  We report the RF performance of several single-cell superconducting
radio-frequency cavities subjected to low temperature heat treatment in
nitrogen environment. The cavities were treated at temperature 120 - 165
$^{\circ}$C for an extended period of time (24 - 48 hours) either in high
vacuum or in a low partial pressure of ultra-pure nitrogen. The improvement in
$Q_0$ with a Q-rise was observed when nitrogen gas was injected at $\sim$300
$^{\circ} $C during the cavity cooldown from 800 $^{\circ}$C and held at 165
$^{\circ}$C, without any degradation in accelerating gradient over the baseline
performance. The treatment was applied to several elliptical cavities with
frequency ranging from 0.75 GHz to 3.0 GHz, showing an improved quality factor
as a result of low temperature nitrogen treatments. The Q-rise feature is
similar to that achieved by nitrogen alloying Nb cavities at higher
temperature, followed by material removal by electropolishing. The surface
modification was confirmed by the change in electronic mean free path and tuned
with the temperature and duration of heat treatment. The decrease of the
temperature-dependent surface resistance with increasing RF field, resulting in
a Q-rise, becomes stronger with increasing frequency and decreasing
temperature. The data suggest a crossover frequency of $\sim 0.95$~GHz above
which the Q-rise phenomenon occurs at 2~K. Some of these results can be
explained qualitatively with an existing model of intrinsic field-dependence of
the surface resistance with both equilibrium and nonequilibrium quasiparticle
distribution functions. The change in the Q-slope below 0.95 GHz may result
from masking contribution of trapped magnetic flux to the residual surface
resistance.
