Crop Field Level Estimation of Nitrogen Input from Fertilizer Use in Jeju Island, South Korea: Management Methods to Prevent Groundwater NO 3 -N Contamination

: The application of synthetic nitrogen (N) fertilizers has boosted crop yields globally. However, it has also imposed on environmental pollution problems. An estimation of actual fertilizer N inputs at the crop ﬁeld level is needed to establish effective N management plans to control groundwater NO 3 -N contamination. Here, a survey to collect the types of cultivated crop and fertilizer application rate was conducted during 2016–2018, covering 44,253 small crop ﬁelds (7730 ha) in the western part (Hanrim and Hankyung regions) of Jeju Island, South Korea. Foreign vegetables, citrus fruits, and bulb vegetables are the major crop types grown in the total cultivated areas of 2165.6 ha, 1718.7 ha, and 944.9 ha, respectively. For several crops (green garlic, potato, and chives), the over-use of N fertilizers is observed, the amount of which is 1.73–4.95 times greater than the standard fertilizer application rate. The highest level of fertilizer N input is observed for bulb vegetables in both the regions (Hanrim: 500.5 kg/ha, Hankyung: 487.1 kg/ha), with nearly 80% of the N fertilizer input turned into surplus N loading. A comparison between a spatial interpolation map of the fertilizer N input and that of the groundwater NO 3 -N concentration implies that the excessive use of synthetic fertilizer results in the degradation of groundwater quality by NO 3 -N. N management plans for the study area are suggested based on the N fertilizer input at the crop ﬁeld level. This study highlights that sustainable N management plans should be arranged at the crop ﬁeld level, considering the spatial heterogeneity of N fertilizer use.


Introduction
Nitrogen (N) is an essential nutrient for plant growth. The mass production of synthetic N fertilizers was initiated using the Haber-Bosch process in the early 1900s, which resulted in a significant increase in crop yield. Subsequently, the global N fertilizer consumption considerably increased from 11.3 Tg N/year in 1960 to 107.6 Tg N/year in 2013 [1]. Although the development of synthetic fertilizers has resulted in a tremendous increase in crop productivity, it has negatively influenced the ecosystems. As crops only use approximately half of the N fertilizer with a 0.45 global nitrogen use efficiency (NUE) [2], the remainder of the applied fertilizer remains in the soil and groundwater, causing adverse environmental problems such as eutrophication [3,4], greenhouse gas emissions [5,6], soil acidification [7], and NO 3 -N pollution in surface water and groundwater [8][9][10].
During the migration of N fertilizers through the unsaturated zone, surplus N is rapidly transformed into NO 3 − through nitrification. As NO 3 − is easily dissolved in water On Jeju Island, only commercial fertilizer sales for each administrative district exist. To our knowledge, there has been no effort to acquire N fertilizer input information in actual crop fields. More reliable and effective management methods against NO 3 -N contamination of groundwater on Jeju Island should be based on fertilizer N input data analyzed at the crop field level. Therefore, the objectives of this study are to (1) survey cultivated crop types and the amount of fertilizer use in the Hanrim and Hankyung regions (located in the western part of Jeju, where severe NO 3 -N contamination in groundwater has been reported), (2) estimate the crop field level N input and surplus N loading, and (3) suggest N management plans considering the spatial variability of N fertilizer input. The construction of the crop field level N input would help elucidate the actual level of N fertilizer over-use and provide information on groundwater management measures that need to be implemented, at the local level, against NO 3 -N contamination.

Hydrogeologic Characteristics of Jeju Island
Jeju Island is the largest island in South Korea, with a total area of 1850 km 2 . The island was formed by multiple volcanic eruptions (from the Late Pliocene to Quaternary), and various features of volcanic rocks (joints, lava tubes, scoria, and clinkers) are observed throughout the island [37]. The porous volcanic rocks that cover most of the island surface induce easy percolation of precipitation into the subsurface medium [38]. The annual precipitation in Jeju averaged for the past 30 years (1991-2020) was 1745.9 mm/year [39], which is 1.34 times higher than that in the Seoul metropolitan area (1303.05 mm/year). These hydrogeological features, characterized by an abundant rainfall amount and porous volcanic rocks, have been attributed to increased groundwater recharge (40.58% of total precipitation; [40]). Owing to the high groundwater recharge rate and lack of surface water resources, water use in Jeju heavily relies on groundwater resources (81.4% of total water use; [40]). Simultaneously, the hydrogeological and anthropogenic features of the island (high permeability of the volcanic aquifer, intensive agricultural activity, and increasing groundwater use) make groundwater highly vulnerable to contamination by surface pollutants.

