Soil erosion and conservation potential of row crop farming in mountainous landscapes of South Korea
Soils play an essential role for mankind because they provide fundamental ecosystem services required for human life, primarily for the production of food by providing the environment for plant growth. However, soils worldwide became highly threatened by human induced degradation, especially as a consequence of accelerated erosion by water during recent decades. In consideration of climate change and an increasing food demand of a rising population, there is an urgent need to conserve the soil resources by implementing effective erosion control measures for agricultural production. The effective implementation of those measures strongly depends on the specific conditions of particular regions and requires the analysis of the existing farming systems and their capability for erosion control. Objective of this thesis is the analysis of the major agricultural practices applied for row crop cultivation in mountainous watersheds of South Korea with respect to water erosion and the identification of their conservation potential. Our first two studies analyze the subsurface flow processes, the runoff patterns, and the associated erosion rates of the widely applied plastic covered ridge-furrow system (plastic mulch), and our third study investigates the impact of herbicide applications on erosion associated with conventional and organic farming. To analyze the flow processes induced by the plastic mulch cultivation, we conducted four irrigation experiments on potato fields that represent a smooth surface, uncovered ridges, and plastic covered ridges with and without a developed crop canopy. With an automatic sprinkler, we irrigated small plots with a dye tracer solution of Brilliant Blue and potassium iodide, collected surface runoff, and excavated soil profiles to visualize the subsurface flow patterns, which were subsequently analyzed by image index functions. We found that the ridge-furrow system, especially when ridges are covered with plastic, decreased infiltration and generated high amounts of surface runoff, whereas a developed crop canopy increased infiltration due to interception and stem flow. The analyses of the subsurface flow patterns show that the plastic covered ridge-furrow system induces preferential infiltration in furrows and planting holes due to its topography and the impermeable covers, but that the impact on flow processes in the soils is relatively small compared to the impact on runoff generation. To identify the patterns of overland flow and the erosion rates associated with the plastic mulch system, we installed runoff collectors to monitor runoff and sediment transport of two potato fields with concave and convex topographies, and we applied the EROSION 3D model to compare the plastic covered ridge-furrow system to uncovered ridges and a smooth surface. We found that plastic mulch cultivation considerably increases soil erosion compared to uncovered ridges as a consequence of high amounts of surface runoff. Our results show that the ridge-furrow system concentrated overland flow on the concave field, resulting in severe gully erosion, but prevented flow accumulation and reduced erosion on the convex field, which demonstrates that the effect of this cultivation strategy is primarily controlled by the field topography and its orientation. To analyze the effects of conventional and organic farming on water erosion, we measured multiple vegetation parameters of crops and weeds of conventional and organic farms cultivating bean, potato, radish, and cabbage, and we simulated long-term soil loss rates with the Revised Universal Soil Loss Equation (RUSLE). We found that organic farming reduced erosion for radish, as a result of an increased weed biomass due to the absence of herbicides, but that it increased erosion for potato due to lower crop coverage, presumably as a consequence of crop-weed competition or herbivory associated with the absence of agricultural chemicals. Although we demonstrated that a developed weed cover in the furrows can potentially decrease the erosion risk for row crops, our results show that the average annual erosion rates of both farming systems exceed by far any tolerable soil loss. In consideration of the generally high soil loss found in our studies, we conclude that the applied farming practices are not capable for effective erosion control and soil conservation in this region. However, based on our findings, we could identify possible modifications of those practices that can help to reduce the risk of erosion in the future. We recommend perforated plastic covers for ridges to reduce runoff generation, and the orientation of the ridge-furrow system along the contours or towards field edges to prevent flow accumulation and gully formation. Additionally, we suggest residue mulching of furrows to protect the soil surface from overland flow, and the cultivation of winter cover crops after harvest to maintain a better soil cover throughout the year.
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