Raw data of layered hydrochemistry and nitrate reduction experiment in Beijing nitrogen attenuation zone profile
Description
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.
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Steps to reproduce
Five boreholes were arranged in the southern part of Tongzhou District (Fig.1b). Sampling was carried out between November and December 2023, during the dry season, when the water in the vadose zone was in its natural undisturbed state. Sediments were collected with a Luoyang shovel in increments of 10 cm, extending to 0.5-2 m below the groundwater table. Immediately after sampling, the sediments were transferred to sealed bags, placed in insulated containers, and quickly transported back to the laboratory. Boreholes C1 and T1 are located in the turf grass test area and orchard test area of the Yongledian Experimental Base of the Beijing Institute of Water Science and Technology (Beijing Irrigation Experimental Center Station), with a groundwater depth of 7.5 m. Borehole G1 is positioned in the forest plantation, 30 m away from Ganggou River, with a groundwater depth of 8.5 m. Boreholes BF1 and BF2 are situated 10 m from Baifeng Ditch, with BF1 located in the upstream forest plantation and BF2 located in the downstream corn plantation, with groundwater depths of 3.7 m and 3.3 m, respectively. During the dry season, the lateral infiltration recharge from the ditches can be ignored due to the small scale of the ditches and the thick sediments. The test parameters included nitrogen components: total soil nitrogen (TSN), soluble nitrogen (NO3-N, NO2-N, NH4-N); carbon content: soil total organic carbon (TOC), dissolved organic carbon (DOC) in sediments; iron and manganese content: Fe(II), Fe(Ⅲ), and Mn extracted by 1M HCl; enzyme activity: nitrate reductase activity (NAR), ferroxidase activity (HP); other physical and chemical properties of sediments: pH, Eh, TDS, dry weight, water content. Iron-rich sediments with low carbon (BF1) and high carbon (BW) were selected for experimental studies to clarify the contribution of different electron donors to denitrification. The sediments were stirred for homogenization, and 100 g of sediment and 500 ml of deionized water were added to a 600 ml serum bottle sealed with a chlorobutyl rubber diaphragm and aluminium crimp. Reactors were assembled with stainless steel needles fixed on each diaphragm, equipped with sterile three-way cocks for sample extraction. To ensure sterility, all solutions, bottles, needles, and diaphragms were autoclaved at 115℃ for 30 min before utilization. After assembly, the reactors were purged with argon for 20 min to remove residual oxygen. The pre-incubation was carried out for 14 days to obtain a stable aquifer environment, and the experiments were performed after adding NO3-N. Meanwhile, a control group without NO3-N were set up. The reactors were maintained in complete darkness throughout the experiment (20°C). Samples were collected and tested in the anaerobic workstation on days 0, 1, 2, 3, 5, 8, 12, 17, 22, 30, and 40. The experiments were designed in two parallel groups, one for sediment sampling and the other for gas sampling, with three replicates for each setup.
Institutions
- China University of Geosciences Beijing School of Water Resources and Environment