4. CONCLUSIONS
A strategy to evaluate soil erosion hazard degree and soil conservation planning for agriculture regions of the BSR of China has been proposed. The innovative aspect of this strategy is the combination of the known relationship between A horizon thickness and crop yield for the BSR with a soil erosion model that bases predictions upon the spatial distribution of the current A horizon thickness and soil properties by soil types. Erosion hazard degree and relative crop yield under different soil conservation scenarios including current tillage (conventional tillage), contour tillage, straw incorporated tillage, no-till farming and two combinations of terracing and “green for grain” practices were evaluated. Knowing that an A horizon thickness of 20 cm is the critical threshold for crop productivity, a soil life expectancy was calculated. Croplands with soil life expectancies of <0 a, 0 a~20 a, 20 a~100 a, 100 a~1000 a and ≥1000 a, were classified as “Damaged”, “High hazard”, “Moderate hazard”, “Low hazard” and “No hazard”, respectively.
The soil erosion model was validated from 137Cs erosion rate measurements for three watersheds of the BSR. The spatial distribution in soil erosion rates for the BSR was linearly and positively correlated with slope gradient and dependent upon the conservation practice scenario. The current tillage practice exhibited the highest soil erosion rates while no-tillage had the lowest soil losses. On average, about 13 cm of A horizon has been eroded away since the establishment of cultivation. Across the BSR, 8% of the sloping croplands are already classified as Damaged which will increase to 27% within 100 years under current practices. The corn yield decreased very fast (0.8% per decade) under the current tillage practice. Despite the effectiveness of no-till to control erosion, this was the third highest practice tested for loss in crop yield due to issues in applying no-till farming in cold regions. The best practice for High hazard, Moderate hazard and Low hazard was Combo 2, applying no-till, contour farming terraces and “green for grain” practices according to the slope gradient and regional temperature (cool or warm). For “High hazard” croplands, conservation practices should be applied immediately as these areas will convert to “Damaged” in just 20 years under current practices. Future research should include other management strategies, such as winter cover crops, strip tillage for flat and ridge system, furrow dikes made from soil or possibly cornstalks, or flat tillage. Such management options could be incorporated into the model predictions of erosion hazard and crop yield once the relationships are experimentally established.
A scheme for optimum spatially and temporally distributed scheduling of soil conservation practices is presented. The plan is designed to maintain all soils to have an A horizon above 20 cm by selecting and scheduling the implementation of conservation practices according to their degree of hazard. Scheduling the implementation of conservation practices according to the township’s maximum erosion hazard will optimize the distribution of labor and serve as a training tool for later-acting townships. Soil erosion estimates are conservative, and as such the soil life expectancy estimates are under-predicted due to not considering gully and wind erosion processes. This study quantifies the urgency in taking action to apply conservation methods in the BSR in order to sustain agriculture while maintaining soil health.