and 0 for the NT. BDs in the surface 3.8 cm averaged 1.34 Mg m-3 for CT and 1.26 Mg m-3 for NT. Correlation coefficients (r) for SOM content versus AS, WDC and BD were 0.92, -0.90 and -0.64, respectively. Regression models indicated that BD, as a single-variable, explained 87% of the variability in RO from the NT treatments. BD alone was less effective in accounting for the variability in RO from CT treatments, but contributed to a three-variable model with AS and WDC to produce an R2 of 0.97. These results indicate that as SOM contents gradually increase in NT treatments, RO decreases due to the development of greater porosity in the near-surface zone attributable to enhanced AS at the soil surface. Thus, surface sealing tendencies are diminished which promotes an increase in infiltration rates.