Biochar Placement Depth Affects Nitrogen Use Efficiency under Fertilizer Deep Placement in Chinese Cabbage

Introduction

Nitrogen (N) is an essential nutrient for crop growth and yield formation and is a critical fertilizer component for securing agricultural productivity. Despite substantial fertilizer inputs, however, only approximately 25–50% of applied nitrogen is recovered by crops [1]. The unrecovered fraction is lost through ammonia volatilization, nitrate leaching, and denitrification, resulting not only in low fertilizer use efficiency but also in significant environmental pollution [2]. In the Republic of Korea, nitrogen fertilizer application per unit cropland area is nearly twice the global average, intensifying both environmental and economic burdens associated with nitrogen losses. Therefore, improving nitrogen use efficiency (NUE) while maintaining stable crop productivity is essential for sustainable agriculture.

Among the agronomic strategies proposed to enhance NUE, fertilizer deep placement has received increasing attention. Fertilizer deep placement involves placing fertilizer at a depth of approximately 25–30 cm below the soil surface, thereby reducing exposure to rapid fluctuations in surface environmental conditions. By supplying nutrients closer to the active root zone and minimizing losses through volatilization and leaching, fertilizer deep placement can reduce nitrogen losses and improve crop nitrogen uptake and yield [3, 4]. In addition, suppression of weed growth under deep placement has been reported to further contribute to improvements in NUE [5, 6].

Biochar has also emerged as a promising soil amendment for improving nitrogen dynamics in agricultural soils. Biochar, produced through pyrolysis of biomass at temperatures exceeding 350℃, is characterized by a porous structure and high specific surface area. These properties can enhance the retention of inorganic nitrogen in soil, thereby reducing nitrogen losses and improving NUE [7, 8]. Biochar applications have also been shown to indirectly enhance nutrient uptake by improving soil physical properties and promoting root development [9].

Although both fertilizer deep placement and biochar applications have independently been shown to improve NUE by reducing nitrogen losses and enhancing soil nitrogen retention, their combined effects remain insufficiently understood. In particular, limited attention has been given to how biochar placement depth influences its interaction with deep-placed fertilizer under field-applicable management conditions. Because the spatial proximity between fertilizer and biochar may affect nutrient availability and root accessibility, optimizing their placement strategy is critical to maximizing synergistic effects. Therefore, this study evaluated the effects of biochar application at different soil depths under fertilizer deep placement on crop growth and NUE.

Results

Growth response of Chinese cabbage

The effects of fertilizer placement and biochar co-application on the biomass and growth characteristics of Chinese cabbage are presented in Table 1. Shoot fresh weight and dry weight were significantly increased in all nitrogen treatments compared with the control (p<0.05). Among nitrogen treatments, shoot fresh and dry weights were similar between top fertilization and deep fertilization treatments. However, under deep fertilization conditions, the treatment with biochar applied to the topsoil showed the highest shoot fresh weight (383.3 g) and dry weight (24.7 g). In contrast, when biochar was applied at the same depth as fertilizer, shoot fresh weight (337.0 g) and dry weight (22.1 g) were relatively lower, although the difference between the two treatments was not statistically significant (p>0.05). In reduced nitrogen treatments, shoot fresh and dry weights were generally decreased, and no significant difference was observed according to biochar placement depth (p>0.05). Root fresh and dry weights showed less variation than shoot biomass. In some nitrogen treatments, root biomass tended to be greater than in control. Root fresh weight did not differ significantly between biochar placement treatments under deep fertilizer application (p>0.05). In contrast, root dry weight was significantly higher when biochar was incorporated at the same depth as the fertilizer compared with topsoil application (p<0.05). A similar pattern was observed under reduced fertilization; however, the differences were not statistically significant (p>0.05). Plant height and head width ranged from 28.2 to 31.2 cm and 12.5 to 14.2 cm, respectively. Both parameters were significantly greater in all nitrogen treatments than in the control, whereas no significant differences were detected among nitrogen treatments (p>0.05).

