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Rising atmospheric carbon dioxide, nitrogen deposition and warmer temperatures may alter the quantity and quality of plant-derived organic matter available to soil biota, potentially altering rates of belowground herbivory and decomposition. Our objective was to simulate future growth conditions for an early successional (loblolly) and late successional (ponderosa) species of pine to determine if the physical and chemical properties of the root systems would change. Seedlings were grown for 160 days in greenhouses at the Duke University Phytotron at 35 or 70 Pa CO2 partial pressure, ambient or ambient + 5 0C temperature, and 1 or 5 mMNH4O3. Roots from harvested seedlings were analyzed for changes in surface area, specific root length, mass, total nonstructural carbohydrates (TNC), and concentrations of macro-nutrients. Surface area increased in both species under elevated CO2, due primarily to increases in root length, and this response was greatest (+138%) in loblolly pine at high temperature. Specific root length decreased in loblolly pine at elevated CO2 but increases in mass more than compensatedfor this, resulting in net increases in total length. TNC was unaffected and nutrient concentrations decreased only slightly at elevated CO2, possibly from anatomical changes to the root tissues.We conclude that future growth conditions will enhance soil exploration by some species of pine, but root carbohydrate levels and nutrient concentrations will not be greatly affected, leaving rates of root herbivory and decomposition unaltered. [ABSTRACT FROM AUTHOR]

The decomposition of plant-derived organic matter exerts strong control over the cycling of carbon and nutrients in terrestrial ecosystems and may be significantly altered by increased precipitation and nitrogen deposition associated with global change. It was the goal of this study to quantify the rate of belowground decomposition in an intact loblolly pine forest, and determine how this was affected by increased availability of water and nitrogen. A randomized complete-block factorial of irrigation and fertilization treatments was installed inan 8 yr old loblolly pine plantation in Scotland county, North Carolina. Fresh root samples of three size classes were buried in fiberglass mesh bags in January, 1994 and recovered at two-month intervals for two years. Samples were analyzed for percent mass remaining and contents of macro-nutrients. Roots decomposed in a two stage process: early in the incubation mass loss was correlated to size class and nutrient concentrations, but this correlation disappeared later in the incubation when rates of mass loss converged for all size classes. Decomposition was seldom affected by the irrigation and fertilization treatments, due to the buffering capacity of soil moisture and complex ecosystem-level responses to fertilization. Net mineralization of N, P, K, Ca, and Mg occurred in the smaller size classes of roots providing a source of these nutrients to the aggrading plantation for an estimated 2 to 15 years. The largest size class of roots was a sink for N, Ca, and Mg for the duration of this study, and was a source of P and K for an estimated 20 and 4 years, respectively. It is concluded that in moist temperate ecosystems belowground decomposition will be less affected by the projected increases in moisture and nutrient availability than will decomposition of the forest floor due to the buffering capacity of the soil. Further, small roots provide important sources of macro-nutrients for several decades to aggrading forests after [ABSTRACT FROM AUTHOR]

We measured CO2 efflux from intact root/rhizosphere systems of 155 day old loblolly (Pinus taeda L.) and ponderosa (Pinus ponderosa Dougl. ex Laws.) pine seedlings in order to study the effects of elevated atmospheric CO2 on the below-ground carbon balance of coniferous tree seedlings. Seedlings were grown in sterilized sand culture, watered daily with either 1, 3.5 or 7 mM NH4+ and maintained in an atmosphere of either 35 or 70 Pa CO2. Carbon dioxide efflux (mu mol CO2 plant-1 s-1) from the root/rhizosphere system of both species significantly increased when seedlings were grown in elevated CO2, primarily due to large increases in root mass. SpecificCO, efflux (mu mol CO2 g root- 1 s-1) responded to CO2 only under conditions of adequate soilnitrogen availability (3.5 mM). Under these conditions, CO2 efflux rates from loblolly pine increased 70% from 0.0089 to 0.0151 mu mol g- 1 s-1 with elevated CO2 while ponderosa pine responded with a 59% decrease, from 0.0187 to 0.0077 mu mol g-1 s-1. Although below ground CO2 efflux from seedlings grown in either sub-optimal (1 mM) or supra-optimal (7 mM) nitrogen availability did not respond to CO2, there was a significant nitrogen treatment effect. Seedlings grown in supra-optimal soil nitrogen had significantly increased specific CO2 efflux rates, and significantly lower total biomass compared to either of the other two nitrogen treatments. These results indicate that carbon losses from the root/rhizosphere systemsare responsive to environmental resource availability, that the magnitude and direction of these responses are species dependent, and maylead to significantly different effects on whole plant carbon balance of these two forest tree species. [ABSTRACT FROM AUTHOR]

Root growth and physiological uptake capacity for NH4+ and NO3- were examined for seedlings of loblolly and ponderosa pine grown for 160 days under two CO2 levels, ambient (35 Pa) and ambient plus 35 Pa (70 Pa). Fractionof biomass allocated to active fine roots as well as total N (NH4+ + NO3-) absorption per unit root dry mass were unaffected by CO2. On a whole-plant basis, elevated CO2 led to a significant increase in N acquisition in loblolly but not in ponderosa pine. However, even in loblolly pine where CO2 significantly increased plant N acquisition, the relative increase, in biomass far exceeded the gain in N, i.e. a 60% increase in total dry weight was accompanied by only a 30% increase in N gain in response to high CO2. We suggest that the commonly reported decline in tissue N concentration of these and other species at high CO2 is largely caused by inabilityof the root systems to sufficiently compensate for increased N demand. Elevated CO2 significantly altered root uptake capacityof the different N forms, i.e., high CO2 significantly increased NO3- absorption rates, but decreased NH4+ absorption rates in both species though the decrease in loblolly was insignificant. However, elevated CO2 increased root respiration rate in loblolly pine while significantly decreasing it in ponderosa pine. This indicates that CO2-induced changes in plant preference for inorganic N forms is not simply regulated by root energy status. If changes in plant preferencefor inorganic N forms represent typical responses to elevated CO2, the results could have important implication, for N dynamics in managed and natural plant communities. [ABSTRACT FROM AUTHOR]