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Frontiers in Agronomy ·

Volume 8 - 2026 | https://doi.org/10.3389/fagro.2026.1929464

Biochar-blended urea enhances wheat yield by integrating fertilizer-nitrogen utilization and photosynthesis

  • Yanrui Wang1,2
  • Yunpu Zheng1
  • Rongjun Bian3
  • Fei Li1
  • Liang Liu1
  • Lihua Hao1
  • Lishu Wang1
  • Haikui Yin1
  • Zhipeng Wang1
  • Yabin Shen1*corresponding author
  • Wei Shi1*corresponding author
  1. 1School of Water Conservancy and Hydropower, Hebei University of Engineering, Handan, China
  2. 2School of Earth System Science, Tianjin University, Tianjin, China
  3. 3Institute of Resources, Ecosystem and Environment of Agriculture, and Center of Biochar and Green Agriculture, Nanjing Agricultural University, Nanjing, China
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Abstract

Excessive nitrogen (N) application poses environmental risks, making improved N use efficiency (NUE) critical for sustainable agriculture. Biochar-based fertilizers provide slow-release N, which can improve crop productivity and NUE. However, the physiological mechanisms linking N fate with plant performance remain poorly understood. Using 15N tracing, this study investigated the effects of biochar-blended urea fertilizer (BF) and conventional urea fertilizer (UF) on fertilizer-N uptake, photosynthetic performance, and grain yield of winter wheat grown in fluvo-aquic soil on the North China Plain under high-N and low-N application rates. Compared with UF, BF significantly increased wheat grain yield by 18% under the high-N rate and 30% under the low-N rate. BF also altered stomatal morphology and spatial distribution in wheat leaves, thereby enhancing stomatal regulation of gas exchange and improving photosynthetic performance, as evidenced by increased net photosynthetic rate (An), transpiration rate (Tr), dark respiration rate (Ra), water use efficiency (WUE), stomatal conductance (Gs), maximum carboxylation rate (Vcmax), and maximum electron transport rate (Jmax), particularly under the low-N rate. In addition, BF significantly enhanced fertilizer-N uptake by 26.4%–40.0% and its translocation to the grain by 29.4%–34.0% compared with UF. Enhanced fertilizer-N uptake was positively associated with photosynthetic performance, and both jointly contributed to increased wheat grain yield. These findings demonstrate that biochar-blended urea enhances fertilizer-N use efficiency and photosynthetic performance, providing an effective strategy to reduce chemical N fertilizer inputs while sustaining wheat productivity. However, long-term field trials are needed to verify the effectiveness and agronomic benefits of biochar-blended urea under practical agricultural conditions.

Keywords

1. Introduction

Nitrogen (N), a vital nutrient, plays a crucial role in plant growth and development and is commonly applied in the form of fertilizers in agriculture to support crop growth, development, ripening, and fruiting processes (Darenova et al., 2022). Approximately 33 million tonnes of N fertilizer are consumed annually in Chinese agriculture (Du et al., 2022; Zhang et al., 2023a), but only 72% of the applied N is effectively retained in or utilized by terrestrial ecosystems (Zhang et al., 2013). It has been reported that annually agriculture in China contributed approximately 24% of global soil N 2 O emissions during 2007–2016 (Yue et al., 2019). Excess reactive N inputs from fertilizers stimulate nitrification and denitrification, increasing the risks of N leaching, N 2 O emissions, and groundwater pollution, thereby threatening environmental quality and ecosystem sustainability (Zhang et al., 2023b).

Biochar, a carbon-rich material produced through the pyrolysis of biomass under oxygen-limited conditions, has been widely studied as a soil amendment because of its high stability, strong adsorption capacity, and recognized potential to improve soil fertility, mitigate climate change, and increase soil carbon storage (Lehmann, 2007; Woolf et al., 2010; Gao et al., 2021; Lehmann et al., 2021; Yang et al., 2022; Elkhlifi et al., 2023; Liu et al., 2023). Despite its positive impact on agricultural production, the high application rates and associated costs of biochar limit its widespread use in agriculture (Clare et al., 2015; Chew et al., 2020). Therefore, modifying biochar to enhance its functionality at lower application rates has attracted increasing attention, with approaches including the incorporation of one or more nutrients through direct mixing, encapsulation, pelleting, and other techniques (Kotaka, 2005; Magrini-Bair et al., 2009). Biochar-based fertilizers could enable N to bind to the biochar carbon matrix and thereby achieve a slow-release effect (Joseph et al., 2013; Shi et al., 2020), which could enhance crop yield and N use efficiency (NUE) compared with conventional chemical fertilizers while reducing greenhouse gas emissions (Joseph et al., 2013; Qian et al., 2014; Yao et al., 2015; Zheng et al., 2017). Application of biochar-based fertilizer increased wheat tiller number, thousand-kernel weight, biomass, and grain yield by 5.50%, 8.46%, 23.85%, and 23.42%, respectively, and increased grain N content (Zhang et al., 2018, 2018b). Similarly, several studies have shown that biochar-based fertilizers can improve yield and grain quality by enhancing N uptake (Shah et al., 2016; Yu et al., 2021; Li et al., 2022). However, other studies have reported limited or no effects of biochar-based fertilizer application on crop yield and grain quality (Akhtar et al., 2014; Silva Gonzaga et al., 2019; Zhu et al., 2019). Therefore, the mechanisms underlying the effects of biochar-based fertilizers on crop productivity remain unclear and require further investigation.

