Effect of a whole-catchment N addition on stream detritus processing

The Bear Brook Watershed in Maine (BBWM) is a paired catchment study investigating ecosystem effects of N and S deposition. Because of the decade long (NH4)(2)SO4 addition, the treatment catchment has higher stream NO3- and enriched foliar N concentrations compared to the reference catchment. We investigated how both stream N and foliar N affect stream detritus processing. Differences in litter processing were assessed by measuring mass loss, tissue softness, and shredder biomass. To examine both stream effects and leaf source, N-enriched and reference litter bags were prepared for 3 leaf species and placed in each stream. Red maple leaves were examined in 1997, 1998, and 1999. Sugar maple and American beech were examined in 1999. In all years, the only stream effects were increased mass loss for sugar maple and higher shredder biomass for red maple in 1998 in the treatment stream. Several leaf source effects were observed. N-enriched leaves of sugar maple, American beech, and red maple in 1998 had significantly higher microbial activity, as indicated by softer tissue, and had higher total mass loss. Further, shredder biomass tended to be highest in N-enriched litter bags. Although significant effects were detected, our results suggest that elevated dissolved N concentrations as a result of N deposition play a minimal role in regulating stream detritus processing at BBWM. Increased foliar N, however, did influence rates of stream detritus processing by increasing microbial activity, and possibly increasing shredder biomass. Our study shows how N deposited on entire catchments can affect litter processing in stream ecosystems.