While some substances, such as heavy metals, pesti­cides, and hormones, are not desirable in soil amend­ments, other substances, such as N and P, are regarded as pollutants in surface waters, yet are essential plant nutrients within agroecosystems. If biochar is used to fil­ter N and P from surface waters before in is incorporated into agricultural soil (Figure 25.3(b)), the process could reclaim a portion of the nutrients that are lost from agro­ecosystems to surface waters, and benefit both crop yield and surface water quality. If effective designer bio­chars are created for the purpose of N and P retention, it is feasible that those biochars, once saturated with N and P, could supply a crop area of some size with an adequate supply of runoff-derived nutrients to replace N and P fertilizer inputs entirely, and becoming effec­tively nutrient-neutral. Furthermore, it is feasible that if an analogous relocation of nutrients is performed where the deposition site is an area that would other­wise not receive fertilizer, and not be a likely source of runoff, such as a well-managed silvicultural system, then a nutrient-negative system could be created (Figure 25.4).

Runoff and water-induced erosion occur when pre­cipitation exceeds the infiltration capacity of a soil, and
gravity carries it downhill over the soil surface into a drainage ditch or natural waterway. Common practices employed to reduce erosion caused by runoff and retain eroded soil particles by slowing the velocity of water flow within drainage ditches are well established. These practices include lining the sides of channels with either large angular rocks or rectangular wire mesh containers filled with smaller rocks, lining the bottom of the chan­nel with grass sod, various bioengineering techniques involving the establishment of trees along the channel edge, and fixing straw bales in order to intercept eroded soil particles (Brady and Weil, 2008). Biochar contained within either reusable synthetic or single use biodegrad­able mesh containers could be used to simultaneously slow and filter overland flow (Figure 25.5). This may be complementary or even preferable to the aforemen­tioned runoff and erosion control methods due to the added benefits of nutrient-saturated biochar application to soil (Figure 25.6). In areas where runoff currently flows directly into natural waterways, an enclosed bio­char overland flow filter (Figure 25.7) fitted with mesh containers of biochar may be an effective option for reducing nutrient losses to surface water, allowing those nutrients to be relocated to soil and ultimately into plant biomass. Similarly, tile drain effluent may be filtered us­ing mesh containers of biochar. Like the biochar over­land flow filter discussed above, end caps on the tile drain with drain holes only in the upper half will in­crease the residence time of nutrient-laden water with the biochar and may increase filtration efficiency (Figure 25.8).