Collaborative Research: Mechanisms of Abiotic Immobilization of Nitrate in Temperate Forest Soils

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Atmospheric deposition of nitrogen to temperate forests has increased in recent decades as a result of increased combustion of fossil fuels. Knowing how nitrogen in air pollution is retained within forests will improve the ability of scientists to anticipate interactions among efforts to mitigate human alterations of regional and global cycles of carbon and nitrogen. For example, the way that forests retain nitrogen affects the ability of forests to help mitigate expected global warming due to increased carbon gases in the atmosphere. Because nitrogen often limits rates of plant growth, increased nitrogen inputs could affect forest growth and health. Most of the additional nitrogen deposited onto the forest from air pollution is retained in the soil rather than being taken up by plants. The biological and chemical processes whereby this incoming nitrogen, dominantly in the form of nitrate, is converted to organically-bound forms that stay in the soil remains unknown. Recent evidence suggests that abiotic reactions (chemical reactions without participation of living organisms) convert nitrate into organic nitrogen in soil. This suggestion challenges a widely held view that microorganisms living in the soils are the dominant agents for nitrogen uptake in soil. This abiotic reaction of nitrate is particularly perplexing because the energetics of nitrate reduction in soils are not favorable under normal conditions of well-drained soils. The fate of the organically bound nitrogen once it gets into the soils is also largely unknown. For example, it is not known how much organic nitrogen is eventually converted back into an inorganic form that plants can use. The proposed studies will be among the first to investigate the fate of dissolved organic nitrogen, which is increasingly recognized as central to the nitrogen cycle of many forests. The experiments use two ongoing nitrogen addition experiments in the Harvard Forest of central Massachusetts and the Howland Forest of central Maine. The first objective is to measure reactions of nitrate in these forest soils that have received nitrogen additions experimentally. The PI's seek to determine whether nitrogen addition has changed the capacity of soils to react with nitrate, either biotically or abiotically. The PI's also present in this proposal the "ferrous wheel hypothesis," that reduced forms of iron in tiny pockets of poorly aerated soil can reduce nitrate to another form of nitrogen, nitrite, which can then react with dissolved organic carbon in the soils to form dissolved organic nitrogen. The second objective is to test this hypothesis in several laboratory experiments, where the concentrations and combinations of hypothesized reactants are varied systematically in a laboratory instrument, called a "redox-pH-stat reactor," which controls the acidity and aeration of the soil sample during the incubation. As a result of its interdisciplinary nature, the research requires the collaborative efforts of an ecologist (Davidson), a chemist (Chorover) and a microbiologist (Dail), all of whom specialize in the study of soils.