It is noteworthy that the slow-release characteristics of biochar-based fertilizers not only mitigate N losses but also ensure a sustained N supply during later growth stages (Shi et al., 2020, 2022). This sustained N subsequently modulates the activity of key photosynthetic enzymes and N allocation within the photosynthetic apparatus, thereby influencing gas exchange, a fundamental process regulating plant growth and carbon assimilation (Ainsworth and Rogers, 2007; Zhang et al., 2011, 2020; Fan et al., 2020). Gas exchange processes, including photosynthesis, dark respiration, and transpiration, play essential roles in plant carbon assimilation, water regulation, and biomass accumulation. Among these processes, photosynthesis is a fundamental determinant of plant productivity and directly influences plant growth and biomass production (Song et al., 2012; Zlatev and Lidon, 2012; Wu et al., 2018). Nitrogen allocation within plants is closely associated with photosynthetic capacity (Zhang et al., 2011; Liu and Li, 2016; Kanno et al., 2017; Liu et al., 2018; Luo et al., 2021; Ye et al., 2022), as N availability and distribution regulate the synthesis of soluble proteins, photosynthetic enzymes, and chlorophyll (Nakaji et al., 2001, 2002; Kochsiek et al., 2006; Xiong et al., 2015). Photosynthetic N can be broadly allocated among the light-harvesting, electron transfer, and carboxylation systems, which are closely related to photosynthetic capacity (Feng et al., 2009; Xu et al., 2012; Elrys et al., 2021). An appropriate N supply can promote light energy use efficiency and photosynthetic enzyme activity. However, excessive N application may disrupt plant nutrient balance, reduce carboxylation and photochemical activity, and consequently inhibit photosynthesis and biomass accumulation (Zhai et al., 2017; Ge and Jiang, 2019; Dong et al., 2023). Therefore, altering N utilization in plants is important for maintaining photosynthetic performance and crop productivity. Biochar can regulate N transformation and cycling in the soil-plant system, thereby affecting photosynthetic rate, transpiration rate, stomatal conductance, and water use efficiency (Liao et al., 2019; Wang et al., 2019; Guo et al., 2021; Wang et al., 2021; Piao et al., 2023; Wu et al., 2024). However, the effects of biochar can vary with application rate and crop type, and excessive biochar application may provide limited benefits or even reduce crop yield (Zhu et al., 2019; Pokovai et al., 2020). Compared with biochar applied alone or in combination with fertilizers, biochar-based fertilizers, which load nutrients onto a biochar carrier, can provide a more sustained nutrient supply, thereby potentially improving fertilizer-N distribution and photosynthetic capacity (Nguyen et al., 2017; Zhang et al., 2021; Wu et al., 2024). For example, biochar-based organic fertilizer enhanced photosynthetic pigment synthesis, photosystem II (PSII) activity, N assimilation, and sucrose synthesis in sugar beet leaves (Chen et al., 2022). However, the linkages among fertilizer-N acquisition and allocation, photosynthetic responses, and grain yield under biochar-blended urea application remain poorly understood.

Therefore, this study investigated the effects of biochar-blended urea on fertilizer-N uptake and allocation, photosynthetic responses, and grain yield of winter wheat under different N application rates. By simultaneously assessing 15 N uptake and distribution along with a range of photosynthetic characteristics, we aimed to elucidate the linkages between fertilizer-N utilization and photosynthetic performance under biochar-blended urea application. These findings provide insights into the mechanisms by which biochar-blended urea regulates N utilization and photosynthetic performance, thereby contributing to an improved understanding of its potential to enhance wheat productivity in dryland soil.

2. Materials and methods

Materials

For the preparation of biochar, this study used maize straw as the raw material. The maize straw was cleaned with deionized water, dried, ground in a grinder, and passed through a 10-mesh sieve. The preparation procedure for biochar is provided in the Supplementary Information (SI 1). This biochar had a pH (H 2 O) of 9.8 and a cation exchange capacity (CEC) of 17.0 cmol kg -1. Its organic carbon (OC) content was 451.1 g kg -1, and its total N, P, and K contents were 6.7, 4.8, and 5.8 g kg -1, respectively, with a total ash content of 19.5%. The material had a Brunauer-Emmett-Teller (BET) surface area of 13.2 m 2 g -1, along with micropore and nanopore volumes of 0.02 and 0.55 cm 3 g -1, respectively. The biochar was then passed through a 20-mesh sieve and stored for subsequent use.

