This dataset contains results from in situ field measurements of riverbed nitrogen transformations in the Hammer Stream, a sandy tributary of the River Rother in West Sussex, UK. Measurements were performed in November 2014 and February, April and July 2015. The data include baseline concentrations of nutrients (NO2, NO3, NH3, PO4), chloride, oxygen, pH, temperature, Fe(II), organic carbon, 15N-N2 and methane (CH4) and nitrous oxide (N2O) sampled from porewater prior to injection of 15N-nitrate.