Date of Award

Winter 2007

Project Type

Thesis

Program or Major

Natural Resources

Degree Name

Master of Science

First Advisor

William H McDowell

Abstract

Rapid suburbanization has caused major land use transformation in New Hampshire, and contributed significantly to nitrogen (N) 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 this study, I measured the rate of in-stream leaf litter decomposition in four basins along a suburbanization gradient located in the Lamprey and Oyster River watersheds in southeastern New Hampshire. Basins varied in population density from 24 to 289 people km-2, and in-stream nitrate concentrations from 0.02 to 1.0 mg L-1 as N. A fine-mesh litter bag technique was used to quantify mass loss, substrate nitrogen and carbon content, fungal and bacterial biomass, and microbial respiration for senesced maple and oak leaves, pine needles, and grass clippings. Rates of decay (k) ranged from 0.0007 to 0.0173 day-1 across the different sites and litter species. Repeated measures analysis showed a significant interaction effect of litter species and time on decomposition. Specifically, maple litter had the highest rates of decay, oak and grass litter had similar and intermediate rates, and pine litter had the slowest rates. The effect of site was not significant on the decay of oak, grass and pine, while maple litter decomposed slower at the more suburban streams. Nitrogen content over time varied by species, but not by site, with N concentrations increasing in maple, oak and pine litter, and decreasing in grass litter. Changes in N mass over time differed by species, with pine gaining N, suggesting N immobilization, and maple, oak and grass loosing N over time, suggesting N mineralization. The effect of site was not significant on N mass dynamics of oak, pine and grass litter, while maple litter at the most suburban stream showed only nominal losses of N mass over time. Microbial biomass and respiration also varied by litter species, while site had no significant effect. The results of this study show that litter decay rates are not accelerated by nitrate enrichment associated with watershed suburbanization. This similarity of processing rates among the four streams suggest that either the ambient streamwater nitrate concentrations at all sites examined were sufficient to fully alleviate substrate-induced N limitation of litter decay, or that other physical or chemical characteristics at the more suburban sites negatively affected decay and masked the potentially stimulating effect of elevated streamwater nitrate enrichment.

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