The properties of bentonite, sepiolite, carboxymethylcellulose sodium (C 6 H 7 O 2 (OH) 2 CH 2 COONa; viscosity > 1500 mPa.s) and chitosan (C 6 H 11 NO 4 X 2; viscosity > 400 mPa.s) were reported in our previous study (Shi et al., 2022). Wood vinegar was provided by Nanjing Qinfeng Straw Biomass Technology Co., Ltd., and its properties were reported previously (Shi et al., 2020). The 15 N-labeled urea (10.00 atom%), purchased from Shanghai Wusheng Biotechnology Co., Ltd., China, was used to prepare the conventional urea fertilizer (UF) and biochar-blended urea fertilizer (BF).

The winter wheat variety used in this study was Luyuan 502. The test soil was collected from farmland adjacent to Hebei University of Engineering, located on the North China Plain. The soil had a sandy loam texture and was classified as fluvo-aquic soil (Supplementary Information: SI 1).

Experimental methods

Preparation of biochar-blended urea fertilizer

The biochar-blended urea fertilizer (BF) was prepared as described in detail in our previous study (Shi et al., 2022). The proportions of the raw materials in the BF were 20% biochar, 40% urea, 25% bentonite, 9% sepiolite, 2% carboxymethylcellulose sodium, and 1% chitosan (w/w). The prepared BF consisted of black pellets 2–3 mm in diameter with a Brinell hardness of 19.15 N mm -2 (Supplementary Information: SI 2).

Column pot experiment

The column pot experiment was conducted using polyvinyl chloride (PVC) columns (15 cm in diameter and 70 cm in height), the inner walls of which were coated with Vaseline. The pot experiments were performed outdoors under natural light and ambient temperature conditions. The columns were filled with test soil according to the soil profile capacity of the agricultural soil, with the soil compacted every 5 cm. The experiment was set up with six treatments: ① no N fertilization (CK); ② high-N conventional urea fertilization (HUF), 0.50 g pot -1; ③ high-N biochar-blended urea fertilization (HBF), 1.28 g pot -1; ④ low-N urea fertilization (LUF), 0.40 g pot -1; ⑤ low-N biochar-blended urea fertilization (LBF), 1.02 g pot -1; and ⑥ low-N biochar and urea mixture fertilization (LBC), 1.02 g pot -1. In addition, phosphorus and potassium were added as KH 2 PO 4 (0.21g pot -1) to each pot, with three replicates per treatment. Throughout the growth stage, the conventional-N treatment received 224 kg N ha -1, 105 kg P 2 O 5 ha -1, and 60 kg K 2 O ha -1, whereas the reduced-N treatment received 179 kg N ha -1, 105 kg P 2 O 5 ha -1, and 60 kg K 2 O ha -1. All fertilizers were applied once as basal fertilizers. The wheat was sown on 28 October 2022, and harvested on 18 May 2023.

Stomatal morphology and distribution

Flag leaves were selected from wheat plants during the heading period to examine stomatal traits. Samples were collected from the middle portions of the adaxial and abaxial leaf surfaces to assess the characteristics of stomatal and epidermal cells. First, we used a degreased cotton ball to clean both the adaxial and abaxial leaf epidermis. Nail varnish was then carefully applied to the middle region between the central vein and the leaf edge, avoiding bubbles, and allowed to dry for 0.5 h. Second, we used tweezers to collect the thin film (approximately 5 × 15 mm) from the leaf surface. The film was mounted on a glass slide and then sealed with a coverslip and glue. Finally, we observed and photographed the stomatal sample with a microscope (DM2500, Leica Corp., Germany) equipped with a digital camera (DFC 300-FX, Leica Corp., Germany). Three fields were randomly selected at 10× and 40× magnification, respectively, and four digital photographs were taken from each field, with three biological replicates and a total of 36 subsamples for each experiment. This study examined the effects of different N fertilizer applications on stomatal distribution patterns on the adaxial and abaxial leaf surfaces of winter wheat using ArcGIS 10.8 software (Supplementary Information: SI 3).

Measuring leaf gas exchange

After a 164-day fertilization treatment, we measured leaf gas exchange parameters and A n - C i response curves during the heading stage from 11 to 14 April 2023, between 9:00 and 11:00 a.m. each day to minimize diurnal variations. A portable photosynthesis measurement system (LI-6800, LI-COR Biosciences, Lincoln, NE, USA) was used to measure leaf gas exchange on the wheat flag leaf, a major determinant of grain filling and yield formation in wheat; a plant was randomly selected from each pot for measurement (Fischer, 2011). Gas exchange parameters included net photosynthetic rate (A n), transpiration rate (T r), stomatal conductance (G s), and dark respiration rate (R a). Water use efficiency (WUE) was calculated as the ratio of A n to the transpiration rate. Before measurement, the LI-6800 leaf chamber was set to Photosynthetic Photon Flux Density (PPFD), temperature, and CO 2 concentration of 1,700 μmol m -2 s -1, 25 °C, and 400 µmol mol -1, respectively. The leaf-to-air vapor pressure deficit (VPD) was kept at 0.9-1.5 kPa to eliminate moisture limitation. The steady-state response of photosynthesis (A n) to internal leaf CO 2 partial pressure (A n - C i curve) was also measured to calculate the maximum rate of photosynthetic Rubisco carboxylation (V cmax) and apparent maximum rate of photosynthetic electron transport (J max; Supplementary Information: SI 4).

