Date of Award

8-2005

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Ecology and Environmental Sciences

Advisor

David Bryan Dail

Second Committee Member

Mary Susan Erich

Third Committee Member

Aria Amirbahman

Abstract

Anthropogenic activities have caused an increase in levels of atmospherically derived ammonium (NH4+) and nitrate (NO3-) entering terrestrial ecosystems. Where this excess inorganic nitrogen (N) has reached N-limited northeastern forests as wet and dry N deposition, its ultimate fate has been immobilization in the soil rather than stimulation of plant biomass production. In experimental forests such as the Harvard Forest in Petersham, Massachusetts, researchers have artificially amended pine and hardwood plots with low (50 kg N/ha/yr) and high (150 kg N/ha/yr) N inputs for almost two decades to study the retention threshold and biological consequences in a system pushed toward "N saturation". Observations at Harvard Forest and other sites suggest that abiotic immobilization of N may be most responsible for the unexpectedly large rates of soil N retention (over 70% of total inputs), challenging a widely held view that microbial processes are the dominant pathways for N immobilization in soil. Observed reduction of labeled nitrate within minutes of addition to sterilized organic horizon soils has led to a possible mechanistic explanation for abiotic NO3- immobilization called the "ferrous wheel hypothesis". The ferrous wheel hypothesis promotes the possibility that ferrous iron reduces NO3- to nitrite (NO2-), which then reacts with dissolved organic matter producing DON (dissolves organic nitrogen). This DON may then become immobilized through interaction with soil constituents such as organic matter and clays. In my experiments, soil samples were taken from the organic and mineral horizons of both N-amended and control plots at the Harvard Forest. Replicate samples of soils from each N-amendment level, horizon, and forest stand type were then autoclaved, while an identical set remained microbially intact or "live". Both live and autoclaved soils were amended with labeled 15NO3- tracer and sub sampled to determine fractionation of 15N contained into NO3-, NH4+, DON and insoluble organic nitrogen pools at both t = 0 (immediately after addition) and 24 hours later (t = 24). Rates of NO3- immobilization had yet to be tested in soils where N inputs were thought to be in excess of plant and microbial demand, and I hypothesized that chronic N inputs such as those at the Harvard Forest could have depleted soil constituents thought necessary for rapid DON formation and thus decreased the soil's capacity to rapidly immobilize 15NO3- inputs via this mechanism. Consistent with my prediction, I observed a trend showing rapid DON formation to be greatest in the unamended control plots followed by those at the low-N and finally high-N level of amendment. This pattern was observed in both 0 horizon and mineral horizon soils. Amended pine plantation soils, which had reached N saturation years before the amended hardwood soils, showed a trend of decreased DON formation in both organic and mineral horizons when compared to hardwood soils. Organic horizon soils tended to have greater DON formation potential from added 15NO3- than underlying mineral soils. This was perhaps owing to greater amounts of some physical moiety of organic matter present in this horizon which is thought to be key to providing the DOC necessary as a ferrous wheel component. Finally, rapid DOIV formation was shown again to occur in autoclave sterilized soils, lending support to the theory that rapid N immobilization in forest soils does not require the actions of soil microorganisms.

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