A preliminary groundwater incubation study dataset to test isotope methods for distinction of various N transformation processes in different pH conditions

pH experiment

2 groundwater samples with high nitrate concentrations ('PZ4' and 'oxytree') were selected and incubated under oxic and anoxic conditions to investigate the effect of pH on nitrogen transformation. pH is a key factor influencing denitrification, as it regulates microbial enzyme activity and N₂O production; pH (7–8) is known to enhance complete denitrification and reduce N₂O emissions. Samples were flushed with N₂ gas for 30 minutes (anoxic) or with synthetic air (oxic), then adjusted to pH 5.0, 7.0, or 9.0 using 0.1 N NaOH or HCl. Each treatment had four replicates. Two types of sterile controls were prepared: one by filtration with sterile filters and addition of 2 mL 0.1 N HgCl₂, and another by sterile filtration followed by autoclaving. All samples were incubated at 16 °C, 100 rpm in the dark for 2 weeks. Glucose was added after one week to stimulate microbial denitrification. Headspace gas samples were analysed for GC and CRDS measurements. Further, Inorganic nitrogen species (NO₃⁻, NO₂⁻, NH₄⁺) were analyzed colorimetrically, and isotopic signatures determined using Pseudomonas aureofaciens for NO₃⁻ and Stenotrophomonas nitritireducens for NO₂⁻ isotopes.