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，大气中向温带森林沉积的氮有所增加。了解空气污染中的氮是如何被保留在森林中的，将提高科学家预测减缓人类对区域和全球碳氮循环的改变的努力之间的相互作用的能力。例如，森林保持氮的方式会影响森林帮助缓解由于大气中碳气体增加而导致的预期全球变暖的能力。由于氮常常限制植物的生长速度，增加氮的投入可能影响森林的生长和健康。由于空气污染而沉积到森林中的额外氮大部分被保留在土壤中，而不是被植物吸收。这些主要以硝酸盐形式进入土壤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尚不清楚。最近的证据表明，非生物反应（没有生物体参与的化学反应）将土壤中的硝酸盐转化为有机氮。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吸收的主要媒介。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。有机结合的氮一旦进入土壤，其命运在很大程度上也是未知的。例如，目前尚不清楚有多少有机氮最终转化为植物可以利用的无机形式。拟议的研究将是首批调查溶解有机氮命运的研究之一，人们越来越认识到溶解有机氮对许多森林的氮循环至关重要。实验使用了两个正在进行的氮添加实验，分别在马萨诸塞州中部的哈佛森林和缅因州中部的霍兰德森林进行。第一个目标是测量这些森林土壤中硝酸盐的反应，这些土壤在实验中接受了氮的添加。PI试图确定氮的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还在这个提议中提出了“铁轮假说”，即在通风不良的土壤的小口袋中，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，然后可以与土壤中的溶解有机碳反应，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，其中假设反应物的浓度和组合在一个称为“氧化还原- ph -stat反应器”的实验室仪器中系统地变化，该仪器在孵育期间控制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（Davidson）、一位化学家（Chorover）和一位微生物学家（Dail）的合作努力，他们都专门研究土壤。

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.