N dynamics in the rhizosphere of Pinus ponderosa seedlings

Transformations of inorganic N were studied in microcosms containing Pinus 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 those observed. We determined the response of mineralization and immobilization rates to increased NH4+ in rhizosphere and bulk soil. Increasing NH4+ addition resulted in proportional increases in immobilization rates in both rhizosphere and bulk soils but did not affect mineralization rates. The increased immobilization rates with increasing NH4+ addition and the failure to predict immobilization rates based on C supply suggested that in the short-term, immobilization rates were limited 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, immobilization of NH4+ and NO3-, nitrification and plant uptake were determined over 48 h. Rates in the root-zone were compared to rates in soils from which roots had been excluded for 2 weeks before labeling. The elimination of active roots from soil regions did not significantly change the production or the total consumption of NH4+ in those regions. The presence of roots reduced microbial consumption of NH4+ by nitrifiers and heterotrophs. Pine roots were successful competitors with microorganisms for limited inorganic N, but were more successful when the N source was NO3- vs NH4+. Plants accounted for 30% of the total NH4+ consumption, but 70% of total NO3- consumption.