The nutrient-dense legume cowpea (Vigna unguiculata) is essential to sustainable agriculture and food security.  The majority of the 500-550 Mt of crop residue produced annually in India are burned, making it a major source of greenhouse gas emissions. A sustainable substitute is provided by turning this biomass into biochar, a carbon-rich byproduct of pyrolyzed organic waste. Biochar enhances the physical, chemical and biological characteristics of soil, such as pH, cation exchange capacity (CEC), water retention, aggregation and microbial activity (Tomczyk et al., 2020). Even under stressful situations, its porous nature promotes microbial growth and improves nutrient availability. Biochar also serves as a long-term carbon sink, remaining in soil for centuries (Yeboah et al., 2016). This study investigates how biochar might improve nutrient dynamics, soil fertility and water retention, making it an important instrument in climate-resilient agriculture.

Description of experimental site
 
The two years (2024-2025) experiment was conducted at Lovely Professional University in Phagwara, Punjab (31.25°N, 75.70°E, 252 m height), a subtropical area with an average annual rainfall of 711 mm, with sandy loam soil. Table 1 displays soil parameters, while Fig 1 displays meteorological data from May to September 2024-2025.




 
Experimental design and treatments
 
The study followed a randomized block design with 13 treatments and three replications. The treatments are T₁ (Absolute control), T₂ (50% RDN), T₃ (75% RDN), T₄ (100% RDN), T₅ (Charcoal biochar 50% RDN), T₆ (Charcoal biochar 75% RDN), T₇ (Charcoal biochar 100% RDN), T₈ (Sugarcane biochar 50% RDN), T₉ (Sugarcane biochar 75% RDN), T₁₀ (Sugarcane biochar 100% RDN), T₁₁ (Rice husk biochar 50% RDN), T₁₂ (Rice husk biochar 75% RDN) and T₁₃ (Rice husk biochar 100% RDN). 
 
Sources of fertilizers, preparation of biochar and urea impregnated biochar
 
Fertilizer recommendations were 41 kg ha^-1 urea and 55 kg ha^-1 SSP. Nutrients were supplied as per percent basis based upon treatments. The biochar used in the experiment was pro­duced from rice husk, sugarcane baggase and charcoal at 600 Degree C and incorporated in soil at the time of land preparation. Characteristics of biochar was: pH-8 C-0.07 dSm^-1, total C-396 g^-1 kg^-1, total N- 4.08 g^-1 kg^-1 biochar, total P-738 m g^-1 kg^-1 biochar, total K-8.35 g^-1 kg^-1 biochar, bulk density-0.117 g cm^-3, particle density-0.2732 g cm³, porosity-60.07%, solid space-39.9%.

To prepare molten urea, urea granules were dissolved in water and heated to 70°C while constantly stirred. Biochar (½ weight per cent) was added and mixed for one hour. For moisture removal, the mixture was oven-dried at 65°C. A 10% starch adhesive was added and it was air-dried, pulverized and used as fertilizer.
 
Parameters of study
 
The crop was established through line sowing and irrigation was applied at regular intervals of 7-10 days to maintain adequate soil moisture throughout the crop growth period. Growth and morphological observations were recorded from five randomly selected and tagged plants in each experimental plot using standard agronomic procedures. The parameters measured included plant height, number of leaves and branches per plant, root length, number of root nodules, chlorophyll index (SPAD value), leaf area index (LAI) and dry matter accumulation. Yield attributes were assessed at crop maturity and included number of pods plant^-1, pod length, pod weight, seed index (100-seed weight) and grain yield. All yield parameters were recorded manually using standard measuring instruments and precision weighing balances. For determination of dry matter, plants were harvested close to ground level, sun-dried to remove surface moisture and subsequently oven-dried at 70°C until constant weight was attained. The oven-dried samples were then weighed and dry matter accumulation was expressed on a plant^-1 basis.
 
Nutrient use efficiency (NUE)
 
NUE represents yield per nutrient applied. A cowpea’s capacity to fix 113.4-250 kg N ha^-1 is contingent upon the climate, variety and soil.