Measuring photosynthetic pigments

A 0.1-g sample of fresh wheat leaf was weighed and placed in a centrifuge tube containing 10 mL DMSO and then wrapped in tin foil to protect it from light. These samples were stored in the dark for 72 h and shaken every 24 h to ensure complete extraction. After extraction, a 3-mL aliquot was collected from each sample, diluted with 6 mL of DMSO, and thoroughly mixed. A spectrophotometer (UV-1750, Shimadzu Corp., Tokyo, Japan) was used to measure the optical density (OD) values of chlorophyll a content (Chl a), chlorophyll b content (Chl b), and total carotenoids at the respective wavelengths of 663 nm, 645 nm, and 470 nm, respectively (Supplementary Information: SI 5).

Measuring non-structural carbohydrates

The anthrone sulfuric acid method was used to measure non-structural carbohydrate content (soluble sugars and starch). The procedures and parameters for non-structural carbohydrates are provided in the Supplementary Information (SI 6).

Plant and soil collection and analysis

Plant biomass was separated into grain, stem, and root fractions, which were washed separately. Fresh wheat leaves were dried at 105 °C for 0.5 h and then at 80 °C to a constant weight. Each biomass fraction was weighed and thoroughly ground, and the resulting powder was used for element content analysis.

Soil was collected at depths of 0–20 cm, 20–40 cm, and 40–60 cm. A portion of each air-dried soil sample was then divided into two parts using the quarter method. One portion was passed through a 2-mm sieve, and the other portion was passed through a 0.15-mm sieve. The total N content and 15 N atom% of the grain, stem, root, and soil were measured by high-temperature combustion and isotope ratio mass spectrometry. The calculation formulas are provided in the Supplementary Information (SI 7).

Data analysis

Before analysis, it was confirmed that all data satisfied the assumptions of homogeneity of variance and normality. Unless otherwise stated, a significant difference was assessed by analysis of variance (ANOVA), and Duncan’s multiple range test was used to compare means at p < 0.05. A split-plot two-way ANOVA was used to test the main effects of fertilizer type and application rate and their interaction on 15 N uptake and residual 15 N in the wheat–soil ecosystem. Statistical analyses and visualizations were conducted using IBM SPSS Statistics (version 19; IBM Corporation, New York, NY, USA), Origin 8.5 (OriginLab, Northampton, MA, USA), and R (version 4.0.5).

3. Results

Plant biomass

Compared with the urea fertilization (UF) treatment (Figure 1a), the biochar-blended urea fertilizer (BF) significantly increased grain biomass by 18.0% under the high nitrogen (N) rate. Under the low N rate, LBF significantly increased grain biomass by 30.5% and 28.3% compared with LUF and LBC, respectively. As shown in Figure 1b, HBF resulted in a 14.5% increase in stem biomass relative to HUF (p < 0.05), whereas no significant variations were observed among the low N treatments. In contrast, root biomass responded strongly to biochar-blended urea under reduced N input, with LBF promoting root biomass by 40.1% compared with LUF (Figure 1c).

Photosynthesis and leaf properties

Stomatal morphological traits and stomatal distribution patterns

Different N fertilization treatments significantly affected the stomatal morphological traits of wheat leaves on both the adaxial and abaxial surfaces (Table 1). Under the high N rate, HBF decreased adaxial surface stomatal density (SD) and stomatal area index (SAI) by 10.7% and 7.2%, respectively, compared with HUF, whereas abaxial SAI increased by 11.6%, with no significant difference in SD. Under the low N rate, compared to LUF, LBF decreased adaxial stomatal width (SW), stomatal area (SA), SD, and SAI by 18.6%, 8.8%, 18.2%, and 25.3%, respectively, while increasing abaxial SA, stomatal circumference (SC), and SAI by 21.9%, 8.5%, and 22.6%, respectively. LBC and LBF showed similar patterns in stomatal length (SL), SW, and SD. Compared with LBF, LBC increased SL by 47.8% and 43.9% and SD by 40.2% and 14.1% on the adaxial and abaxial surfaces, respectively, but decreased SW by 25.2% and 45.9%, respectively. These results indicate that N fertilizer type had significant effects on SL, SW, and SD. Low N fertilization decreased SL but increased SW and SA on both leaf surfaces compared with CK. Moreover, low N fertilization reduced SAI and stomatal shape index (SPI) on the adaxial surface but increased both indices on the abaxial surface.

