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This is the second paper in a series aiming at the analysis of nitrogenabundances in O-type stars, to enable further constraints on the earlyevolution of massive stars. We investigate the NIV lambda4058 emission lineformation, provide nitrogen abundances for a substantial O-star sample in theLMC, and compare our findings with recent predictions from stellar evolutionarymodels. Stellar and wind parameters are determined by line profile fitting ofhydrogen, helium and nitrogen lines, based on synthetic spectra calculated byFASTWIND. We derive nitrogen abundances for 20 O- and 5 B-stars, by analyzingall nitrogen lines present in the available optical spectra. The dominatingprocess responsible for emission at NIV lambda4058 in O-stars is the strongdepopulation of the lower level of the transition, which increases as afunction of Mdot. Unlike the NIII triplet emission, resonance lines do not playa role for typical mass-loss rates and below. The bulk of our sample O-starsseems to be strongly nitrogen-enriched, and a clear correlation of nitrogen andhelium enrichment is found. By comparing the nitrogen abundances as a functionof vsini ('Hunter-plot') with tailored evolutionary calculations, we identify aconsiderable number of highly enriched objects at low rotation. Due to the lowinitial abundance, the detection of strong Nitrogen enrichment in the bulk ofO-stars indicates that efficient mixing takes place already during the veryearly phases of stellar evolution of LMC O-stars.

Nitrogen line spectroscopy in O-stars -- II. Surface nitrogen abundances for O-stars in the Large Magellanic Cloud