Statistical analysis
 
The data were subjected to analysis of variance (ANOVA) for significant differences between factors such as growth and yield parameter. Significant F-tests were obtained (0.05 probability level) and the factor separation was achieved using CV STAT 2020.

The results obtained from the study in terms of growth and assessment studies were presented in tables and necessary analysis was done.
 
Growth parameters
 
Plant height was significantly influenced by nitrogen levels and biochar application. The lowest plant height (164.65 cm) was recorded under the absolute control, while the highest plant height (306.68 cm) was observed with sugarcane biochar combined with 100% recommended dose of nitrogen (RDN). Biochar-amended treatments produced taller plants than non-amended treatments at comparable nitrogen levels, indicating a synergistic effect of biochar and nitrogen fertilization. The number of leaves plant^-1 followed a similar pattern, with the minimum leaf count (66.49) under the absolute control and the maximum (122.71) under sugarcane biochar at 100% RDN. Leaf area index (LAI) was also significantly higher in biochar-treated plots, recording the lowest value (6.40) in the control and the highest value (8.42) in biochar treatments receiving 100% RDN, reflecting improved canopy development and photosynthetic efficiency (Table 2). Root growth and nodulation were markedly enhanced by biochar application (Table 3). The lowest root length (8.02 cm) and minimum number of nodules (20.34 plant^-1) were observed in the control, whereas sugarcane biochar at 75-100% RDN produced the longest roots (11.17 cm) and maximum nodulation (32.79 plant^-1). Enhanced nodulation suggests improved rhizobial activity and biological nitrogen fixation under biochar-amended soils. Physiological performance, expressed as SPAD chlorophyll index, was lowest (29.55) in the control and highest (40.87) with sugarcane biochar at 100% RDN, indicating improved nitrogen assimilation. Similarly, dry matter accumulation was minimum (34.23 g plant^-1) under the control and maximum (73.07 g plant^-1) with sugarcane biochar at 100% RDN, reflecting superior growth and biomass production (Table 3). Overall, nitrogen application significantly improved growth parameters, while biochar further improved these effects by enhancing soil nutrient availability, moisture retention and root–microbial interactions. Among the biochars, sugarcane biochar consistently performed superior to charcoal and rice husk biochars, likely due to its higher nutrient content and better soil-conditioning properties, in agreement with earlier findings (Phares et al., 2020; Xiang et al., 2017; Weber and Quicker, 2018; Sharma et al., 2025; Thu et al., 2023).




 
Yield attributes and nutrient use efficiency (%)
 
Yield attributes responded positively to increasing nitrogen levels and biochar application. The lowest number of pods plant^-1 (17.16), shortest pod length (11.89 cm) and minimum pod weight (4.37 g) were recorded in the absolute control, whereas the highest values for these parameters (29.07 pods plant^-1, 23.27 cm pod length and 8.15 g pod weight) were obtained with sugarcane biochar at 100% RDN. The lowest seeds per pod (13.50) and minimum seed index (11.79 g) were observed in the control, while the maximum seeds per pod (16.70) (Table 4) and highest seed index (16.21 g) were recorded under sugarcane biochar at 100% RDN, indicating improved seed filling and assimilate partitioning. Grain yield was significantly affected by treatments, with the lowest yield (610.59 kg ha^-1) in the control and the highest yield (1242.44 kg ha^-1) under sugarcane biochar combined with 100% RDN. Biochar-amended treatments at higher nitrogen levels consistently outperformed nitrogen-only treatments, demonstrating the beneficial role of biochar in improving yield formation. Nutrient use efficiency (NUE) was lowest (0%) in the control and highest (50.33%) with sugarcane biochar at 100% RDN (Table 5). The substantial improvement in NUE under biochar application indicates reduced nitrogen losses and enhanced nitrogen retention and uptake. In summary, nitrogen fertilization improved yield attributes and grain yield, while biochar integration-particularly sugarcane biochar-further enhanced productivity and nutrient use efficiency. The results confirm that combining biochar with nitrogen fertilization is an effective strategy for improving legume yield and sustainability by optimizing nutrient utilization and reducing nitrogen losses (Laird et al., 2010; Azeem et al., 2020; Njonjo et al., 2019, Arunkumar and Thippeshappa, 2023). The significance of biochar in increasing biomass, yield and legume productivity has been confirmed by earlier research (Rab et al., 2016; Saxena et al., 2013; Solaiman et al., 2010; Berihun et al., 2017, Ramamoorthy et al., 2024).




