In Situ Soil Nitrogen Dataset Development from West of the Baiyangdian Wetland for High-Nitrogen Accumulation Farmland Research

LV Qi^1,2  WANG Shiqin^1*  TAN Kangda^1,2  LIU Bingxia¹  ZHENG Wenbo¹

1. Center for Agricultural Resources Research, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences/Hebei Key Laboratory of Agricultural Water-Saving, Shijiazhuang 050022, China；

2. University of Chinese Academy of Sciences, Beijing 100049, China

Abstract: The western agricultural region of Baiyangdian Wetland has long been subjected to intensive agricultural production, irrigation, drainage, and nitrogen fertilizer application, leading to significant nitrogen accumulation in soil profiles and its migration into the deep vadose zone. Soil nitrate and nitrite nitrogen are critical indicators for assessing nitrogen transport, transformation, and the potential risk of groundwater contamination. To investigate the dynamics of water and nitrogen transport in the vadose zone under different fertilization regimes, we conducted long-term in situ monitoring from June 2023 to September 2024 in wheat-maize rotation fields in Dulian Village, Anxin County, Xiong’an New Area. Three management strategies were implemented, including conventional fertilization, 25% nitrogen reduction fertilization, and biochar combined with 25% nitrogen reduction fertilization. Soil samples were collected at 11 vertical soil layers across 5 sampling periods from different soil depths (0–10, 10–25, 25–50, 50–75, 75–100, 100–150, 150–200, 200–250, 250–300, 300–350, 350–400 cm). Soil water content, nitrate nitrogen, and nitrite nitrogen were determined using the oven-drying method and KCl extraction-automated discrete chemical analysis, respectively. This study established a measured soil nitrogen dataset for high-nitrogen accumulation farmlands in western Baiyangdian Wetland. The dataset includes: (1) geolocation of the farmland plot; (2) vertically soil texture classification; (3) soil water content; (4) measured nitrate nitrogen values in different soil layers; and (5) measured nitrite nitrogen values in different soil layers. The dataset is archived in .shp and .xlsx formats, comprising 9 data files with data size of 41.8 KB (compressed into 1 file with 35.4 KB).

Keywords: Baiyangdian Wetland; high-nitrogen farmland; vadose zone; nitrate nitrogen; nitrite nitrogen

DOI: https://doi.org/10.3974/geodp.2026.05.10

Dataset Availability Statement:

The dataset supporting this paper was published and is accessible through the Digital Journal of Global Change Data Repository at: https://doi.org/10.3974/geodb.2026.04.07.V1.

1 Introduction

Nitrogen fertilizers are foundational to global agricultural productivity, while rational nitrogen application can significantly boost crop yields and enhance the soil fertility; however, their chronic overapplication has precipitated a widespread ecological challenge characterized by massive soil nitrogen surpluses and profound nitrate accumulation within soil profiles^[1]. Because nitrate is highly mobile and water-soluble, it readily leaches deep into the vadose zone via irrigation and precipitation infiltration, severely elevating the risk of groundwater contamination^[2,3]. Concurrently, nitrite—though typically present at lower bulk concentrations—acts as a critical intermediate in nitrification and denitrification pathways, serving as a highly sensitive proxy for active soil nitrogen transformations and shifting redox environments^[4,5]. The vadose zone functions as the critical biogeochemical interface governing these dynamic processes^[6]. The vadose zone functions as the critical biogeochemical interface governing these dynamic processes. Consequently, stratifying nitrate and nitrite distributions within this deep profile is essential for mechanistically understanding agricultural nitrogen loss.

The Baiyangdian Wetland basin, a critical freshwater wetland and the ecological linchpin of the Xiong’an New Area, exemplifies this challenge. The farmland area in the western part of Baiyangdian is closely connected with the water environment of the lake, and factors such as nitrogen fertilizer input, irrigation regimes, rainfall infiltration, and changes in groundwater depth during agricultural production may all affect the migration and accumulation of soil nitrogen in the vadose zone of farmland^[7,8]. Its adjacent farmlands are subjected to intensive wheat-maize rotations characterized by marked seasonal transitions and heavy fertilization, creating a hydrological regime where high surface nitrogen inputs drive continuous deep vertical leaching^[9]. Obtaining multi-temporal and multi-depth measured data of soil water content and nitrogen under various fertilization regimes in farmlands of western Baiyangdian Lake provides an essential foundation for assessing nitrogen environmental impacts of farmland in this area..

