The current conceptual model for the nitrogen (N) cycle of northeastern US forests stresses the importance of inorganic N forms (i.e., ammonium and nitrate) for tree growth and forest productivity. However, organic forms of N are also be taken up by plants. The overall objective of this proposal is to quantify the importance of organic N as a nutrient source in temperate forest ecosystems of the northeastern U.S. To achieve this objective, the factors that affect the production and turnover of organic N in the soil will be studied, focusing specifically on amino acids and amino sugars. The balance between the production and turnover of organic N will be assessed to determine the types and forms of N taken up by dominant tree species. These objectives will be met by establishing plants in single-species stands on two types of soil parent materials in western Connecticut and Massachusetts. Field and laboratory studies will be used to measure the pools, fluxes and uptake of organic N in the soil. The availability of N in the soil limits the productivity of forest ecosystems and their capacity to sequester rising concentrations of carbon dioxide, a greenhouse gas. By understanding the factors that affect the form and the quantity of N taken up by forest trees, this proposal will shed light on the processes regulating the availability of a key growth limiting soil resource and the degree to which forest ecosystems can be relied upon to absorb CO2 from the earth's atmosphere.
The availability of nitrogent (N) from the soil controls the productivity of forests across the United States. The productivity of forests is very important to human welfare and the economy. By absorbing carbon dioxide (CO2) from the atmosphere, trees help slow the rate at which fossil fuel emissions of CO2 accumulated and therefore help slow climate change. In this project we studied the factors affecting the production and turnover or a particular form of N in the soil called organic N. It is referred to as this because there are carbon atoms attached to the N as opposed to inorganic forms of N like ammonium and nitrate, which is typically used as fertilizer on lawns and agricultural fields. We made many important discoveries. First, we used sophisticated molecular tools to show that common tree species throughout the eastern US could take up organic forms of N from the soil directly without their conversion to ammonium or nitrate. This was the first ever demonstration of this outcome for eastern species. Second, we found that fungi growing in soils that form symbiotic relationships with trees and their roots were critical in determining how much organic N was taken up from the soil. This is important because the different types of symbiotic fungi take up more or less organic N. In particular, we found that organic N uptake was greatest in trees with associate with ectomycorrhizal fungi. Such fungi colonize the roots of tree species like hemlock, oak, birch and beech. Third we found that increasing soil temperature could accelerate the formation of organic N in the soil, but that the biggest increases in organic N production occurred depended on the activity of the soil microbes themselves. This means that climate warming could increase the rate of tree growth, but that the magnitude of this effect depends upon changes in the activity of soil microorganisms, which include fungi but also bacteria and other prokaryotic organisms. Lastly, we collected soil samples from climate-change studies that experimentally manipulated temperature and rainfall. The objective of this research was to determine whether climate change could in fact increase the growth rates of vegetation from many different biomes (e.g., arctic tundra, confer forests, hardwood forests, grasslands, deserts). Very importantly, we found that the controls over organic N production were similar across the country. In particular, we found increases in organic N production when changes in climate simultaneously increased the temperature and moisture content of soils, whereas changes in climate that warmed and dried soils caused moderate-to-large decreases in the production of organic N. This means that the C-storage potential of vegetation and their associated soils is highly dependent depending upon how the climate changes. Simply stated, we cannot assume that plants will grow more rapidly with warmer temperatures and rising levels of CO2. There are other factors that also control this response.
