| Disturbance events, such as wildfire or clearcutting, exert profound influence over the structure, composition and functioning of forest ecosystems. Since nitrogen (N) is generally considered the nutrient most limiting for plant growth in boreal and cold temperate forests, it is critical to understand how disturbances may alter soil N availability and how plants subsequently access that N. In this dissertation, I utilized the jack pine (Pinus banksiana) ecosystem of northern Michigan to ask three fundamental questions regarding the effects of disturbance: (1) do the initial recoveries of soil carbon (C) and N pools and dynamics differ following wildfire vs. clearcutting harvesting?; (2) how do available forms of organic and mineral soil N change over time following wildfire?; and, (3) how does community composition of ectomycorrhizal fungi on jack pine roots change following disturbance?;Clearcutting is replacing wildfire as the major disturbance in many forest types, but the implications for soil C and N cycling are little understood. In chapter 3, I compared the initial recovery of C and N pools and dynamics in clearcut, wildfire-burned stands, and intact jack pine stands. Amongst other findings, I observed higher potential nitrification in the clearcut vs. wildfire-burned stands, due to differences in microbial gross nitrate consumption rates. Since the production of nitrate can lead to leaching losses of N and base cations, this finding has potential implications for the recovery of these forests following clearcutting.;Secondly, studies of successional changes in soil N availability have almost exclusively focused on minerai N; yet, plants can take up forms of organic N, such as amino acids, at biologically important rates. In Chapter 4, I investigated changes in amino acid- and minerai N availability along a ten-site chronosequence of jack pine stands, varying in age from 4 to 60 y post-wildfire. Overall, my results suggest that heterotrophic consumption, not production via proteolysis, controls soil amino acid availability. Moreover, since I found that amino acid N exceeds mineral N in a time period where jack pine growth rates and N demand are highest, I speculate that amino acid N may be important to the N economy of these forests.;Lastly, symbioses between host plants and mycorrhizal fungi can enhance rates of amino acid uptake and allow plants to access complex organic N forms. In Chapter 5, I investigated changes in the belowground community composition of ectomycorrhizal fungi (EMF) using molecular methods along a six-site chronosequence of jack-pine dominated stands (5, 11, 19, 23, 47 and 56 y post-wildfire). Overall, the results of this chapter show that EMF community composition can shift dramatically within the first decades of forest stand development. This compositional shift was primarily driven by higher relative abundances of Rhizopogon and Thelephora taxa in sites age 5 and 11, and increases in Cortinarius in older stands. Since EMF taxa can vary in ability to access N for their plant host, this shift in community may have implications for jack pine nutrition over stand development. |
