Assessing Progress in Nutrient Management with Large-scale Nutrient Mass Balances: Ten Year Case Study of Delaware, USA.

Nutrient mass balances are used worldwide to guide strategic nutrient management planning efforts for farms, watersheds, states, regions, and countries. We conducted agricultural nutrient mass balance analyses for Delaware for 1996-2006 for nitrogen (N) and phosphorus (P). Our goal was to document how efforts to improve agricultural nutrient management, particularly since passage of the 1999 Delaware Nutrient Management Act, have affected statewide and county-level nutrient mass balances. Our findings indicate changes in nutrient management practices since ~2000 are significantly reducing nutrient surpluses. For example, comparisons of the amounts of N and P available for land application with the amounts recommended for optimum crop yields showed statewide total N and P surpluses for 2004-2006 were reduced by 60% (N) and 52% (P) vs. 1996-1998. Statewide comparisons of fertilizer and manure N and P use with nutrient removal in crop harvest showed reductions in nutrient surpluses of 55% (N) and 97% (P) in 2004-2006 vs. 1996-1998. Major causes for declining nutrient surpluses were: (i) reductions in fertilizer N and P tonnages of 30% for N and 60% for P in 2004-2006 vs. 1996-1998; (ii) decreases in manure N and P, caused mainly by declining livestock inventories and lower N and P contents in broiler litters due to dietary modifications; and (iii) a state manure relocation program that exports excess manure N and P out of Delaware, estimated to be  ~21% of the litter N and 18% of the litter P generated in 2006.  Implications of these changes in nutrient balance for water quality, key environmental problems still remaining, and future directions in statewide nutrient management will be presented.