Long-Term Effect of Different Carbon Management Strategies on Water Flow and Related Processes for Three Loamy Soils

The decline in organic matter of arable land, induced and accelerated by modern agriculture, has been identified as a threat to sustained soil quality. In this article, we studied strategies to counter this decrease by building up soil organic carbon (SOC) levels in the soils using several approaches. They included return of organic matter, such as straw, animal manure, and slurry, to the soil and diverse crop rotations with, for example, grass-clover leys. We used three field sites in Denmark with different C repletion strategies to assess and quantify the effect of these approaches on preferential flow and loss of colloids during heavy irrigation events. The field sites were all under long-term management and therefore represent up to 30 years of pairwise different management strategies. One field in each field pair was managed with a more C-repleting strategy (HighC) than the other (LowC). Only small differences in SOC contents were identified, and none of the management strategies had succeeded in building up SOC pools large enough to saturate the soil with C. Only at one field site was the content of water-dispersible colloids lower in the HighC than the LowC treatment. Preferential flow patterns showed a rapid breakthrough of irrigation water but only little or negligible effect in reducing the risk of colloid loss and chemicals leaching from the root zone, despite 20 to 30 years of different management strategies with this aim. Previous studies reported in the literature have emphasized high SOC contents and grass ley soils to significantly reduce nonequilibrium water flow. Our results thus indicate the need for more effective management options than those addressed in this study.