Agricultural Activities in the Study Area
The Hanrim and Hankyung regions, the site of this study, are located in the western part of Jeju Island ( Figure 1). After the readjustment of arable land, performed through the 1960s-1970s [41], agricultural practices on the island were concentrated in the study area. In the Hanrim and Hankyung regions, the agricultural land contributed to the largest proportion of the total surface area (Hanrim: 42.6 km 2 of 90.8 km 2 , 46.9%; Hankyung: 43.2 km 2 of 79.3 km 2 , 54.4% as of 2015). On Jeju Island, two to three crops a year, in general, are grown; major cultivated crops include onion, cabbage, white radish, garlic, and sesame [9]. The amount of synthetic N fertilizer in 2018 was 841 t/year for the Hanrim region and 1164 t/year for the Hankyung area, accounting for 5.6% and 7.7% of the total N fertilizer sales on the island, respectively [28].

The NO 3 -N Contamination in Groundwater of the Study Area
The average NO 3 -N level in groundwater of the Hanrim and Hankyung regions (4.93 mg/L and 10.00 mg/L, respectively) was reported to be greater than 1.95 mg/L of the baseline concentration of NO 3 -N in Jeju [40]. Previous studies also found severe groundwater contamination by NO 3 -N in the study area, with NO 3 -N levels exceeding the maximum contamination level (MCL, NO 3 -N drinking water standard of 10 mg/L) [9,30,42,43]. Using a N stable isotope analysis, the major source of NO 3 -N contamination was identified as synthetic fertilizers applied in agricultural fields [9,30,42]. Additionally, groundwater contamination by animal wastes was partly observed in the Hanrim area, where livestock farms are predominantly distributed ( Figure 1) [42,43].

The NO3-N Contamination in Groundwater of the Study Area
The average NO3-N level in groundwater of the Hanrim and Hankyung regions (4.93 mg/L and 10.00 mg/L, respectively) was reported to be greater than 1.95 mg/L of the baseline concentration of NO3-N in Jeju [40]. Previous studies also found severe groundwater contamination by NO3-N in the study area, with NO3-N levels exceeding the maximum contamination level (MCL, NO3-N drinking water standard of 10 mg/L) [9,30,42,43]. Using a N stable isotope analysis, the major source of NO3-N contamination was identified as synthetic fertilizers applied in agricultural fields [9,30,42]. Additionally, groundwater contamination by animal wastes was partly observed in the Hanrim area, where livestock farms are predominantly distributed ( Figure 1) [42,43].

Surveying the Application Rates of N Fertilizers and Types of Cultivated Crops
The N fertilizer input in the Hanrim and Hankyung study areas was estimated through a two-step survey. Farm data (crop type and fertilizer use) of some agricultural areas (226 ha) in the Hankyung region ( Figure 1) were obtained from the 1st step of the survey. For the entire study area (7730 ha), cultivated crop types were investigated through the 2nd step survey. Then, total N input by the fertilizer uses in the entire study area calculated by applying the farm data obtained from the 1st step of the survey for the same crops over the entire study area (the 2nd step survey).
The 1st step of the survey was conducted from December 2016 to September 2018 by interviewing the farmers using a questionnaire (Table S1), which covered 185 farmhouses corresponding to 925 crop fields (the 1st-6th surveys were conducted for the same crop field). The questionnaire surveying the individual crop field information included the crop name, harvest period, crop yield, harvest labor, non-cultivation period, and chemical fertilizer usage.

Surveying the Application Rates of N Fertilizers and Types of Cultivated Crops
The N fertilizer input in the Hanrim and Hankyung study areas was estimated through a two-step survey. Farm data (crop type and fertilizer use) of some agricultural areas (226 ha) in the Hankyung region ( Figure 1) were obtained from the 1st step of the survey. For the entire study area (7730 ha), cultivated crop types were investigated through the 2nd step survey. Then, total N input by the fertilizer uses in the entire study area calculated by applying the farm data obtained from the 1st step of the survey for the same crops over the entire study area (the 2nd step survey).
The 1st step of the survey was conducted from December 2016 to September 2018 by interviewing the farmers using a questionnaire (Table S1), which covered 185 farmhouses corresponding to 925 crop fields (the 1st-6th surveys were conducted for the same crop field). The questionnaire surveying the individual crop field information included the crop name, harvest period, crop yield, harvest labor, non-cultivation period, and chemical fertilizer usage.
For the 2nd step of the survey, information on the types of cultivated crops for each agricultural field in the entire study area was acquired using a field survey in November 2018. Considering that cultivating two to three crops a year is a typical agricultural practice on Jeju Island, the latter survey covered the N fertilizer input in the second half of 2018 (September 2018 to February 2019). The information on cultivated crops in the total 44,253 crop fields was collected through the 2nd step of the survey.