In recent years, reduced nitrogen fertilization and biochar co-application have been regarded as important farmland management practices for reducing the risk of nitrogen leaching, enhancing nitrogen use efficiency, and ameliorating soil physicochemical properties^[10,11]. Compared with conventional fertilization, a 25% reduction in nitrogen application directly decreases exogenous nitrogen input; biochar co-application with reduced nitrogen may alter soil water movement and nitrogen migration characteristics^[12] by improving soil pore structure, enhancing soil water and nutrient retention capacity, and affecting microbial nitrogen transformation processes^[13].

Currently, for the high nitrogen accumulation farmland area in Baiyangdian, existing studies^[14,15] indicate that long-term stratified measured data of soil profile nitrogen under different fertilization regimes are relatively scarce, making it difficult to support precise nitrogen management and non-point source pollution control. We established permanent experimental plots in a typical farmland in Anxin County, located within 10 km of the Baiyangdian Wetland. To isolate the mechanistic effects of fertilization, all plots shared identical cropping and field management conditions. Treatments included conventional fertilization, a 25% reduced nitrogen fertilization, and a biochar-amended reduced nitrogen treatment. Sampling was conducted across five critical phenological and hydrological periods utilizing a high-resolution vertical stratification strategy (11 depth intervals down to 400 cm) for soil water content, nitrate nitrogen, and nitrite nitrogen concentrations. A measured dataset of soil nitrogen was constructed for the high nitrogen accumulation agricultural area in western Baiyangdian, aiming to reveal the spatiotemporal differentiation and variation patterns of soil nitrogen, and to provide data support and a scientific basis for agricultural nitrogen emission reduction and non-point source pollution control in the Baiyangdian watershed.

2 Metadata of the Dataset

The metadata of the In situ soil nitrogen dataset from west of the Baiyangdian Lake for high-nitrogen accumulation farmland research^[16] is summarized in Table 1. It includes the dataset full name, short name, authors, year of the dataset, temporal resolution, spatial resolution, data format, data size, data files, data publisher, and data sharing policy, etc.

Table 1  Metadata summary of the In situ soil nitrogen dataset from west of the Baiyangdian Lake for high-nitrogen accumulation farmland research

3 Methods

3.1 Study Area

The long-term in situ experiment was conducted in the intensively managed farmlands of Dulian Village, Anxin County, located within the Xiong’an New Area (115°51′52″E, 38°51′57″N). Positioned less than 10 km from the western periphery of Baiyangdian Wetland in the central North China Plain, the site represents a typical high-yield agricultural zone characterized by flat terrain and a continuous winter wheat-summer maize rotation system^[14]. The region experiences a temperate monsoonal continental climate heavily governed by the East Asian monsoon. Mean annual temperature is 12.6 ℃^[18], with a mean annual precipitation of approximately 480.8 mm, the vast majority of which is concentrated during the summer months between June and September.

3.2 Field Experimental Design and Sampling

In June 2023, three permanent experimental plots were established within a designated crop quality and yield improvement demonstration zone (Figure 1). Each plot spanned 22 m in length, 10 m in width, and was bordered by 2-m guard rows, yielding a total experimental area of 0.075 ha. Specifically, the nitrogen (N) application rates using compound fertilizer in the conventional fertilization (CF) treatment were 210 and 207 kg N/ha for winter wheat and summer maize, respectively. In the 25% reduced nitrogen fertilization (RF) treatment, the N rates for winter wheat and summer maize were reduced to 157.5 and 155.25 kg N/ha, respectively. For the biochar combined with reduced nitrogen fertilization (BRN) treatment, biochar granules were additionally applied at a rate of 225 kg/ha based on the RF nitrogen application rates. To strictly isolate the environmental effects of varying nitrogen inputs, all treatments were maintained under identical cropping systems and baseline field management conditions. The long-term monitoring period for the plots extended from June 2023 to September 2024, covering different crop growth stages and the periods affected by fertilization, irrigation, and rainfall within the wheat-maize rotation system.

During soil sampling, 3 replicate samples were collected from plots under the same fertilization regime using a soil auger at depths of 0–10, 10–25, 25–50, 50–75, 75–100, 100–150, 150–200, 200–250, 250–300, 300–350, and 350–400 cm, respectively. Each sample was placed separately in an aluminum box and a self-sealing bag; the aluminum boxes were additionally sealed with waterproof tape at the closure and immediately stored in a refrigerator at 4 ℃.

