This dataset presents data collected along a four-station transect sampled in January 2023 in the upwelling system offshore of Chile (36°S) during the MAPUCHE cruise SO296-2 aboard the R/V Sonne. Phosphate, nitrite, ammonium and nitrate concentrations were analyzed. Nitrate plus nitrite and nitrate δ15N (vs. AIR) and δ18O (vs. VSMOW) measurements have been performed using the denitrifier method (Weigand et al., 2016) on seawater samples. Samples for nitrate δ15N and δ18O were treated with sulfamic acid before isotope analysis to remove nitrite (Granger and Sigman, 2009). δ18O-nitrate values were additionally corrected for the changes in salinity (Knapp et al., 2008).δ15N-nitrite values were calculated using a mass balance and δ15N values before and after nitrite removal. This dataset also includes values of nitrification rates, measured using the tracer incubation method (Damashek et al., 2016).