Raw data of layered hydrochemistry and nitrate reduction experiment in Beijing nitrogen attenuation zone profile

Nitrogen attenuation zones (NAZ), characterized by high surface total soil nitrogen (TSN) and groundwater intrinsic vulnerability (Inv) but low groundwater total nitrogen (TN) concentrations, were found in a regional shallow groundwater nitrogen pollution study. The relatively high Fe2+ (1.50 mg/L) and Mn2+ (0.35 mg/L) concentrations in these zones prompted the hypothesis that they might be involved in the NO3-N reduction within nutrient-deficient groundwater. However, whether this process occurred and its contribution remained ambiguous.
Consequently, five boreholes were sampled in the southeastern suburb of Beijing, a typical NAZ in the North China Plain. The results indicated that the high TSN but low groundwater TN concentrations were primarily attributable to NO3-N attenuation during infiltration, with silty clay in the vadose zone and the water table fluctuation zone (WTF zone) serving as crucial layers for attenuation. Enzyme activity and microbial analysis confirmed that Fe(II) in the WTF zone participated in NO3-N reduction. Electron balance analysis further discovered that dissolved organic carbon (DOC) was the predominant electron donor for NO3-N reduction in the vadose zone (51.44%-92.98%), whereas Fe(II) prevailed in the WTF zone (27.4%-52.43%), consistent with the enzyme activity results. Notably, incubation experiments revealed that Fe(II) was the primary electron donor when DOC/NO3-N<0.7 and DOC/Fe(II)<0.023 in sediments.
These findings underscored the significant role of Fe(II) in NO3-N reduction. Future studies could provide a basis for differentiated fertilizer nitrogen regulation by dividing the NAZ and exploring the nitrogen loading thresholds that the nitrogen attenuation capacity of different NAZ can support.