N DYNAMICS IN THE RHIZOSPHERE OF PINUS-PONDEROSA SEEDLINGS

Transformations of inorganic N were studied in microcosms containing P inus ponderosa seedlings. We assessed the potential for immobilization and mineralization of NH4+ in soils collected from adjacent to coarse , fine and young roots and in soil more than 5 mm from any root. Rates of mineralization and immobilization of NH4- in soils collected from adjacent to roots were > 50% higher than those in soils more than 5 mm from any root. C input estimates suggest that soils adjacent to fine or young roots could have supported immobilization rates higher than t hose observed. We determined the response of mineralization and immobi lization rates to increased NH4+ in rhizosphere and bulk soil. Increas ing NH4+ addition resulted in proportional increases in immobilization rates in both rhizosphere and bulk soils but did not affect mineraliz ation rates. The increased immobilization rates with increasing NH4+ a ddition and the failure to predict immobilization rates based on C sup ply suggested that in the short-term, immobilization rates were limite d by NH4+ supply rather than C availability. In intact microcosms, we determined the transformation rates of inorganic N using a combined N- 15 pool dilution and tracer approach. The rates of mineralization, imm obilization of NH4+ and NO3-, nitrification and plant uptake were dete rmined over 48 h. Rates in the root-zone were compared to rates in soi ls from which roots had been excluded for 2 weeks before labeling. The elimination of active roots from soil regions did not significantly c hange the production or the total consumption of NH4+ in those regions . The presence of roots reduced microbial consumption of NH4+ by nitri fiers and heterotrophs. Pine roots were successful competitors with mi croorganisms for limited inorganic N, but were more successful when th e N source was NO3- vs NH4+. Plants accounted for 30% of the total NH4 + consumption, but 70% of total NO3- consumption.