Spatial optimization of nutrient recovery from dairy farms to support economically viable load reductions in the Chesapeake Bay Watershed

CONTEXT: To promote circularity in agricultural systems, the utilization of aquatic vegetation for ecological wastewater treatment is a potential mechanism to capture and upcycle nutrients. Agricultural wastewater is an excellent growing medium for aquatic plants like duckweed, offering opportunities for wastewater treatment and conversion of harvested biomass into bio-based products, including protein-rich livestock feed, which can potentially replace conventional soil-based crops such as alfalfa.OBJECTIVE: We hypothesize that nitrogen (N) and phosphorus (P) loadings to the Chesapeake Bay Watershed (CBW) can be reduced via replacing alfalfa cultivation with manure-grown duckweed by: a) reducing excess manure application on agricultural fields; b) reducing synthetic fertilizer application on alfalfa croplands; and c) decreasing the release of fixed N back into the environment from the decomposition of alfalfa crop residue.METHODS: This study developed an optimization framework to identify locations where alfalfa-to-duckweed replacement could be theoretically employed to minimize N and P loads into the CBW. A relative effective-ness (RE) indicator representing landscape-specific nutrient delivery capacity was included within the frame-work. Using county-level data on alfalfa yields, cropping area, and nutrient inputs from alfalfa croplands and dairy manure, we identified alfalfa cultivation areas that could be removed and replaced with full or partial duckweed cultivation and land conservation for optimal benefits.