Nitrogen uptake and use efficiency of Chinese cabbage

The effects of fertilizer placement and biochar co-application on plant nitrogen concentration, nitrogen uptake, and shoot NUE are presented in Table 2. Shoot total nitrogen concentration was significantly higher under deep fertilizer placement (3.50%) than under conventional top fertilization (3.28%) (p<0.05). Although nitrogen uptake and shoot NUE were numerically greater under deep placement than under top fertilization, these differences were not statistically significant (p>0.05). Between biochar co-applied treatments, the response varied depending on placement depth of biochar. The top biochar treatment under fertilizer deep placement resulted in the highest nitrogen uptake (0.57 g plant⁻¹) and NUE (31.7%) among all treatments (p<0.05). In contrast, co-placement of biochar at the same depth as the fertilizer numerically reduced nitrogen concentration, uptake, and shoot NUE compared with fertilizer deep placement alone, while the difference was not statistically significant (p>0.05). Under reduced nitrogen conditions, shoot N concentration and N uptake generally decreased, while shoot NUE showed a less pronounced reduction depending on biochar placement depth.

Physicochemical properties of soil at harvest

The physicochemical properties of soil after cultivation are presented in Table 3. Soil pH, EC, available phosphorus, and total carbon showed significant differences among treatments (p<0.05). Soil pH ranged from 7.38 to 7.82 and available phosphorus ranged from 468.2 to 474.8 mg kg⁻¹. Compared with the initial soil (Table 4), no substantial differences in overall soil properties were observed across all treatments.

Among inorganic nitrogen forms, NO₃⁻ concentration was significantly higher in nitrogen treatments compared with the control (p<0.05), with the highest value (5.31 mg kg⁻¹) observed in the deep fertilization treatment. In contrast, NH₄⁺ concentration ranged from 5.8 to 6.67 mg kg⁻¹ in nitrogen treatments and was significantly lower than in the control (7.90 mg kg⁻¹) (p<0.05).

Discussion

In this study, fertilizer deep placement tended to increase root fresh and dry weights compared with top fertilization (Table 1), but this enhancement did not translate into significant improvements in shoot biomass or NUE (Table 2). This may be because localized N placement stimulated root growth near the fertilizer zone [10], but the resulting root response was not sufficient to enhance shoot biomass and N uptake under the present pot conditions [11]. In contrast, the effect of biochar co-application differed depending on its placement depth. When biochar was applied to the topsoil under deep fertilization, shoot growth and NUE tended to improve compared with deep fertilization alone. Biochar can improve soil pore structure and enhance water retention and aeration due to its porous nature [12], which can alleviate physical constraints in the rhizosphere and promote plant growth [13]. However, soil physical properties could not be reliably measured under the pot-scale experiment; thus, the contribution of biochar-induced physical improvement remains tentative and should be verified under field conditions.

Considering that changes in soil chemical properties, including inorganic nitrogen concentration, pH, and EC, were relatively minor in this study (Table 3), the enhanced nitrogen uptake and NUE observed under top biochar application are more likely attributable to improvements in soil–root interactions than to direct chemical changes in soil properties. By contrast, when biochar was co-placed with fertilizer at the same deep layer, root development was stimulated but did not result in corresponding increases in shoot biomass. This pattern may be associated with the high adsorption capacity of biochar, which can retain inorganic N near the fertilizer placement zone and thereby reduce its immediate availability for shoot N uptake [14]. This tendency was also observed under reduced nitrogen input, although the response varied depending on biochar placement depth. Under limited N supply, inorganic N retained by biochar may be less readily available for plant uptake, which could partly explain the weaker shoot N uptake and NUE responses. This interpretation is consistent with previous findings that biochar adsorption may reduce plant-available nitrogen under limited nitrogen supply conditions [8, 15].