Estimation of Surplus N Loading
We estimated the surplus N loading to evaluate the influence of the excessive use of fertilizer on the aquifer system. The surplus N loading could be calculated using Equation (1) as follows: N surp = N fer + N soil − N uptake (1) where N surp is the surplus N loading (kg/ha), N fer is the N input from the fertilizer (kg/ha), N soil is the residual N amount of the soil layer (kg/ha), and N uptake is the amount of N adsorbed by crops (kg/ha) in the cultivated field. In Equation (1), N refers to the total nitrogen. Oh et al. [44] estimated the residual amount of N in the soil layer in the Hankyung agricultural area using the measured NO 3 -N concentrations from the lysimeters and the soil water content. In their study, the N soil was calculated as 43 N kg/ha (±18.4 N kg/ha), 35 N kg/ha (±10.7 N kg/ha), 62 N kg/ha (±14.0 N kg/ha) at 0.4 m, 0.7 m, 1.0 m depth of the installed lysimeters, respectively. We used the N soil at 0.4 m to calculate the N surplus loading, considering the average root zone depth of the major crops.
Referenced data for the N uptake amount by crop type were collected from the literature survey and are listed in Table S2. We collected uptake data for 12 crops, including white radishes, barley, garlic, broccoli, cabbage, kohlrabi, onions, potatoes, dry-field rice, chives, wheat, and tomatoes [45][46][47][48][49][50][51][52]. For the red cabbage and Brussel sprouts, the amount of N uptake was assumed to be the same as that for cabbage (Table S2). For several crops that did not have a reference N uptake value (citrus fruits, sesame, beans, blueberries, pumpkins, and deodeok), we used the average value of the collected N uptake (138 N kg/ha ± 30.1 N kg/ha). Table 1 contains information on the cultivated crops, cultivation area, fertilizer usage amount for each crop obtained from the survey in the Hankyung area, and the ratio of the N fertilizer use divided by the standard N fertilizer use (F actual /F standard ). The standard N fertilizer usage for each crop type provided by the agricultural research and extension services of the Jeju Special-Self Province. A F actual /F standard ratio greater than one indicates excessive use of chemical fertilizers.

Surveyed Information of N Fertilizer Usage by Cultivated Crops
A total of 39 crops was cultivated in the surveyed area receiving 168,932 kg of synthetic N fertilizer. White radish was the most often cultivated crop in 709 crop fields, with a cultivated area of 168 ha, followed by barley and garlic in 541 and 396 crop fields (139 ha and 89 ha, respectively). Green garlic (495 kg/ha), Brussel sprout (368 kg/ha), chives (364 kg/ha), and onion (346 kg/ha) showed the highest amounts of chemical fertilizers. Sesame (55 kg/ha), house citrus (55 kg/ha), bean (61 kg/ha), and wheat (34 kg/ha) revealed a relatively low application rate of chemical fertilizer per unit area. In Figure 2, green garlic accounted for the highest ratio of F actual /F standard (4.95), nearly five times greater than the standard fertilizer amount. Additionally, the potato and chives showed considerably large F actual /F standard ratios of 3.29 and 1.73, respectively.

