Nitrogen and sulfur input-output budgets in the experimental and referencewatersheds, Bear Brook Watershed in Maine (BBWM)

Dry (NH4)(2)SO4 (1,800 eq(-1) ha(-1) yr(-1)) has been applied to the wester n of two contiguous 10 ha catchments at the Bear Brook Watershed in Maine ( BBWM) since November, 1989. The initial rapid and significant response in b oth S and N in West Bear, compared to the reference East Bear, slowed after three years. Annual S retention of the total experimental treatment decrea sed from 86 to 34%, with a seven year cumulative retention of 59%. Hydrolog y influences the export flux of S; S is retained more in dry seasons and dr y years. The annual retention of N has decreased from 96 to 81%, with a cum ulative retention of 82%. The export of N from the reference watershed has declined from 178 to 23 eq(-1) ha(-1) yr(-1) during the treatment period. T he treatment N (as NH4) initially stimulated nitrification, and caused pre- existing N to be lost in runoff rather than the treatment N. Retention of t he treatment N has decreased to approximately 80%. The majority of the reta ined N is stored in the soil, but the reasons for the decreased flux from t he reference watershed are not known.