Mycorrhizal Regulation of Ecosystem Response to Chronic Nitrogen Deposition

从大气中沉积的易用形态的氮的数量远远超过植物所能利用的数量。这主要是城市地区燃烧化石燃料产生的硝态氮，但一些农业区容易出现氨氮的高沉积。大气氮沉降的影响首先在荷兰的斯堪的纳维亚半岛和美国的工业东北部地区被注意到，在那里它与树木，特别是针叶树的死亡有关。这些树表现出特有的反应，包括针叶含氮量高和细根丢失。目前，高水平的大气氮沉积正在向美国其他地区扩散，包括洛杉矶盆地和科罗拉多前线山脉。在多年的氮素添加后，许多森林显示出达到了不能再吸收氮素、树木死亡增加、硝酸盐氮渗入地下水位的地步。最初，植物被认为吸收和储存森林中大部分沉积的氮。但使用氮同位素作为示踪剂的研究表明，氮储存在森林地面中。这将注意力集中在两类占据森林地面的有益真菌上：外生菌根真菌和内生菌根真菌。这两类植物都是植物的补充根系，外生菌根真菌可以在覆盖针叶树和橡树根尖的真菌组织的鞘中储存沉积的氮。这个项目提出了三种问题，以了解外生菌根真菌在氮素储存和树木死亡中的作用。首先，外生和内生菌根真菌的种类数量和相对丰度是否受到长期大气氮沉降的影响？第二，外生菌根真菌和内生菌根真菌的代谢活性是否受到大气氮沉降的影响？外生菌根真菌储存氮素的能力是否随着细根数量的减少而降低？第三，大气氮沉降对外生菌根生长和死亡的影响是否与内生菌根树相同？为了回答这些问题，研究人员将在一项为期五年的研究中，量化芝加哥大都市区的菌根和植物对氮沉积梯度的反应。芝加哥大都市区适合提出这些问题，因为在类似的土壤和植被上，慢性氮沉积沿着从西向东的梯度增加了24%。在每个地点，研究人员将使用红栎树(Quercus rubra，外生菌根)和糖枫(Acer Sucharum，内菌根)来测试氮素是否改变(A)内外菌根真菌的种类和丰度，(B)外生菌根和内生菌根真菌的代谢活动和氮素储存能力，(C)外生菌根和内菌根吸收氨和硝酸盐的比速率，(D)在17至42 kg ha-1和两次环境氮沉积速率为17至42 kg ha-1a-1的环境和两次环境氮沉积速率下，红橡树和糖枫幼苗下土壤的存活、生长和N淋失。

Readily usable forms of nitrogen are being deposited from the atmosphere in much greater quantities than plants can use. This is mainly nitrate nitrogen produced by the combustion of fossil fuels in urban areas, but some agricultural areas are subject to high deposition of ammonium nitrogen. Effects of atmospheric nitrogen deposition were first noticed in Scandinavia, the Netherlands, and, in the U.S., in the industrial Northeast where it is associated with the death of trees, particularly conifers. These trees showed characteristic responses which included high nitrogen in their needles and loss of fine roots. Currently high levels of atmospheric nitrogen deposition are spreading to other parts of the U.S. including the Los Angeles Basin and the Colorado Front Range. After many years of nitrogen additions many forests show evidence of reaching a point where no further nitrogen can be taken up, tree mortality increases, and nitrate nitrogen leaches into the water table. Initially, plants were thought to take up and store most depositional nitrogen in forests. But studies using nitrogen isotopes as tracers suggested that the nitrogen was stored in the forest floor. This focused attention on two groups of beneficial fungi that occupy forest floors: ecto- and endomycorrhizal fungi. Both groups function as supplementary root systems for plants, and ectomycorrhizal fungi may store depositional nitrogen in sheaths of fungal tissue covering root tips of conifers and oaks. This project asks three kinds of questions in order to learn the role of ectomycorrhizal fungi in nitrogen storage and tree mortality. First, are species numbers and relative abundance of ecto- and endomycorrhizal fungi affected by chronic atmospheric nitrogen deposition? Second, is the metabolic activity of ecto- and endomycorrhizal fungi affected by atmospheric N deposition? Is the ability of ectomycorrhizal fungi to store N decreased when numbers of fine roots decline? Third, does atmospheric nitrogen deposition have the same effect on growth and mortality of ectomycorrhizal as endomycorrhizal trees? To answer these questions the investigators will quantify mycorrhizal and plant responses across the nitrogen deposition gradient in the Chicago metropolitan area in a five-year study. The Chicago metropolitan area is suitable for asking these questions because chronic nitrogen deposition increases 24% along a west to east gradient on similar soils and vegetation. At each site the investigators will use red oak (Quercus rubra, ectomycorrhizal) and sugar maple (Acer saccharum, endomycorrhizal) to test whether nitrogen alters (a) the species and abundance of ecto- and endomycorrhizal fungi, (b) the metabolic activity and nitrogen storage ability of ecto- and endomycorrhizal fungi, (c) the specific rates of ammonium and nitrate uptake by ecto- and endomycorrhizal roots, (d) survivorship, growth, and N leaching from soil under red oak and sugar maple seedlings at ambient and twice-ambient nitrogen deposition rates ranging from 17 to 42 kg ha-1 year-1.