Cultivated Crops in the Hanrim and Hankyung Areas
To estimate the spatially varying N input distribution at the crop field level, a GIS map of the cultivated crop for the second half of 2018 was delineated over the study site ( Figure 3). During this process, 68 different crops were categorized into nine major crop types, including citrus fruits, foreign vegetables, bulb vegetables, food crops, green vegetables, root vegetables, fruit vegetables, special-use crops, and other crops. Details of the cultivated crops and major crop types are provided in Table 2. In the entire study area, foreign vegetables accounted for the largest cultivation area, with 2165.6 ha (29.0% of the total area), followed by citrus fruits (1718.7 ha, 23.0%) and bulb vegetables (944.9 ha,

Cultivated Crops in the Hanrim and Hankyung Areas
To estimate the spatially varying N input distribution at the crop field level, a GIS map of the cultivated crop for the second half of 2018 was delineated over the study site ( Figure 3). During this process, 68 different crops were categorized into nine major crop types, including citrus fruits, foreign vegetables, bulb vegetables, food crops, green vegetables, root vegetables, fruit vegetables, special-use crops, and other crops. Details of the cultivated crops and major crop types are provided in Table 2. In the entire study area, foreign vegetables accounted for the largest cultivation area, with 2165.6 ha (29.0% of the total area), followed by citrus fruits (1718.7 ha, 23.0%) and bulb vegetables (944.9 ha, 12.7%). In the Hanrim area, the number of foreign vegetables (1262.3 ha) was higher than that in the Hankyung area (903.3 ha). In the Hankyung, the citrus fruits (1082.2 ha) and bulb vegetables (652.6 ha) were the major cultivated crops compared to those in the Hanrim region (citrus fruits: 636.5 ha, bulb vegetables: 292.3 ha). Additionally, the root vegetables were intensively cultivated in Hankyung (Hanrim: 15.1 ha, Hankyung: 252.6 ha). The food crops (Hanrim: 431.3 ha, Hankyung: 421.7 ha) showed similar occupancy for both sites in terms of the total cultivation area.
The citrus fruit fields were mainly located throughout the Hankyung region and western parts of Hanrim ( Figure 3). The foreign and bulb vegetables were cultivated in the low elevation area along the seaside, and the green vegetable fields were widely distributed on the seaside of the northern part of the Hanrim. Food crops were majorly raised in the central regions of Hanrim and Hankyung. Root vegetables, such as white radish and carrot, were the major crops grown in the western part of Hankyung. The special-use crop field was mostly concentrated in the seaside zone between the Hanrim and Hankyung regions, whereas the fruit vegetable fields were scattered across the entire study area.

Crop Field Level N Input and Surplus N Loading
The estimated N input from the cultivated fields of the study area (Figure 4a) ranged from 0.0 to 1584.6 kg, with an average value of 45.1 kg. A high N input, greater than 272.0 kg, was mainly located in the southern part of Hankyung and showed a scattered distribution across the Hanrim area. The total N input from the Hankyung area (1131.9 t) was 1.3 times higher than that from the Hanrim area (862.3 t), because of the larger distribution of total cultivated land in Hankyung (Table 3). In both study regions, a significant N input was For the unit areal N input, a large quantity of fertilizer N was applied on the foreign and bulb vegetables in both regions (foreign vegetables: 380.9 kg/ha in Hanrim, 386.7 kg/ha in Hankyung; bulb vegetables: 500.5 kg/ha in Hanrim, 487.1 kg/ha in Hankyung). The fertilizer N input was estimated based on data from the second half of 2018. Therefore, an approximately 2-3 times greater fertilizer N input might be loaded into the subsurface because of the double-triple cropping practice for 1 year.
The estimated surplus N loading (Figure 4b) showed a spatial distribution similar to that of N inputs. Smaller amounts of surplus N loading (0.0-1339.7 kg, average: 29.7 kg) than those of the N input were generally observed because of N removal by crop uptake. Comparisons between the estimated N input and surplus N showed that the cultivated crops absorbed approximately 30-40% of N from the applied fertilizer, indicating that the remaining 60-70% of the applied N would infiltrate into the subsurface and cause groundwater pollution (Table 3). Additionally, Table 3  bution across the Hanrim area. The total N input from the Hankyung area (1131.9 t) was 1.3 times higher than that from the Hanrim area (862.3 t), because of the larger distribution of total cultivated land in Hankyung (Table 3). In both study regions, a significant N input was estimated for foreign vegetables (Hanrim: 480.8 t, Hankyung: 349.3 t), citrus fruits (Hanrim: 177.3 t, Hankyung: 318.1 t), and bulb vegetables (Hanrim: 146.3 t, Hankyung: 317.9 t).

