Sorbent-amended compost filter socks in grassed waterways reduce nutrient losses in surface runoff from corn fields

Surface runoff from row-crop fields frequently has high concentrations of sediment, nutrients, and pesticides, particularly in the first few events after tillage and agrochemical application. Compost filter socks placed in grassed waterways can further reduce sediment concentration as runoff is transmitted offsite but are generally ineffective in removing dissolved chemicals. Therefore, we investigated the effect of adding a proprietary sorbent, Nutriloxx, to filter socks filled with composted bark and wood chips on sediment, nutrient, and glyphosate concentrations in runoff. Surface runoff from one tilled and one no-till watershed planted to corn (Zea mays L.) was routed into two parallel, 30 m (99 ft) long, grassed waterways. Three, 46 cm (18 in) diameter filter socks filled with Nutriloxx-amended compost were placed 5 m (16.5 ft) apart across the upper half of one waterway and in the lower half of the paired waterway. Automated samplers were used to obtain samples above and below the treated waterway segments in the 2009 and 2010 crop years. The effectiveness of the grassed waterways and filter socks was highly dependent on tillage treatment and timing and size of the runoff events. In 2009, there were no sizable events during the early growing season. Consequently, erosion was minimal, and no significant effects on sediment concentration were detected. Averaged for both watersheds, however, the amended filter socks contributed to an additional 28% reduction in dissolved phosphate-phosphorus (PO4-P) concentration compared to waterway segments without filter socks (significant at p = 0.05). The filter socks, however, significantly increased sulfate (SO4) concentrations up to 20-fold in the first sampled event, but SO4 concentrations declined rapidly with subsequent events. Similarly, the filter socks increased concentrations of calcium (Ca), potassium (K), and sodium (Na), but this was not significant in all instances. In 2010, runoff-producing rainfall occurred frequently during the growing season, and the filter socks significantly decreased sediment and PO4-P concentrations from the tilled watershed. In addition, large reductions in ammonium-nitrogen (NH4-N) concentrations were noted (average > 7-fold), but field observations suggested that this was due to physical trapping of eroded coated-urea fertilizer prills rather than sorption. The filter socks continued to contribute to significantly increased SO4 concentrations from both watersheds. Filter socks can effectively reduce sediment losses when used in agricultural applications, and adding selective sorbents can increase their ability to retain nutrients. However, losses of sorbent components need to be considered.