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

近几十年来，由于化石燃料燃烧的增加，大气中氮在温带森林中的沉积增加。 了解空气污染中的氮是如何保留在森林中的，将提高科学家预测各种努力之间相互作用的能力，以减轻人类对区域和全球碳和氮循环的改变。 例如，森林保留氮的方式影响森林帮助缓解由于大气中碳气体增加而预期的全球变暖的能力。 由于氮往往限制植物生长的速度，增加氮的投入可能会影响森林的生长和健康。 大部分因空气污染而沉积在森林中的额外氮被保留在土壤中，而不是被植物吸收。 这种主要以硝酸盐形式进入的氮转化为留在土壤中的有机结合形式的生物和化学过程仍然未知。 最近的证据表明，非生物反应（没有生物体参与的化学反应）将硝酸盐转化为土壤中的有机氮。 这一建议挑战了一个广泛持有的观点，即生活在土壤中的微生物是土壤中氮素吸收的主导因素。 硝酸盐的这种非生物反应特别令人困惑，因为在排水良好的土壤的正常条件下，土壤中硝酸盐还原的能量学是不利的。 有机结合态氮进入土壤后的命运也很大程度上是未知的。 例如，目前还不知道有多少有机氮最终转化为植物可以使用的无机形式。拟议的研究将是第一批调查溶解有机氮命运的研究之一，溶解有机氮越来越被认为是许多森林氮循环的核心。 实验使用两个正在进行的氮添加实验在马萨诸塞州中部的哈佛森林和缅因州中部的豪兰森林。第一个目标是测量这些森林土壤中的硝酸盐的反应，已收到氮添加实验。 PI的目的是确定氮添加是否改变了土壤与硝酸盐反应的能力，无论是生物的还是非生物的。 PI还提出了“铁轮假说”，即在通气不良的土壤中，铁的还原形式可以将硝酸盐还原为另一种形式的氮，亚硝酸盐，然后可以与土壤中溶解的有机碳反应，形成溶解的有机氮。 第二个目标是在几个实验室实验中测试这一假设，其中假设的反应物的浓度和组合在实验室仪器中系统地变化，称为“氧化还原-pH-stat反应器”，其控制培养期间土壤样品的酸度和通气性。 由于其跨学科的性质，这项研究需要一位生态学家（戴维森），一位化学家（乔罗弗）和一位微生物学家（戴尔）的共同努力，他们都专门从事土壤研究。

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.