Uses of Nitrogen Fertilizers, and NO 3 -N Contamination in Groundwater
In agricultural areas, intensive use of synthetic fertilizers is directly associated with groundwater quality degradation by NO 3 -N [9,11,16,53]. On Jeju Island, farmers tend to apply larger amounts of fertilizers than the standard fertilizer application rate, because of the highly permeable volcanic aquifer and thin thickness of the topsoil layer [54]. For example, the average N fertilizer usage amount (323.9 kg/ha) in the study site was 1.6 times higher than that of the N fertilizer sale amounts in South Korea (203.4 kg/ha per year, [27]), although the estimation from Table 3 covered only the second half period of the year. Additionally, the overall NUE at the study site (30-40%) was relatively low compared to that in the European countries (60-65%, [55]) and similar to the NUE in China (39%, [56]). The abundant surplus of N (average: 211.8 kg/ha) in the study area would travel to subsurface and induce NO 3 -N pollution in groundwater. In the crop fields in the North China Plain [11], nearly 56-91% of total N input (629-3656 kg/ha/yr) turned into the surplus N and it heavily contaminated the shallow groundwater with a maximum NO 3 -N level of 274.4 mg/L.
To estimate NO 3 -N concentration leaching into the subsurface aquifer caused by the surplus N loading in the study area, we assumed that most of the surplus N would dissolve in recharged rainwater and infiltrate the aquifer (average precipitation rate in the Hankyung area: 1396 mm, recharge rate: 36.5%, [40]). Significant losses of NO 3 -N by biogeochemical reactions were not considered because of the high DO (dissolved oxygen) levels of the groundwater samples (average: 9.0 mg/L, [31]) and the high permeability of the aquifer at the study site [38]. The results showed that a total of 1314.9 t of surplus N for the six months would reach the groundwater table, with an average NO 3 -N concentration of 33.5 mg/L, exceeding the MCL. In the agricultural land in The Netherlands [57], the surplus N loading of 223 kg/ha/year, which was similar to our study, elevated the groundwater NO 3 -N level over 12 mg/L and area with groundwater NO 3 -N exceeding the MCL was 29% of the total agricultural land. In the Hanrim and Hanyung areas, various studies have shown that the sources of severe NO 3 -N contamination in groundwater come from chemical fertilizers applied to agricultural fields, based on a N stable isotope analysis [9,30,32,42,58].
To estimate the spatial trend of N inputs throughout the study area, spatial interpolation was conducted using ArcGIS 10.6 [59]. The 100 × 100 m grids covering the study site were generated, and for each grid, the total N input estimated from Figure 4a was applied. As a result, we obtained a spatial interpolation map of the N input (Figure 5a). A higher application rate of N fertilizer was observed in the northern seaside area of Hanrim and across the entire Hankyung area (Figure 5a). A similar spatial pattern between the interpolation map of the N input and the groundwater NO 3 -N distribution map in 2018 (Figure 5b) was observed with a statistically significant correlation coefficient, R, of 0.32 (p-value < 0.01). An elevated NO 3 -N level in groundwater over 5 mg/L matched well with the area having a higher usage of the N input, which implied, again, that the intensive use of chemical fertilizer resulted in the enrichment of the NO 3 -N concentration in groundwater. Exceptionally high NO 3 -N concentrations, greater than 10 mg/L, were observed in the western part of Hanrim despite the relatively low N loadings in the area. This site is characterized by the intensive distribution of livestock farms (as shown in Figure 1), indicating that another NO 3 -N source from animal waste could affect the local groundwater quality in this area [42,43].

Crop Field Level N Management Strategy
Sustainable N management can be achieved by increasing NUE and crop production and lowering N losses [2,60]. In several European countries, where the Nitrate Directive has been enforced since 1991, decreasing N input trends in recent years have been ob-