Figure 1  Map of the geographical location and experimental layout of sample plots

3.3 Soil Chemical Analysis

Soil samples stored in aluminum moisture tins were oven-dried at 105 ℃ for over 8 h until a constant weight was reached. The weights of the fresh soil plus tin, dry soil plus tin, and the empty tin were recorded separately to calculate the soil water content. For inorganic nitrogen extraction, the homogenized fresh soil samples were subjected to extraction utilizing a 2 mol/L KCl solution at a 1:10 (w/v) soil-to-solution ratio. The resulting suspensions were subsequently centrifuged and filtered. Concentrations of nitrate (NO₃^‒-N) and nitrite (NO₂^‒-N) were precisely quantified using an automated discrete chemical analyzer (SmartChem 200).

3.4 Data Processing

The soil water content was calculated using the following Equation:

                                                                                                (1)

where  represents the gravimetric soil water content (%);is the mass of the fresh soil plus the aluminum tin (g);is the mass of the dry soil plus the aluminum tin (g);is the mass of the empty aluminum tin (g).

The NO₂^‒-N concentration was determined using the diazotization-coupling colorimetric method. Under acidic conditions, the nitrite in the extract reacts with sulfanilamide to undergo a diazotization reaction, and subsequently couples with N-(1-naphthyl) ethylenediamine dihydrochloride to form a magenta azo dye. The instrument calculates the NO₂^‒-N concentration in the extract based on the measured absorbance and a standard curve.

For NO₃^‒-N determination, the nitrate in the extract is first reduced to nitrite via a cadmium reduction module. The resulting nitrite is then measured following the aforementioned colorimetric procedure, and the final NO₃^‒-N concentration is obtained by subtracting the original background NO₂^‒-N content of the sample.

The soil NO₃^‒-N and NO₂^‒-N contents were calculated using the following Equation:

                                                                                                              (2)

where  is the soil NO₃^‒-N or NO₂^‒-N content (mg/kg);  represents the concentration of the extract determined by the automated discrete chemistry analyzer (SmartChem200) (mg/L);  is the volume of the extract (L);  is the dilution factor; and  is the mass of the oven-dried soil (kg).

The mass of the oven-dried soil was calculated from the fresh soil mass based on the gravimetric soil water content:

                                                                                                                  (3)

whereis the mass of the fresh soil (kg)，andis the gravimetric soil water content (%).

Origin 2025 was used to construct graphs of these three indicators within the 0–400 cm soil profile.

4 Data Results

4.1 Dataset Composition

The dataset comprises a spatial shapefile (.shp) delineating the precise geographical coordinates of the experimental plots, coupled with a comprehensive tabular dataset (.xlsx) containing 4 distinct spreadsheets. One spreadsheet catalogs the baseline physical properties of the soil profile, specifically texture and bulk density. The remaining spreadsheets house the high-resolution empirical measurements for soil water content (SWC), nitrate nitrogen (NO₃^‒-N) and nitrite nitrogen (NO₂^‒-N). The temporal scope encompasses 5 strategic sampling campaigns conducted between June 2023 and September 2024. To ensure statistical robustness, all temporal data report individual measurements from biological triplicates, accompanied by their calculated means and standard deviations.

4.2 Data Results Analysis

Table 2 summarizes the mean values of soil water content, NO₃^‒-N and NO₂^‒-N in the soil profile under different treatments across various sampling periods. Furthermore, Figure 2 illustrates the dynamic changes of these parameters at different depths throughout the monitoring period.

Temporally, SWC exhibited pronounced seasonal variability. Moisture levels reached a minimum in early summer (June 2023, 21.2%) and peaked significantly during the autumn months (September and November 2023, 27.9% and 27.7%, respectively). The SWC in March and September 2024 was recorded at 26.5% and 25.8%, respectively. This seasonal pattern directly reflects the integrated effects of concentrated monsoonal precipitation, periodic agricultural irrigation, and shifting crop water consumption during the growing seasons^[20,21]. Vertically, a distinct moisture gradient emerged between the shallow and deep horizons. The mean SWC in the 0–10, 10–25, and 25–50 cm layers was 24.3%, 24.7%, and 24.6%, respectively, whereas the deep vadose zone (200–300 cm) was identified as a relative water-enriched stratum, averaging 29.7%. Across the management regimes, mean SWC followed the hierarchy of BRN (27.08%)>RF (26.91%)>CF (25.75%). The elevated moisture profile under the BRN treatment confirms that biochar co-application exerts a positive, functional effect on soil hydrological retention, thereby improving the soil water-holding capacity^[22].

