Forests play an important role in mitigating climate change by removing carbon dioxide (CO2) from the atmosphere via photosynthesis and storing it in plant tissues and soil organic matter (SOM). Plant roots are a major conduit for transporting recently fixed CO2 belowground, where carbon (C) remains in SOM or returns to the atmosphere via respiration of soil microbes. Compared to aboveground plant processes related to the C cycle, there is little understanding of how belowground plant-C allocation to roots, symbiotic root fungi and secretions into the soil influence the gain or loss of C from the soil. Further, the uncertainty in the timing and amount of root growth that occurs in forests is a barrier to understanding how root activity responds to global change and feeds back to the C cycle. Therefore, the objective of my research is to quantify the timing and magnitude of C allocation to roots and soil via data compilation, field studies and modeling across broad spatial scales. Using data compilation at the global scale, I show that root and shoot phenology are often asynchronous and that evergreen trees commonly have later root growth compared to deciduous trees using meta-analysis across four biomes. At the plot scale, field studies in a mid-latitude forest demonstrate that deciduous stands allocate more C belowground earlier in the growing season compared to a conifer stand. The difference in phenology between stands can be attributed to the timing of root growth. At the root scale, zymographic analysis demonstrates that microbial extracellular enzyme activity is concentrated near the surface of roots and that the rhizosphere can extend well beyond 2 mm from the root surface. Finally, I developed a new model of microbial physiology and extracellular enzyme activity to assess how climate change may affect plant - microbe interactions and soil organic matter decomposition. I show that increases in temperature and the quantity of C inputs substantially alter decomposition. Collectively, these results demonstrate the importance of belowground allocation to the C cycle of terrestrial ecosystems.

The purpose of this study was to determine the effects of amendments (control, (1x) compost, (2x) compost, (TS) topsoil, and 1x+TS) and microtopography (level, pit and mound) on three parameters (plant species composition, above-ground characteristics of Taxodium distichum, and plant root characteristics) of vegetation growing at a created tidal freshwater swamp in Virginia. None of the soil treatments met the traditional vegetation criteria for federal wetland jurisdictional determination, which only considers dominant species. When the same criteria were used for all of the species, the control, 1x, and 2x treatments met jurisdictional criteria. Considering these findings, vegetative criteria should be re-evaluated for young created wetlands. Compost addition produced the highest proportion of obligate wetland species (30%) while topsoil additions created the lowest proportion of wetland obligates (11%) and the highest proportion of upland plants. The 1x treatment generated the greatest species evenness and lowest weighted average (2.57). Topsoil treatments had the lowest diversity and evenness. Therefore, compost amendment is recommended to increase hydrophytes without compromising evenness and diversity. Bald cypress in pits were taller, had larger trunk diameter and basal trunk swelling than trees growing at higher elevations. Roots growing in mounds were more numerous with greater length than roots at lower elevations. Root length and count were highest for the control soil treatment. Amended treatments may have had lower rooting values because nutrient supply was adequate. Overall, incorporating microtopography and compost during wetland creation had a positive effect on vegetative function in this system. / Master of Science