Crop Field Level N Management Strategy
Sustainable N management can be achieved by increasing NUE and crop production and lowering N losses [2,60]. In several European countries, where the Nitrate Directive has been enforced since 1991, decreasing N input trends in recent years have been observed with increases in NUE and a stable crop yield [60,61]. For example, in Denmark, where the usage of agricultural N fertilizers is strictly regulated by the annual permission of N amounts at a farm scale, the average N surplus has been reduced from 170 kg/ha/year to below 100 kg/ha/year during the past 30 years and has brought a positive impact on the aquatic environment [62]. This demonstrates that an ultimate direction to relieve the NO 3 -N contamination may reduce N inputs from the over-use of synthetic fertilizers in agricultural areas. However, on Jeju Island, the management of NO 3 -N contamination in groundwater only focuses on the regulations of groundwater well installation near potential contaminant sources and the reorganization of poorly grouted wells [63]. Therefore, in this study, we suggested an effective N management strategy based on crop field level N input estimation.
To establish more efficient N management measures against groundwater contamination from the fertilizers, we categorized the N inputs at the crop field level into three management classes (MC1-MC3). EU countries regulate organic fertilizer use based on the criterion limiting N loading [64]. In this study, we adopted the EU criteria to delineate the management class based on N fertilizer inputs. First, we developed a histogram showing the total cultivated areas for each interval of the N input (<100 kg/ha/year, 100 -<170 kg/ha/year, 170 -<250 kg/ha/year, 250 -<300 kg/ha/year, 300 -<350 kg/ha/year, and ≥350 kg/ha/year), and assessed its cumulative percentage ( Figure 6). We defined the management class based on two inflection points, which corresponded to 170 kg/ha/year and 250 kg/ha/year. The management class 1 (MC1) regions were defined as cultivated fields receiving N input over 250 kg/ha/year. Class 2 (MC2) and class 3 (MC3) regions corresponded to the crop fields with a N input between 170 and 250 kg/ha/year and less than 170 kg/ha/year, respectively ( Figure 7).    The MC1 regions (total area: 1024 ha) are most likely to impose NO3-N pollution on groundwater; thus, the reinforced NO3-N management plans need to be implemented, including (1) a restriction on the cultivation of crops that require a high amount of ferti- The MC1 regions (total area: 1024 ha) are most likely to impose NO 3 -N pollution on groundwater; thus, the reinforced NO 3 -N management plans need to be implemented, including (1) a restriction on the cultivation of crops that require a high amount of fertilizer use, (2) an adjustment of the fertilizer application rate after analyzing the soil N content, (3) cultivating catch crops to uptake residual N in soils after the crop yield, (4) monitoring soil N content, and (5) raising awareness programs for farmers to reduce fertilizer use. For the MC2 regions (total area: 2745 ha), more relaxed management measures could be adopted, including (1) using the standard fertilizer amount, (2) cultivating catch crops, (3) monitoring soil N, and (4) raising awareness programs. For the MC3 regions (total area: 10,848 ha), we recommend applying the present regulations without additional measures if total N inputs do not show significant increases.
Efforts to reduce N losses from fertilizer use hardly achieve an immediate effect in a short time. The subsurface aquifer system shows a delayed response to changes in the surface environment, which can be explained by the concept of groundwater residence time [36]. In Jeju, the groundwater residence time is estimated to be between 2 and 53 years (average: 19 years) [65], indicating that several decades are required to relieve the NO 3 -N pollution in groundwater. This demonstrates that reduction in the N loading is an essential action to solve the NO 3 -N problems; thus, the N management plans on Jeju Island should be conducted from a long-term perspective. Moreover, under the crop cultivation system in Jeju, which is based on small agricultural fields, a reliable map of the crop field level N input could be developed for establishing effective NO 3 -N management plans by reflecting the local heterogeneity in fertilizer N use. This can be an administrative ground for farmers to understand better and follow the NO 3 -N management regulations.

Conclusions
We estimated the crop level application rate of synthetic N fertilizers and types of cultivated crops during the second half of 2018, in 44,253 small crop fields covering an area of 7730 ha through an intensive survey analysis. Sixty-eight crops were cultivated in the study area, receiving 1994.2 t (323.9 kg/ha) of N fertilizer input. The results showed that several types of crops (green garlic, potato, and chives) used a 1.73-4.95 times higher amount of N fertilizer than the standard fertilizer application rate. The cultivated crops in the study area consumed approximately 35% of the fertilized N input (112.1 kg/ha), and the remaining N (211.8 kg/ha) remained in the subsurface as the surplus N loading. A total of 1314.9 t of surplus N loading could degrade the groundwater quality by enriching the NO 3 -N level to 33.5 mg/L. This study also confirmed that the over-use of N fertilizer shows a similar spatial distribution to the groundwater NO 3 -N level, with statistical significance (R: 0.31, p-value < 0.01). To increase the NUE and decrease the surplus N, crop field level N management was suggested, in which different management plans were established based on the input amounts of N fertilizer. This study emphasized the importance of constructing actual farm data (a type of cultivated crop and the fertilizer application rate) at a crop field level to understand the behavior of fertilizer over-use and prepare proper measures to mitigate groundwater pollution induced by agricultural activities.