In addition to altering stomatal morphology, N fertilization also modified the spatial distribution of stomata on wheat leaves (Figure 2). On the adaxial leaf surface, stomata exhibited a regular spatial distribution, with the observed pattern falling below the lower 95% envelope) at small spatial scales of 20–150 μ m (Figure 2a). Under CK, the stomatal distribution shifted from regular to random at 150 μ m, whereas this transition occurred at a scale of 110–150 µ m under N fertilization, indicating that N fertilization reduced the spatial scale of regular stomatal distribution. Among the N-fertilized treatments, HUF showed the lowest Lhat(d) value at the 80- µ m scale (–6.19), indicating the most regular stomatal distribution. Overall, UF treatments exhibited more regular stomatal distributions than BF treatments, while LBC showed the least regular distribution. On the abaxial surface, stomata also showed a regular spatial distribution at 30–170 µ m, with the distribution pattern changing from regular to random at approximately 140 μm under CK. LBF showed the lowest Lhat (d) value at 80 µ m (–7.37), indicating a more regular stomatal distribution than the other treatments at small spatial scales.

Leaf gas exchange

Low N application treatments significantly affected the net photosynthetic rate (A n) of wheat (Figure 3a). LBF significantly increased the A n by 85.3% and 37.8% compared with LUF and LBC, respectively, whereas no significant difference was observed between HUF and HBF treatments. In addition, compared with LUF and LBC, LBF significantly increased the transpiration rate (T r) and stomatal conductance (G s) (Figures 3b, c), while no difference was observed between HUF and HBF. Compared with LUF and LBC, T r and G s were significantly increased under LBF by 181.8% and 209.8% and by 144.0%, 268.6%, respectively. Compared with CK, N fertilization increased the water use efficiency (WUE) of winter wheat (Figure 3d), with increases of 19.4% and 14.7% under HBF compared with CK and HUF, respectively, and increases of 48.9% and 19.4% under LBF compared with CK and LBC, respectively. Therefore, LBF resulted in the highest WUE, accompanied by enhanced photosynthetic efficiency in winter wheat. However, N fertilization decreased the dark respiration rate (R a) by 16.0%–35.8% compared with CK (Figure 3e). Notably, BF treatments maintained higher R a than UF treatments, with increases of 28.1% under HBF compared with HUF and 13.4% under LBF compared with LUF.

As shown in Figure 4, N fertilization affected the maximum electron transport rate (J max) and maximum carboxylation rate (V cmax). Compared with CK, J max significantly increased by 29.4%, 15.6%, and 25.0% under HUF, HBF, and LBF, respectively, but decreased by 12.0% under LUF. Moreover, the value under HBF was 10.6% lower than that under HUF, whereas that under LBF was 25.0% higher than that under LUF (Figure 4a). V cmax under LBF was 31.9% and 23.7% higher than that under LUF and LBC, respectively (Figure 4b). The J max / V cmax ratio reflects the balance between electron transport and Rubisco carboxylation capacity and can be used to indicate photosynthetic resource allocation (Figure 4c). HUF and HBF exhibited significantly higher J max / V cmax ratios than the other treatments, suggesting that high N application promoted greater investment in electron transport relative to carboxylation capacity in wheat.

Chlorophyll content and non-structural carbohydrates

Compared with CK, LUF and LBF significantly decreased chlorophyll a (Chl a) by 23.0% and 26.2%, respectively, whereas LBC had a 20.4% higher Chl content than LBF (Figure 5a). Under the high N application rate, BF significantly increased the contents of chlorophyll b (Chl b) and total chlorophyll (Chl t) by 22.6% and 22.7%, respectively, compared with UF (Figures 5b, c), with no difference under the low N rate. In addition, carotenoid content (CX) was highest under the HBF (Figure 5d), being 9.2% higher than under HUF. These results suggest that BF application exerted a greater positive effect on the chlorophyll content of winter wheat leaves than UF, particularly under the high N application rate.

In Figure 6a, starch content of wheat leaves significantly decreased by 53.1% and 33.2% under LBF and LBC compared with CK, respectively. In addition, starch content under LBC was significantly higher than that under LBF by 42.5%, whereas no significant difference was observed under the high N rate. Grain starch content increased by 96.2%, 27.8%, 31.3%, and 57.7% under HBF, LUF, LBF, and LBC, respectively, compared with CK (Figure 6b), while that under HBF was 136.2% higher than that under HUF. There was no significant difference between LBF and LUF, but grain starch content under LBC was 20.1% higher than that under LBF. Compared with CK, the treatments under the low N rate enhanced soluble sugar content, with the content under LBF being 56.0% and 74.7% higher than that under LUF and LBC, respectively (Figure 6a). Nevertheless, the soluble sugar content of wheat grains exhibited a decline under all the treatments compared with CK (Figure 6b) and was significantly lower under the BF treatment than under the UF treatment. However, HUF decreased by 55.9% compared to HBF, and LBF decreased by 22.9% compared to LUF. Compared to LUF, the total non-structural carbohydrate (NC) content of leaves under LBF increased by 26.3% (Figure 6a). The results demonstrated that BF promoted the synthesis of NC in wheat leaves more than UF, especially under low N application. However, unlike in leaves, grain NC content showed a decreasing trend compared with CK, with significant reductions of 14.6% and 14.1% under HBF and LBF, respectively, compared with HUF and LUF.

