Nitrogen (N) is essential for ecosystem productivity, yet frequently constrains ecosystem primary production. Previously, the majority of research on N cycling has focused on inorganic N biogeochemistry. More recently, research investigating dissolved organic N (DON) has found it also plays a significant role in N biogeochemistry, both as a vector for N loss from terrestrial ecosystems and as a source of plant-available N, which suggests DON is an important component of the terrestrial N cycle. This dissertation research investigated the role of DON as an N source for temperate trees and as a vector of N loss, using northen hardwood forests of varying tree species composition and soil properties as a basis for study. First, in Chapter 2 I investigated DON uptake by four tree species that commonly occur in either low or high N availability forests. I grew tree seedlings in a greenhouse and labeled them with 15N-enriched amino acids (organic N source), 15N-ammonium, and 15N-nitrate (inorganic N sources). I found that specific uptake rates of amino acid-N were similar across all tree species. However, high N availability species took up NH4+ twice as fast as low N availability species, suggesting amino acid-N was relatively more important to low versus high N availability species. Low N availability species also acquired up to 4 times more total N from amino acids compared to inorganic N sources. These results suggest plant species dominance in a habitat is linked to their ability to use the most available N pool. Second, in Chapter 3 I investigated dissolved organic matter (DOM) leaching losses from forests that spanned a gradient of N availability and tree species composition. I collected soil solutions for three years with lysimeters and analyzed them for dissolved organic carbon (DOC), dissolved inorganic nitrogen (DIN), and DON composition and fractionated DOC in the solutions into hydrophobic and hydrophilic compounds. I also evaluated the characteristics of soils in the forests. I found that DON losses at 100 cm soil depth were not related to increasing soil N stocks across forests, contrary to my expectations. Surprisingly, DOM losses at this same soil depth demonstrated a unimodal pattern of DOC:DON, with relatively low DOC:DON in DOM losses from low and high N availability forests and relatively high DOC:DON from intermediate N availability forests. These patterns likely resulted from the different source and sink strengths of forest soils for DOM as forest floor composition and soil characteristics changed. Finally, in Chapter 4 I evaluated how soil characteristics impacted the chemistry of soil waters leaching from soil cores collected from the above forests. I leached 0-10, 0-25, and 0-50 cm deep cores with a common organic matter solution and analyzed the solutions for DOC, DIN, DON, and hydrophobic/hydrophilic fractions. I also measured multiple physical and geochemical characteristics of the soil cores. Soil depth had a stronger impact on DOM chemistry compared to forest differences. DOM concentrations in soil core leachate decreased with soil depth due to the removal of hydrophobic compounds. Noticeably, DON concentrations increased between the input organic matter solution and 10 cm soil depth, which was accompanied by 67-fold increase in the hydrophilic fraction of DON. These results demonstrated that soil has a strong ability to influence the quantity and quality of DOM leaching through forest soils.
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