Impacts of Fine-roots on Terrestrial Net Primary Productivity and Soil Nutrient Cycling

Large uncertainties remain in fine-root longevity, and contribution to terrestrial nutrient cyling. In my dissertation, I utilized a carbon isotope tracer at a long-term Free-Air CO2 Enrichment (FACE) experiment in a Liquidambar styraciflua plantation to examine properties of fine-roots including longevity and sources of carbon for growth and respiration. Soil cores were sequentially sampled for multiple growing seasons following cessation of CO2 fumigation. Fine-roots were extracted from soil and separated by diameter. Newly produced roots were produced from exclusively new photosynthate. Fine-root carbon was replaced more slowly, with about half of the carbon remaining after two full growing seasons. Model fitting found at least two turnover rates for carbon occur in the fine-root population, with 10% of carbon quickly being turned over (< 3 months) and 90% turning over more slowly (> 2 years). In a follow-up study, I utilized a potentially more functional approach by separating roots by root branching order. Results indicate that branching order and root nitrogen concentration correlate with root longevity. Thus, easily measurable traits such as nitrogen concentration may help elucidate root longevity in different species or at larger spatial scales. Knowledge of the longevity and standing biomass of fine-roots is essential for quantifying fine-root contribution to terrestrial NPP and forest nutrient cycling. An extensive literature review was conducted to examine fine-root biomass within branching orders, with just 10 reports in the literature. Even with this sparse data-set, it is clear that environmental conditions such as nutrient and water availability impact fine-root biomass distribution. Increased studies quantifying the amount of biomass in roots by branching order will be needed to fully calculate fine-root contribution to terrestrial carbon and nutrient cycling. A final contribution from my thesis is the creation of a dynamic vegetation model that optimizes both above- and below-ground biomass allocation with respect to changing environmental conditions. Results from empirical research, including that done in my other chapters, conclude that biomass allocation is plastic with respect to abiotic conditions. No current simple modeling scheme adequately captures this plasticity. It is hoped that the model developed here can make progress towards that goal.