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.