The findings indicate that the effectiveness of biochar co-application under deep fertilizer placement depends on both placement depth and nitrogen supply level. Therefore, optimizing both fertilizer rate and biochar placement is necessary to enhance crop growth and NUE, rather than simple simultaneous application. Additionally, the relatively high initial soil pH may have limited the liming effect of biochar compared with its potential effect in acidic soils. Thus, the response to biochar placement may vary under different soil pH conditions or when biochar with different physicochemical properties is used. Biochar properties vary depending on feedstock materials and pyrolysis conditions; therefore, further studies using biochar with contrasting properties are needed to clarify how biochar characteristics influence plant N uptake and NUE under different placement strategies.

Conclusion

This study evaluated the effects of biochar placement depth under fertilizer deep placement on the growth and NUE of Chinese cabbage. Topsoil application of biochar enhanced nitrogen uptake and NUE, whereas deep co-placement of biochar with fertilizer showed limited effects on shoot growth and NUE. These responses varied depending on nitrogen supply level. The results suggest that the interaction between fertilizer placement and biochar location plays an important role in determining crop nitrogen utilization under deep fertilization systems. Therefore, optimizing the spatial arrangement of biochar relative to deep-placed fertilizer may be an effective management strategy to improve NUE and reduce nitrogen losses in agricultural soils. These findings provide a scientific basis for developing management strategies to enhance NUE while reducing environmental burdens in agricultural systems.

Materials and Methods

Experimental Materials

Surface soil (0–20 cm) was collected from an upland field at the National Institute of Agricultural Sciences (Wanju, Republic of Korea). The collected soil was air-dried, homogenized by gentle crushing, and passed through a 2-mm sieve prior to the experiments. A commercial wood-derived biochar, manufactured using an indirectly heated screw kiln system, was obtained from Ocean & Farm Tech (Hampyeong, Republic of Korea) [16]. The physicochemical properties of the initial soil and biochar are summarized in Tables 4 and 5, respectively.

Pot experiments

The pot experiment was conducted in a glasshouse at the National Institute of Agricultural Sciences. Wagner pots (1/3000 a) were filled with 9 kg of prepared soil to depths of approximately 25 cm. The pots were equipped with drainage holes at the bottom to allow free drainage of excess water and prevent waterlogging. The experiment was arranged in a completely randomized design with three replicates per treatment. Chinese cabbage (Brassica rapa L.) was transplanted at one plant per pot and grown for approximately 12 weeks. Soil moisture was maintained through daily irrigation, resulting in an average moisture content of approximately 30% (w/w). Fertilizer application rates were determined according to the standard fertilization guidelines of the Rural Development Administration, equivalent to N–P₂O₅–K₂O=32–16.6–19.8 kg 10a⁻¹. Nitrogen fertilizer was applied in three split applications, consisting of one basal application and two topdressings at 15-day intervals after transplanting. For practical pot-scale application, each N application was rounded to 0.4 g N pot⁻¹. Thus, full-N treatments received a total of 1.2 g N pot⁻¹.

Three fertilizer placement regimes were established: (i) conventional top fertilization (TF), in which basal fertilizer was incorporated into the topsoil (topsoil depth was 5−10 cm); (ii) deep fertilizer placement (DF), in which basal fertilizer was applied at 25 cm depth; and (iii) reduced deep fertilizer placement (RDF), which was identical to DF except that the second topdressing was omitted, resulting in a total N input of 0.8 g N pot⁻¹, approximately 33% lower than the full-N treatments. Biochar was applied at a rate equivalent to 200 kg 10a⁻¹ (dry weight basis). Depending on placement depth, biochar was either incorporated into the topsoil layer (TB) or applied at the same 25 cm depth as the deep-placed basal fertilizer (DB). A non-fertilized control without fertilizer or biochar application (NF) was included. Treatment combinations are summarized in Table 6.

