Date of Award

Spring 1999

Project Type

Dissertation

Program or Major

Natural Resources

Degree Name

Doctor of Philosophy

First Advisor

John D Aber

Abstract

Theories of nitrogen retention suggest that N cycling and loss should increase with ecosystem successional age and with chronic N deposition over time (N saturation). These factors both affect northeastern U.S. forests, most of which receive elevated rates of N deposition and have experienced past disturbances by wind, logging, fire, or agriculture. This work examined the long-term (80--110 year) effects of land-use history on nitrogen cycling and loss in the White Mountains, New Hampshire. Historical land-use maps were used to identify a network of watersheds and plots containing burned, logged, or old-growth forests. Nitrate-N fluxes from old-growth watersheds exceeded those from historically disturbed watersheds, yet losses from all forested watersheds were low ( ≤ 2.1 kg ha-1 yr-1). Land-use history did not affect DON losses, which comprised 28--87% of total N fluxes, and increased with losses of dissolved organic carbon and with conifer forest cover. Old-growth stands had lower soil C:N ratios and twice the nitrification rates of historically disturbed stands. Nitrification increased as soil C:N ratio decreased, and stream nitrate concentrations increased with soil nitrification. Theories of N retention were further tested by remeasuring 28 streams sampled 23 years previously. Mean nitrate concentrations declined by 68%; calcium, by 28%; magnesium, by 26%; and sulfate, by at least 22%. Nitrate concentrations declined in all streams, but old-growth watersheds had higher nitrate concentrations than successional watersheds in both years. Sulfate and base cation deposition have decreased since the 1970s but N deposition has not. Climate variability and its effects on biotic N retention may be responsible for the low nitrate losses observed in all stream, overriding expected increases due to chronic N deposition or forest aging. Century-old disturbances influenced spatial patterns of C:N ratio and nitrate production and loss, but climate may control temporal patterns of nitrate loss on the scale of months to decades. If the current, low losses of N are due to a high capacity to absorb N, forest ecosystems may continue to take up N for decades to centuries before reaching late-stage N saturation; if due to interannual climate variability, large losses of nitrate may occur much sooner.

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