Cropping system partially offsets tillage-related degradation of soil organic carbon and aggregate properties in a 30-yr rainfed agroecosystem

Abstract

Soil tillage increases the susceptibility of agricultural soils to erosion and organic carbon losses, but tillage effects could be mitigated through other management practices such as crop rotation. Here, we evaluated the 30-year impacts of tillage intensity and cropping system on surface soil bulk density, nutrient availability, dry aggregate size distribution, and water-stable aggregation. This study was established in 1980 in eastern Nebraska USA, and included six tillage treatments of varying intensity (no-till, ridge till, disk till, subsoil rip, chisel plow, moldboard plow) and four crop rotation treatments (continuous soybean [Glycine max (L.) Merr.]; soybean-corn [Zea mays L.]; corn-soybean, continuous corn) in a randomized block design with six replicates. Surface soils were sampled in 2011 and soil aggregate properties assessed, including occluded particulate organic matter (oPOM) in micro/macroaggregates (0.053–0.5 mm) and mega-aggregates (>2.0 mm). Because of significant treatment differences in bulk density, soil properties were converted to an equivalent soil mass (ESM) basis to more accurately assess management effects. After 30 years, only the main effects of tillage and crop rotation were significant for most measured soil properties. Surface soil organic carbon (SOC) stocks (ESM for ∼0−30 cm soil depth) decreased with tillage intensity, and stocks were higher when corn was included in the cropping system. Dry aggregate size distributions shifted towards smaller size classes as tillage intensity increased and whenever corn was included in the cropping system. As a result, aggregate mean weight diameter (mm) followed a similar trend. Soil stocks of water-stable mega-aggregates also decreased with increasing tillage intensity. In near-surface soils (0–7.5 cm), highly-erodible aggregate oPOM was highest in no-till soils and was more sensitive to tillage disturbance (56–69% loss) than mega-aggregate oPOM (5–35% loss). Even in no-till soils, highly-erodible aggregate oPOM concentrations decreased under continuous corn compared to rotated systems likely due to greater frequency of fertility management-related soil disturbances (i.e. fertilizer injection annually vs every two years). These results suggest that cropping systems that maximize plant carbon inputs can partially mitigate soil erosion risks due to long-term tillage, but that other crop management-related soil disturbances (i.e. method of fertilizer application) could limit the mitigating effect of cropping system.