Mycorrhizal Regulation of Ecosystem Response to Chronic Nitrogen Deposition

University Of Illinois At Chicago, Chicago IL

Investigators

Abstract

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.