15 N uptake and residues in wheat-soil system

15 N uptake by wheat

The 15 N recovery in different wheat tissues varied among treatments (Figure 7a). BF application promoted total fertilizer- 15 N recovery by wheat, with total recovery reaching 58.0% under HBF and 65.1% under LBF, compared with 45.9% under HUF and 46.5% under LUF. Regardless of fertilizer type or N application rate, 15 N recovery followed a consistent pattern among wheat tissues, with the highest proportion recovered in grain, followed by stem and root. In particular, grain accounted for 38.5% and 45.1% of the applied 15 N under HBF and LBF, respectively, exceeding the corresponding values under HUF (29.4%) and LUF (34.0%). By contrast, stem 15 N recovery did not differ significantly among treatments. Root 15 N recovery showed a different response, reaching 4.8% under LBF, which was significantly higher than that under HUF and LUF (p < 0.05). Two-way ANOVA further revealed that fertilizer type exerted a significant effect on 15 N recovery in grain (p < 0.01) and root (p < 0.05), whereas the effects of N application rate and its interaction with fertilizer type were not statistically significant.

Residual 15 N in soil

The distribution of residual 15 N along the soil profile differed markedly among fertilizer treatments (Figure 7b). In the 0–20 cm layer, HBF retained 33.8% of the applied 15 N, compared with 23.8% under HUF, corresponding to a 42.0% increase in residual 15 N under BF. At the low N rate, however, residual 15 N did not differ significantly between LBF and LUF. A contrast emerged in the 20–40 cm layer, where residual 15 N accounted for 16.2% and 8.3% under HUF and LUF, respectively, whereas only 5.0% and 3.7% remained under HBF and LBF, respectively. Thus, a greater proportion of fertilizer-N was detected in the deeper 20–40 cm layer under UF than under BF. In contrast, no significant treatment differences were detected in the 40–60 cm layer. Two-way ANOVA demonstrated significant main effects (p < 0.01) of fertilizer type and N application rate on residual 15 N in both the 0–20 cm and 20–40 cm layers, while their interaction was also significant (p < 0.01).

Strong linkages between 15 N fate and photosynthetic response on wheat biomass

The relationships between winter wheat biomass and various physiological and biochemical indices (Figure 8) indicated that grain biomass was significantly correlated with grain 15 N recovery (15 N REC), root 15 N REC, and grain total non-structural carbohydrate (TNC), while stem biomass and root biomass were significantly correlated with grain sugars and leaf starch, respectively. In addition, Spearman’s correlation coefficient analysis showed that grain 15 N REC was positively correlated with the net photosynthetic rate (A n), dark respiration rate (R a), water use efficiency (WUE), and maximum rate of photosynthetic Rubisco carboxylation (V cmax). In addition, positive relationships were observed among grain 15 N REC, stem 15 N REC, root 15 N REC, leaf sugar, leaf TNC, and grain starch, whereas grain sugar and grain TNC showed negative associations with grain 15 NREC.

The structural equation model (SEM) further characterized the relationships among fertilizer type, fertilizer-N recovery, gas exchange, and grain biomass (Figure 9). Fertilizer type, including urea and BF, had a significant positive effect on 15 N REC recovery in grain, stem, and root, with a path coefficient of path coefficient of 0.45. Fertilizer-N recovery was also strongly associated with gas exchange parameters, including A n, WUE, and V cmax, with a path coefficient of 0.81. Notably, the path coefficient for gas exchange (0.50) was comparable to that for 15 N REC (0.46) in the SEM model, indicating that gas exchange and fertilizer-N uptake had similar contributions to grain biomass. Overall, both photosynthesis and fertilizer-N uptake were positively associated with grain biomass, while SEM analysis further revealed that fertilizer-N uptake was significantly associated with photosynthetic performance in winter wheat. However, it should be noted that the SEM fit indices appear unusually ideal, which may be partly attributable to the relatively small sample size (n = 15) and the near-saturated nature of the model. These indices may reflect overfitting rather than a truly perfect fit. Accordingly, the SEM results should be interpreted primarily as exploratory evidence of the relationships among fertilizer-N recovery, photosynthetic performance, and grain biomass. Further validation using larger datasets and field-scale experiments will be necessary to determine whether these structural relationships remain consistent under more complex environmental conditions.

