Influence of catchment suburbanization on leaf litter decomposition and microbial activity in four streams of southeastern New Hampshire

Rapid suburbanization has caused major land use transformation in New Hampshire, and contributed significantly to nitrogen enrichment in aquatic ecosystems. However, we do not understand how increased nitrogen concentration in lotic environments affects key ecosystem-level processes such as decomposition.  In an ongoing study, we are measuring the rate of in-stream leaf litter decomposition in four basins along a suburbanization gradient located in the Lamprey River watershed, southeastern New Hampshire. Basins vary in population density from 24 to 300 people km^-2 and in stream nitrate concentrations from 0.02 to 1.7 mg L^-1 as N.  A fine-mesh litter bag technique was used to quantify decomposition rate of litter from senesced maple and oak leaves, pine needles, and grass clippings.  We are also measuring microbial respiration, fungal and bacterial biomass, and substrate nitrogen and carbon content of the litter.  After 126 days of in-stream incubation, decomposition rates (k) ranged from 0.0011 to 0.0118 day^-1 across the different sites and litter species.  Preliminary analysis shows that species is a more important factor in determining decomposition rate than site (p=0.001 for species effect and p>0.05 for site).  Specifically, maple had the highest k (0.0108 ± 0.0022), and pine had the lowest (0.0014 ± 0.0001; p<0.05 by Tukey’s post-hoc comparison).  Oak and grass were similar and intermediate between pine and grass (0.0054 ± 0.0008 and 0.0052 ±0.0007 respectively).  These results suggest that substrate quality, rather than stream nitrate concentration, controls the rates of decomposition.