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

近几十年来，由于化石燃料的燃烧增加，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，这些努力旨在缓解人类对区域和全球碳和氮循环的改变。例如，森林保持氮的方式影响了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。由于氮素往往会限制植物的生长速度，增加氮素投入可能会影响森林的生长和健康。由于空气污染而沉积到森林中的大部分额外氮素保留在土壤中，而不是被植物吸收。这些主要以硝酸盐形式进入的氮被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。这一建议挑战了一种广泛持有的观点，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有利的。有机结合氮一旦进入土壤，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会转化回植物可以利用的无机形态。拟议中的研究将是首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。这些实验使用了在马萨诸塞州中部的哈佛森林和缅因州中部的豪兰森林进行的两个正在进行的氮素添加实验。第一个目标是测量这些在实验中接受了氮素添加的森林土壤中硝酸盐的反应。PI试图确定氮素添加是否改变了土壤与硝酸盐反应的能力，无论是生物上的还是非生物的。PI在这项建议中还提出了“铁轮假说”，即在通风不良的土壤中，还原形式的铁可以将硝酸盐还原为另一种形式的氮--亚硝酸盐，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验证这一假设，在这些实验中，假设的反应物的浓度和组合在实验室仪器中系统地变化，该仪器被称为“氧化还原-pH-稳态反应器”，该仪器在培养期间控制土壤样品的酸度和曝气量。由于其跨学科性质，这项研究需要一名生态学家(Davidson)、一名化学家(Chorover)和一名微生物学家(Dail)的共同努力，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.