Physicochemical analysis

The pH and electrical conductivity (EC) of soil and biochar were measured in a 1:5 (w/v) sample to deionized water suspension. Total carbon (TC) and total nitrogen (TN) of soil, biochar and plant samples were analyzed using a CN analyzer (Vario MACRO cube, Elementar, Germany). Available phosphorus (Av. P₂O₅) was extracted by the Lancaster method and measured colorimetrically at 720 nm. Exchangeable cations were extracted with 1 M ammonium acetate (pH 7.0) and analyzed by ICP-OES. Soil inorganic nitrogen was extracted with 2 M KCl and measured by ion chromatography.

At harvest, Chinese cabbage plants were separated into shoots and roots, oven-dried at 70℃ for 48 h, and dry weights were measured. Nitrogen uptake was calculated for each replicate using plant dry weight and nitrogen concentration, and treatment means were obtained from replicate values. Shoot NUE was determined using the following equation [17]

where U_treated is shoot nitrogen uptake in fertilized plants, U_control is shoot nitrogen uptake in control, and N_applied is the amount of nitrogen applied.

This NUE value represents apparent shoot N recovery based on shoot N uptake and does not directly quantify N losses through leaching, volatilization, or denitrification.

Statistical analysis

Differences among treatments were analyzed using one-way analysis of variance (ANOVA). When significant differences were detected (p<0.05), means were compared using Tukey’s HSD test. Statistical analyses were performed using R Studio.

Data Availability: All data are available in the main text or in the Supplementary Information.

Author Contributions: Shin Y: Writing-original draft, Visualization, Oh BW: Conceptualization, Investigation, Data curation, Park HR: Investigation, Visualization, Data curation, Lee M: Investigation, Visualization, Lee HS: Investigation, Visualization, Kim J: Writing-review & editing, Jeong YJ: Data curation, Writing-original draft, Conceptualization, Supervision.

Notes: The authors declare no conflict of interest

Acknowledgments: This study was carried out with the support of the “Field test of application of deep fertilizer and biochar in converted cropland (Project No.: PJ01677841)” and “2026 the RDA Fellowship Program” of National Institute of Agricultural Sciences, Rural Development Administration of the Republic of Korea.

Additional Information:

Supplementary information The online version contains supplementary material available at https://doi.org/10.5338/KJEA.2026.45.09

Correspondence and requests for materials should be addressed to Young-Jae Jeong.

Peer review information Agricultural and Environmental Sciences thanks the anonymous reviewers for their contribution to the peer review of this work.

Reprints and permissions information is available at http://www.korseaj.org

Tables & Figures

Table 1. Growth characteristics and biomass of Chinese cabbage

²Data represent the mean of three replicates, and different letters within each column indicate significant differences among treatments according to Tukey’s HSD test (p<0.05). ¹NF, no fertilizer; TF, top fertilizer; DF, deep fertilizer; TB, top biochar; DB, deep biochar; RDF, reduced deep fertilizer.

Table 2. Nitrogen concentration, nitrogen uptake and shoot nitrogen use efficiency of Chinese cabbage

¹NF, no fertilizer; TF, top fertilizer; DF, deep fertilizer; TB, top biochar; DB, deep biochar; RDF, reduced deep fertilizer. ²Shoot NUE was calculated using shoot N uptake and the actual N input for each treatment. ³Data represent the mean of three replicates, and different letters within each column indicate significant differences among treatments according to Tukey’s HSD test (p<0.05).

Table 3. Physicochemical properties of soil at harvest

¹NF, no fertilizer; TF, top fertilizer; DF, deep fertilizer; TB, top biochar; DB, deep biochar; RDF, reduced deep fertilizer. ²Data represent the mean of three replicates, and different letters within each column indicate significant differences among treatments according to Tukey’s HSD test (p<0.05).

Table 4. Physicochemical properties of initial soil

Table 5. Physicochemical properties of biochar

Table 6. Description of experimental treatments

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