DECOMPOSITION OF ROOTS IN LOBLOLLY-PINE - EFFECTS OF NUTRIENT AND WATER AVAILABILITY AND ROOT SIZE CLASS ON MASS-LOSS AND NUTRIENT DYNAMICS

The decomposition of plant-derived organic matter exerts strong contro l over the cycling of carbon and nutrients in terrestrial ecosystems a nd may be significantly altered by increased precipitation and nitroge n deposition associated with global change. It was the goal of this st udy to quantify the rate of belowground decomposition in an intact lob lolly pine forest, and determine how this was affected by increased av ailability of water and nitrogen. A randomized complete-block factoria l of irrigation and fertilization treatments was installed in an 8 yr old loblolly pine plantation in Scotland county, North Carolina. Fresh root samples of three size classes were buried in fiberglass mesh bag s in January, 1994 and recovered at two-month intervals for two years. Samples were analyzed for percent mass remaining and contents of macr o-nutrients. Roots decomposed in a two stage process: early in the inc ubation mass loss was correlated to size class and nutrient concentrat ions, but this correlation disappeared later in the incubation when ra tes of mass loss converged for all size classes. Decomposition was sel dom affected by the irrigation and fertilization treatments, due to th e buffering capacity of soil moisture and complex ecosystem-level resp onses to fertilization. Net mineralization of N, P, K, Ca, and Mg occu rred 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 d uration 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 ec osystems belowground decomposition will be less affected by the projec ted increases in moisture and nutrient availability than will decompos ition of the forest floor due to the buffering capacity of the soil. F urther, small roots provide important sources of macro-nutrients for s everal decades to aggrading forests after large-scale disturbances suc h as harvesting of aboveground biomass.