 
Principal component analysis
 
PCA analysis between growth and yield attributes
 
PCA biplot and PCA screen plot analysis
 
In PCA biplot, the majority of the variation in cowpea growth and yield attributes is captured by PC1 and PC2 as seen in Fig 2. Plant height, dry weight, LAI, secondary branches, pod weight and grain production are all highly correlated with PC1, suggesting that these factors are interdependent in terms of productivity. Seed index and pod length are more in line with PC2, indicating a clear contribution to variability and greater influence is indicated by longer vectors.



PCA analysis between growth and weather attributes
 
Correlation plot of variables vs PC and PCA of variables
 
In the correlation plot (Fig 3), temperature and wind have a significant detrimental impact on cowpea growth through PC1, but precipitation, humidity and dew point have a minor impact on PC2.


 
PCA analysis between yield and weather attributes
 
Correlation plot of variables vs PC and PCA biplot analysis
 
The PCA biplot in Fig 4 demonstrates that yield features are positively aligned, whereas weather factors cluster independently. Factors associated to moisture, such as humidity and dew point, exhibit inverse trends with temperature, wind and pressure. Stronger contributions are shown by longer vectors, which show differing climate effects on cowpea output in different environmental settings.


 
Network visualization of correlations among plant growth parameters
 
Strong positive correlations between fresh weight, root nodules, plant height and chlorophyll index are shown in Fig 5, suggesting improved growth. While there was a positive correlation between large and small nodules, there was a trade-off between fresh weight, nodule size and root nodules.


 
SEM and biochar morphology
 
FE-SEM (JSM-7610F Plus) was used to examine the surfaces of biochar following gold/silver plating. Binary image processing was used at magnifications of 10 µm (Fig 6) to 100 µm (Fig 7) to identify pore structures. Volatile loss led to cracking and shrinkage at 800°C. While the surfaces of charcoal and rice husk biochars were uneven and partially disintegrated the cellular structures of sugarcane biochar were preserved. Sugarcane biochar had more distinct, smaller pores (44.000-70.257 px, Fig 6c), enhancing surface area for nutrient and water absorption. In contrast, charcoal (146.820-191.927 px) and rice husk biochars (150.037-216.148 px) had larger, fewer pores and partially collapsed structures as presented in Table 6. Sugarcane biochar’s porous architecture supports its superior soil performance.

Nitrogen fertilization significantly improved growth, yield attributes and grain yield of the legume crop, while biochar application further enhanced these responses. The absolute control recorded the lowest performance, confirming the essential role of nitrogen in crop productivity. Sugarcane biochar combined with 100% recommended dose of nitrogen produced the highest growth, yield and nutrient use efficiency. Biochar improved nodulation, chlorophyll content and nitrogen utilization by enhancing soil nutrient retention and rhizosphere activity. Overall, integrating sugarcane biochar with nitrogen fertilization is an effective and sustainable strategy for improving legume productivity and resource-use efficiency.
 
Funding agency
 
No funding received to assist with the preparation of this manuscript.
 
Availability of data (Data transparency)
 
The author declares the all the data and materials as well as software application or custom code support their published claims and comply with field standards.
 
Author contributions
 
Kritika wrote the paper, material preparation, field data collection, Arshdeep Singh did the data analysis and formatting and Shimpy Sarkar did the corrections in the manuscript.

There is no conflict of interest among the authors.