Evaluation of Biochar and Compost as Soil Amendments for Nutrient and Sediment Runoff Control

An established Best Management Practice (BMP) for stormwater management in Virginia is the tilling of compost into roadside soils to improve soil matrix stability and porosity, encouraging infiltration and reducing runoff. An alternative soil amendment that functions similarly and may also offer additional nutrient-sorbing properties is biochar, the carbon-rich solid component of pyrolyzed organic material. Biochar has been widely studied as a soil amendment in both agricultural and stormwater-management contexts, but its properties can vary considerably depending on many factors, including soil type. This laboratory-scale soil column study compares the hydrologic performance of compost-amended, biochar-amended, and unamended soils during and after synthetic storm events, and assesses the nutrient and sediment content of effluent draining from the soil mixtures. Four differently-textured soils from throughout Virginia were tested. Both soil amendments achieved some enhanced hydrologic function in all four soil types. On average, biochar-amended soils offered the best moisture retention and resistance to compaction, while compost-amended soils demonstrated the fastest infiltration and drainage rates and also leached the least nitrogen. Unamended soils generated the effluent with the lowest phosphorus, and biochar-amended soils also removed a significant percentage of influent phosphorus, while phosphorus removal from compost-amended soils was much less substantial. Considering biochar’s advantage over compost in terms of phosphorus transport in light of specific phosphorus-restricting regulation in Virginia, biochar may offer an opportunity for more appropriate large-scale application as a roadside soil amendment throughout the state.