  This is the second paper in a series aiming at the analysis of nitrogen
abundances in O-type stars, to enable further constraints on the early
evolution of massive stars. We investigate the NIV lambda4058 emission line
formation, provide nitrogen abundances for a substantial O-star sample in the
LMC, and compare our findings with recent predictions from stellar evolutionary
models. Stellar and wind parameters are determined by line profile fitting of
hydrogen, helium and nitrogen lines, based on synthetic spectra calculated by
FASTWIND. We derive nitrogen abundances for 20 O- and 5 B-stars, by analyzing
all nitrogen lines present in the available optical spectra. The dominating
process responsible for emission at NIV lambda4058 in O-stars is the strong
depopulation of the lower level of the transition, which increases as a
function of Mdot. Unlike the NIII triplet emission, resonance lines do not play
a role for typical mass-loss rates and below. The bulk of our sample O-stars
seems to be strongly nitrogen-enriched, and a clear correlation of nitrogen and
helium enrichment is found. By comparing the nitrogen abundances as a function
of vsini ('Hunter-plot') with tailored evolutionary calculations, we identify a
considerable number of highly enriched objects at low rotation. Due to the low
initial abundance, the detection of strong Nitrogen enrichment in the bulk of
O-stars indicates that efficient mixing takes place already during the very
early phases of stellar evolution of LMC O-stars.
