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

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