4. Discussion

Photosynthetic response as a potential path for increasing wheat yield

Biochar has been shown to promote biomass accumulation by improving plant N utilization and photosynthetic performance, and a positive relationship between photosynthesis and plant biomass accumulation has been widely established (He et al., 2020). However, compared with the physical mixture of biochar and conventional urea (LBC), biochar-blended urea (BF) was found to further enhance plant N utilization and photosynthetic performance by regulating stomatal morphological characteristics and spatial distribution patterns. Stomata play an important role in regulating leaf gas exchange through changes in stomatal number, size, and distribution (Li et al., 2018; Kimura et al., 2020), and appropriate biochar application can alleviate environmental stress and improve photosynthetic capacity (Zhang et al., 2024). Under low N application, BF increased stomatal width (SW) on the abaxial leaf surface, which increased the stomatal area (SA) (Table 1) and subsequently promoted the net photosynthetic rate (A n) of winter wheat (Figure 3a). Wider stomatal openings may facilitate CO 2 diffusion into leaves, allowing Rubisco to fix more CO 2 and reducing the relative contribution of photorespiration, thereby enhancing photosynthetic carbon assimilation (Xu, 2015). In addition, the minimum Lhat(d) value on the abaxial leaf surface of winter wheat was lower at –7.37 (Figure 2), indicating a more regular spatial arrangement of stomata than on the adaxial surface and suggesting that the abaxial surface may contribute more to leaf gas exchange in winter wheat. A more regular stomatal distribution may facilitate CO 2 diffusion, thereby improving photosynthetic efficiency (Wang et al., 2024). Moreover, stomatal distribution was more regular under low N than under high N application (Figure 2b), which may explain the greater improvement in gas exchange efficiency under low N. Consequently, A n under LBF was significantly higher than that under LUF, indicating that BF could enhance photosynthesis under a reduced N rate by optimizing stomatal traits and distribution patterns.

Leaves are important organs for gas exchange in plants, while photosynthesis is essential for plant growth and development and ultimately affects biomass accumulation and yield (Dong et al., 2023). Numerous studies have shown that biochar application can enhance A n by promoting stomatal conductance, transpiration rate, and other gas exchange processes, thereby promoting plant growth and yield (Basri et al., 2016; He et al., 2020; Ren et al., 2021). In the present study, no significant differences in A n, T r, or G s were observed between HUF and HBF (Figure 3), whereas A n, T r, R a, WUE, and G s were significantly higher under LBF than under LUF, which may be related to the greater N recovery by wheat and higher residual N rate in the 0–20 cm soil layer under BF than under UF (Figure 7). The slow-release characteristics of BF may enhance the N‐supplying capacity of the soil, allowing wheat to utilize the applied N more efficiently (Chen et al., 2022). Consequently, the increased N availability may promote N accumulation in leaves, which are the primary sites of N storage and typically allocate more than half of their N to photosynthetic organs (Liu et al., 2016; Dong et al., 2023). This may facilitate the synthesis of photosynthetic enzymes and electron-transport proteins involved in carbon assimilation, thereby supporting photosynthetic activity. In addition, the larger stomatal opening and more regular spatial distribution of stomata under LBF may facilitate rapid CO 2 diffusion into the leaf and its subsequent transport to photosynthetic sites. These stomatal adjustments could increase the effective area for gas exchange and improve stomatal conductance, thereby contributing to the higher A n observed under LBF.

Beyond the improvements in stomatal regulation and gas exchange, the enhanced photosynthetic performance under BF was also reflected in the photosynthetic carboxylation capacity. A higher maximum rate of photosynthetic Rubisco carboxylation (V cmax) reflects a greater potential for CO 2 fixation and carbon assimilation (Walker et al., 2014; Stocker et al., 2025). V cmax was significantly higher under LBF than under the other treatments (Figure 4b), which may have enhanced carbon assimilation and contributed to greater seed, stem, leaf, and root biomass accumulation. These findings are consistent with the structural equation modeling (SEM) analysis. The higher grain yield under BF than under UF was positively associated with improved fertilizer-N uptake and photosynthetic capacity. In addition, the non-significant direct effect of fertilizer type on gas exchange in the SEM suggested that BF may not directly regulate photosynthetic processes but rather indirectly enhance photosynthetic performance by improving fertilizer-N recovery and allocation. Enhanced N acquisition can support the synthesis of key photosynthetic components, thereby promoting carbon assimilation. Together, these results provide further support for the potential of biochar-based fertilizers to reduce N inputs while maintaining crop productivity and improving N use efficiency. However, long-term field trials are needed to further verify these effects under field conditions.

Mitigating the environmental risk caused by BF on reducing soil nitrogen leaching

