Cumulative sediment, nitrogen and phosphorus losses from bare and compost-amended fill slopes
Constructed slopes are often amended with compost in order to reduce erosion and improve vegetation establishment. These composts can contain soluble nutrients which may become a source of pollution to nearby water bodies. This study examined annual sediment and nutrient losses in surface runoff from nine potential types of compost treatments. These included treatments that tested differences between two types of compost (mature and immature) as well as various methods of compost application, including surface blankets and incorporation. Over an entire rain season, there were no significant differences in sediment and nutrient (NO3-, NH4+ and P) losses between the two compost types when they were applied as surface blankets. In treatments where a compost blanket was applied over the top of compost tilled into the soil there was a significant reduction in sediment loss compared to treatments where compost was just tilled into the soil, without a blanket overlay. This treatment reduced sediment loss to a level that was comparable to vegetated treatments. Although this treatment had twice the amount of compost as the treatments with just a surface blanket of compost, it lost less than a fourth as much nitrate in its runoff over the year as either compost type when they were applied as a blanket over bare soil. The greatest sediment loss came from treatments in which compost was tilled into the soil with no additional compost surface blanket. Effects of different treatments were confounded by a gradient in slope hydrology that resulted in a relationship between runoff volume and plot position along the slope. Our statistical analysis took this into account and the data presented represent our attempt to control for this nuisance variable. Our findings suggest that compost can be used as a soil amendment and as an erosion control measure on constructed slopes at a minimal risk of nutrient loss if it is done in the proper manner. We recommend that compost be incorporated into the soil to facilitate greater infiltration and that a blanket of compost or woody mulch be placed over the soil to provide surface erosion protection.