A watershed nitrogen and phosphorus balance: The upper Potomac River basin

Nitrogen and phosphorus mass balances were estimated for the portion of the Potomac River basin watershed located above Washington, D.C. The total nitrogen (N) balance included seven input tource terms, six sinks, and one “change-in-storage” term, but was simplified to five input terms and three output terms. The phosphorus (P) balance had four input and three output terms. The estimated balances are based on watershed data from seven information sources. Major sources of nitrogen are amimal waste and atmospheric deposition. The major sources of phosphorus are animal waste and fertilizer. The major sink, for nitrogen is combined denitrification, volatilization, and change-in-storage. The major sink for phosphorus is change-in-storage. River exports of N and P were 17% and 8%, respectively, of the total N and P inputs. Over 60% of the N and P were volatilized or stored. The major input and output terms on the budget are estimated from direct measurements, but the change-in-storage term is calculated by difference. The factors regulating retention and storage processes are discussed and research needs are identified.