Soil organic matter content and nutrient turnover in thin Black Oxbow soils after intensive conservation management
Cultivation and frequent fallowing has a significant impact on soil organic matter (SOM) concentration and soil bulk density. Conservation tillage systems, such as direct seeding, minimize mixing and disturbance of the surface soil which, in turn, is thought to improve SOM characteristics. This study was initiated to determine the extent to which four highly diverse management systems conserve SOM and influence potential nutrient supply. Conservation managements investigated in this study included a brome grass seed-down and a direct seeded cereal-oilseed rotation. A third site was selected from an area managed under conventional fallow-wheat (with intermittent fertilizer) from 1930 to present. Uncultivated native prairie was also analyzed and used as a control. Concentrations of organic C, N and S were highest in the native sod, followed by brome grass, direct seeded and conventional fallow-wheat. Soil bulk density indicated a significant increase in the mass of soil in the 0 to 5 cm rooting layer with increased disturbance and soil mixing. The inverse trend between % SOM and bulk density resulted in management having no significant effect on the mass of organic nutrients in the top 5 cm of the rooting volume (CV 9.6%). More important to crop growth however, is the potential supply of mineral nutrients from SOM, which was significantly influenced by conservation management. Managements which improve residue input and reduce soil mixing, although not changing the actual mass of nutrients per rooting layer, enhance the turnover of organically held N and S in the 0 to 5 cm root zone.
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