Effects of Slope Ratio, Straw Mulching, and Compost Amendment on Vegetation Establishment and Runoff Generation

Abstract

Soil erosion management is a major environmental challenge facing highway construction. This study was undertaken to evaluate the effectiveness of compost use in lieu of topsoil for final grade turfgrass establishment on highway slopes. Two compost types, biosolids and greenwaste, and four compost/topsoil blends were compared with a topsoil standard (TS; with straw and fertilizer application) in their ability to reduce soil and nutrient loss and improve vegetation establishment. A series of greenhouse studies and field tests were conducted to analyze the effects of slope ratio, straw mulching, and compost mixing ratio on runoff by observing green vegetation (GV) establishment, runoff volume generation, and nutrient and sediment export. GV was measured using an innovative image segmentation and classification algorithm coupled with machine learning approaches with varying block size and classification acceptance thresholds. Algorithm classifications were compared to manual coverage classifications with R-squared values of 0.86 for GV, 0.87 for straw/dormant vegetation, and 0.96 for exposed soil, respectively. Straw mulching (≥95% straw cover) reduced evaporation rates and soil sealing and increased soil roughness and field capacity (FC), which significantly reduced volume runoff (34-99%) and mass export of sediment and nutrients (81-91%). With mulching, no statistical differences were found in GV establishment among the compost and TS treatments (≥95% cover in 60 days) while non-mulched media cover reached a maximum of 35%, due to limited moisture availability. Composted material (excluding 2:1 compost: topsoil mixtures) had higher hydraulic conductivity, FC, and shear strength than TS which, combined with straw mulching, reduced total runoff volume by 33-72%. This led to sediment and nutrient mass reductions of 57-97% and 6-82%, respectively, from standard TS. A general increase in runoff generation and decrease in GV was seen with slope ratio increase (41-96% more nutrient and sediment export and 81-97% lower GV from 20:1 to 2:1 slopes). However, benefits displayed at 25% slope were reduced at shallower slopes and enhanced at greater slopes. The use of compost as an additive or replacement to TS, with straw mulching, was seen to reduce runoff generation and improve runoff quality from the TS standard and is suggested as possible alternatives.