Soil NO₃^‒-N concentrations exhibited extreme spatiotemporal heterogeneity, ranging from 3.17 to 354.23 mg/kg, with an overall mean of 17.60 mg/kg. Temporally, a critical accumulation phase occurred during the autumn and winter seasons, with mean NO₃^‒-N concentrations surging to a distinct peak of 33.30 mg/kg in November 2023. Vertically, NO₃^‒-N was overwhelmingly localized within the shallow upper root zones. The 10–50 cm depth interval functioned as the primary accumulation sink, recording mean concentrations of 37.58 mg/kg (10–25 cm) and 45.44 mg/kg (25–50 cm) before attenuating significantly with increasing profile depth. Crucially, the alternative management practices successfully mitigated nitrate accumulation; the mean NO₃^‒-N profile content followed the order of CF (18.97 mg/kg)> RF (17.52 mg/kg)> BRN (15.97 mg/kg). This confirms that both reduced nitrogen inputs and biochar amendments are highly effective strategies for depressing the overall nitrate burden in the vadose zone.

Although bulk NO₂^‒-N concentrations were orders of magnitude lower than nitrate, their vertical distribution and temporal evolution were remarkably synchronous with NO₃^‒-N. Elevated NO₂^‒-N activities were strictly confined to the 10–50 cm layer and peaked concurrently in November 2023. This distinct synchronization highlights an intensified phase of active nitrogen accumulation and microbial transformation within the shallow soil profile during the post-harvest period. Among the treatments, conventional fertilization (CF) consistently yielded the highest NO₂^‒-N contents, whereas levels were comparatively suppressed under both the RF and BRN regimes.

Table 2  Statistical summary of soil profile parameters under different fertilization treatments across various sampling periods

Crucially, NO₃^‒-N and NO₂^‒-N exhibited a highly significant positive correlation, demonstrating robust spatiotemporal synchrony across the soil profile. This geochemical coupling was exceptionally pronounced during the November 2023 accumulation peak within the 10–50 cm stratum, capturing an intensified phase of active nitrogen biotransformation in the shallow subsurface. In stark contrast, bulk soil water content was largely decoupled from the distribution of both inorganic nitrogen forms, indicating that physical moisture gradients are not the dominant drivers of nitrogen migration^[23]. The pronounced spatiotemporal heterogeneity of the vadose nitrogen pool is co-regulated by a complex, coupled interplay of anthropogenic and biogeochemical factors—namely, fertilization phenology, crop assimilation, intrinsic soil texture, localized microbial cycling^[15,24], and the episodic redistribution of nutrients triggered by routine precipitation and agricultural irrigation^[25].

Figure 2  Dynamic changes of soil water content, NO₃^‒-N and NO₂^‒-N in the soil profile under different fertilization treatmentsf

5 Discussion and Conclusion

Based on a long-term field monitoring experiment under 3 fertilization management practices conducted from June 2023 to September 2024, this study systematically analyzed the dynamic changes of nitrogen in the soil profile of agricultural areas with high nitrogen accumulation in western Baiyangdian. Soil texture, water content, and nitrate and nitrite nitrogen contents were measured. This dataset can provide data support for identifying nitrogen leaching risks in farmland in western Baiyangdian, optimizing nitrogen-reduction fertilization, and constructing water and nitrogen transport models for the vadose zone.

Based on the systematic analysis of the empirical dataset, the findings reveal that soil nitrogen within the high-accumulation farmlands west of Baiyangdian Wetland is overwhelmingly dominated by NO₃^‒-N. Across the 0–400 cm profile, nitrate is predominantly concentrated in the upper 100 cm layer, posing a pronounced risk of deep vertical leaching under CF practices. Among the varied management regimes, the intensity of nitrogen accumulation followed a clear hierarchy: CF>RF>BRN. This effectively demonstrates that both reduced exogenous nitrogen input and biochar co-application serve as viable functional interventions to mitigate inorganic nitrogen accumulation and alleviate potential leaching risks. Moving forward, subsequent research should integrate multidimensional variables—including precipitation, irrigation regimes, fertilization rates, crop nitrogen uptake, and groundwater depth—to further quantitatively evaluate the impacts of diverse agricultural practices on nitrogen leaching and associated aquatic environmental risks.

Author Contributions

Wang, S. Q. conducted the overall design of the dataset. Liu, B. X. established the field experiment. Lv, Q. conducted the sample collection and laboratory experiments, processed the data, compiled the dataset, and wrote the data paper. Tan, K. D. and Zheng, W. B. supervised and assisted in sample collection.

Conflicts of Interest

The authors declare no conflicts of interest.

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