Submitted to: Soil & Tillage Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 9, 2004
Publication Date: August 1, 2004
Citation: Moorman, T.B., Cambardella, C.A., James, D.E., Karlen, D.L., Kramer, L.A. 2004. Quantification of tillage and landscape effects on soil carbon in small Iowa watersheds. Soil & Tillage Research. 78(2):225-236.
Interpretive Summary: Increasingly, watersheds are being considered as the geographic unit for evaluating the relationship between agricultural practices and water quality. This study was conducted to evaluate the effects of two tillage systems on soil organic matter content in three watersheds in southwest Iowa. Ridge tillage was more effective in retaining soil within the watershed than conventional plowing and disking tillage system. However the retention of soil organic matter over time was not improved by ridge tillage. Respiration by soil microorganisms was the primary cause of carbon loss. There was also evidence that soil and organic matter were eroded from the steeper side slopes and deposited at the base of the slopes with the watersheds. A topographic index was identified which appears to predict organic matter contents in soil profiles. The research documents changes in organic matter and the loss of organic matter from 1972 to 1995. Long-term records of this type improve our ability to predict changes in soil carbon and soil quality. This information will be useful to scientist seeking to understand the effects of tillage systems on soils and the impact of agricultural practices on carbon storage by soils.
Knowledge of the long-term effects of tillage on soil organic carbon is important to our understanding of sustainable agricultural systems and global carbon cycles. In landscapes susceptible to erosion, tillage can exacerbate losses of soil and carbon by increasing erodibility and stimulating microbial respiration. We measured long-term changes in soil carbon and soil loss in three small watersheds cropped to continuous corn (Zea mays L.) with moldboard and disk tillage or ridge tillage. The ridge tillage system had greater carbon contents in the surface soil than the disk tillage soils, but ridge tillage was not different from the conventional tillage in carbon retention over time. The ridge tillage system, however, was more effective in retaining soil within the watershed. Microbial respiration by soil microorganisms accounted for 97% of the carbon loss in the ridge tilled watershed compared to carbon loss in eroded sediment (3%). Terrain analysis was used to segment the landscape into landform elements. Less total carbon was present in the soil profiles of backslope elements than in footslope or toeslope elements, reflecting the combined effects of soil erosion and deposition within the watersheds. Profile carbon content was also positively correlated with the Wetness Index, a compound topgraphic attribute, that indentifies areas of the landscape where runoff water and sediment would accumulate.
Keywords: ridge-till, erosion, organic matter, terrain analysis, wetness index, loess