Nitrogen Cycling Under Increased N Loads in Two Forested Ecosystems

Abstract

The coupled role of the terrestrial nitrogen (N) and carbon (C) cycles is of increasing importance, particularly in light of global climate change. Forested ecosystems contain over half of the terrestrial C pool and currently absorb as much as 30% of anthropogenic CO2 emissions. Many forests, particularly Douglas-fir forests in the Pacific Northwest, are growth limited with respect to N. Fertilizers are therefore applied in commercially managed forests to increase productivity and timber yields. However, high N inputs can saturate the biological demand for N and lead to increased rates of N loss through leaching and gas loss via denitrification and volatilization. To improve N use efficiency one approach is the application of Enhanced Efficiency Fertilizers (EEFs) designed to limit N losses to volatilization. In this dissertation, the recovery rates of three different EEFs including Environmentally Smart Nitrogen (ESN), N-(n-butyl) (NBPT), and Arborite coated urea fertilizer (CUF), are compared to the recovery rates of an unformulated urea fertilizer. Using 15N enriched fertilizer I determined both short term N losses (0-4 weeks after fertilization) and how the applied N is partitioned between ecosystem pools one year after fertilization. After four weeks, a mean of 60% of the applied N was recovered in the soil and forest floor. Retention rates were not significantly improved with the use of EEFs. After 1-year, N recovery was low across all sites and fertilizer types with a mean of 39.0%. The remaining N could not be accounted for in the pools we sampled. The largest fertilizer pool was the top 20cm of mineral soil. Again, after 1-year, no differences were found between EEFs and the standard urea fertilizer. Monitoring of nitrogen (N) inputs to forested ecosystems in China has historically been limited despite known negative effects that high N additions can have on environmental and human health. To better understand terrestrial patterns of foliar, soil, and wood δ15N relative to soil N concentrations (%N) in a heavily polluted region, two Chinese parasol tree (Firmiana simplex) groves adjacent to a industrialized basin were examined the Loess Plateau, approximately 80km east of the city of Xi’an, Shaanxi Province, China. The δ15N values of ecosystem pools sampled in this study are substantially lower than what is typically observed in temperate forests around the world. Results suggest greater foliar uptake of N in the tree canopies bordering the industrialized basin compared to trees located at the forest interior. N concentrations and δ15N of soil and plant tissue were inversely correlated along this transect and indicate higher inputs of N from δ15N depleted sources in the basin. Tree ring δ15N values declined between 2003 and 2014 during a time of rapid development and expansion of industry in the basin. The projects in the Pacific Northwest and on the Loess Plateau in China are two separate studies. The differences in experimental design and objectives of the two studies does not facilitate comparisons between them.