Regulation of fertilizer-N fate by biochar combined with fertilizers is critical for improving N use efficiency and reducing potential environmental risks associated with N losses in agricultural systems. Biochar-based fertilizers provide a potential approach to optimizing N dynamics by enhancing soil N retention, regulating N release, and improving the coordination between soil N availability and crop N demand (Joseph et al., 2010; Fiorentino et al., 2019; Lehmann et al., 2021). Fertilizer-N was predominantly allocated to wheat grain, followed by the stem and root (Figure 7a), suggesting that BF facilitated the uptake and internal translocation of fertilizer-N, thereby improving the recovery rate of applied fertilizer-N. In addition to enhancing plant N uptake and translocation, biochar-based fertilizer may also regulate the retention and redistribution of fertilizer-N in soil. The consistently higher performance of BF compared with LBC suggested that the beneficial effects were not solely derived from biochar addition but were also associated with the integrated biochar-based fertilizer structure, which may enhance N retention and regulate N release more effectively. Consistent with previous studies (Steiner et al., 2008; Liu et al., 2015; Kalu et al., 2021; Liu et al., 2021; Ding et al., 2022; Feng et al., 2024), BF improved soil N retention by increasing residual 15 N in the topsoil (0–20 cm) and reducing it in the subsoil (20–60 cm) relative to UF, thereby inhibiting deep N leaching. The mechanisms underlying the reduction of N leaching by biochar-based fertilizers involve both physicochemical and biological processes. The adsorption of NH 4 + by functional groups on biochar surfaces has been recognized as a primary physicochemical pathway for N immobilization in soil (Taghizadeh-Toosi et al., 2012; Liu et al., 2021). In addition, synergistic N retention may arise from the interaction between biochar and incorporated clay minerals. As revealed by our previous EELS and EDS analyses, clay minerals derived from bentonite and sepiolite were closely associated with the porous biochar, and N was detected not only in the carbon matrix but also in the mineral phases, indicating dual adsorption sites (Shi et al., 2020). Furthermore, biochar-based fertilizer may promote microbial N assimilation (Shi et al., 2022), thereby contributing to biological N retention and reducing N loss via leaching (Bass et al., 2016). Future studies could further characterize soil N fractions to better understand the changes in soil N forms.

Although the soil column experiment conducted during a single growing season provided a controlled approach for quantifying root distribution and fertilizer-derived N dynamics, it cannot fully capture the complexity and temporal variability of field environments. Factors such as soil structural variability, natural precipitation, and heterogeneous root growth may influence N transport and plant uptake under actual agricultural systems. Moreover, the relatively small number of replicates (three) may have reduced the statistical power of the study, although consistent treatment effects were observed across the major measured variables. Therefore, further multisite field experiments with increased replication across multiple growing seasons, involving multiple wheat varieties and different soil types, are required to validate the applicability of these findings and assess the long-term effects of biochar-based fertilizer on N utilization under practical production conditions.

5. Conclusions

Biochar-blended urea (BF) enhanced wheat grain yield by improving photosynthetic performance and fertilizer-N utilization, particularly under the reduced N application rate, indicating that the benefits of BF for improving N utilization were associated with its integrated structure rather than the simple co-application of biochar and urea. BF increased stomatal area and the regularity of stomatal distribution on the abaxial leaf surface, thereby enhancing stomatal regulation of gas exchange and increasing photosynthetic capacity. Moreover, BF significantly increased fertilizer-N uptake and its translocation to the grain, and enhanced fertilizer-N uptake was positively associated with key photosynthetic traits, including the net photosynthetic rate, water use efficiency, and maximum carboxylation rate, ultimately resulting in higher grain yield. Notably, the low-N combined with biochar-blended urea (LBF) treatment achieved the optimal balance between reduced N input and sustained grain productivity, demonstrating its potential as an effective strategy for improving N use efficiency while maintaining yield. Overall, this study provides physiological evidence that biochar-blended urea enhances the coordination between fertilizer-N utilization and photosynthetic performance, highlighting LBF as a promising nutrient management practice for reducing chemical N fertilizer inputs while sustaining wheat productivity in fluvo-aquic soils on the North China Plain. Nevertheless, long-term field trials are needed to validate the effectiveness of biochar-blended urea under practical agricultural conditions.

Statements

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The handling editor JC declared a past co-authorship with the author RB.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This work was founded by the Natural Science Foundation of Hebei Province (D2024402015), and by the Natural Science Foundation of Hebei Province (E2025402107), supported by the Norwegian Ministry of Foreign Afairs (CHN-2152, 22/0013 SINOGRAIN III), National Natural Science Foundation of China (52209049) and Natural Science Foundation of Hebei Province (E2024402128), Natural Science Foundation of Hebei Province (C2024402017).

Data availability

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found below: The data that support the findings of this study are openly available in Zenodo at https://doi.org/10.5281/zenodo.21215033.

Author contributions

YW: Conceptualization, Data curation, Methodology, Writing – review & editing, Visualization, Writing – original draft. YZ: Supervision, Conceptualization, Writing – review & editing. RB: Writing – review & editing, Conceptualization. FL: Validation, Writing – review & editing, Visualization. LL: Investigation, Writing – review & editing, Resources. LH: Methodology, Writing – review & editing. LW: Writing – review & editing. HY: Writing – review & editing, Data curation, Visualization. ZW: Visualization, Data curation, Writing – review & editing. YS: Supervision, Writing – review & editing, Conceptualization. WS: Data curation, Methodology, Supervision, Conceptualization, Writing – review & editing, Project administration, Funding acquisition, Visualization.

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Citation

Wang, Y., et al. (2026). Biochar-blended urea enhances wheat yield by integrating fertilizer-nitrogen utilization and photosynthesis. Frontiers in Agronomy, Volume 8. https://doi.org/
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8 - 2026

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Copyright © 2026 Wang, Zheng, Bian, Li, Liu, Hao, Wang, Yin, Wang, Shen and Shi.

License: https://creativecommons.org/licenses/by/4.0/

*Correspondence: Yabin Shen; Wei Shi

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