Carbon and nitrogen dynamics in the rhizosphere of Pinus ponderosa seedlings

The rhizosphere is a dynamic soil region characterized by dense microbial populations and enhanced rates of microbial processes. The rhizosphere may be especially important in determining the spatial distribution of carbon and nitrogen cycling in forest soils. I have investigated the flow of carbon from roots to the soil, the quantity and metabolic status of bacteria and fungi, and the production and consumption of inorganic nitrogen in the rhizosphere of Pinus ponderosa seedlings. The role of plant/microbial competition for inorganic nitrogen in determining the availability of nitrogen to plant assimilation was assessed.; I examined the flow of recently fixed photosynthate from roots to the soil using a {dollar}sp{lcub}14{rcub}{dollar}C pulse labelling technique. The highest concentration of recently fixed photosynthate carbon in the soil was adjacent to the young root tip region. Fine mycorrhizal roots had the highest rate of carbon loss to the soil per unit carbon assimilated by the root. Mycorrhizal hyphae played an important role in the redistribution of recently fixed photosynthate throughout the soil. The input of plant carbon to the soil by rhizodeposition was an important energy source for the microbial community even in soil not directly adjacent to the root.; I used direct microscopy and vital staining to determine the quantity and metabolic status of bacteria and fungi in rhizosphere soils. Approximately 50% of the microbial biomass exhibited evidence of potential activity. Fungi were enhanced adjacent to fine mycorrhizal roots compared to bulk soil while bacteria were more responsive than fungi to the labile carbon inputs in the young root zone.; In short-term {dollar}sp{lcub}15{rcub}{dollar}N experiments, I observed that rates of mineralization and NH{dollar}sb4sp+{dollar} immobilization were higher in soils harvested from adjacent to roots than in soils harvested from greater than 5 mm from any root. Results from intact microcosms suggest that NH{dollar}sb4sp+{dollar} supply and competition between roots, heterotrophs and nitrifiers for NH{dollar}sb4sp+{dollar} were the direct controls on NH{dollar}sb4sp+{dollar} immobilization rates rather than the supply of, recently fixed carbon by rhizodeposition. Plants were more successful competitors for NO{dollar}sb3sp-{dollar} than for NH{dollar}sb4sp+{dollar}. The results of the carbon flow study and the importance of plant/microbial competition for limited nitrogen supply suggest that mycorrhizal symbioses play an important role in below-ground carbon and nitrogen dynamics in conifer systems.