Effect of sulphur and babool (Prosopis juliflora) biochar on soil properties of sesame (Sesamum indicum L.) grown in loamy sand soil

Introduction

Sesame (Sesamum indicum L.) is one of the oldest known oilseed crops in the world, known by various regional names -Gingelly in India, Benniseed in Africa, Benne in South America, Gengelin in Brazil, Semsem in Hebrew, and Tila in Sanskrit. It belongs to the family Pedaliaceae, comprising 60 species under 16 genera, of which S. indicum (2𝑛 = 26) is the principal cultivated species. The plant grows 50-250 cm tall depending on cultivar and environment, is annual and self-pollinated, and matures in 85-90 days. The crop is particularly sensitive to water deficit at the seedling, flowering and seed-filling stages, which strongly determines final yield (Fazeli et al., 2006). Sesame seeds are often called "the seeds of immortality" owing to their rich content of natural antioxidants; the oil exhibits high resistance to oxidative rancidity because of endogenous antioxidants such as sesamol, sesamolinol and sesaminol (Alpaslan et al., 2001) [1] . The oil (48-52%) is rich in protein (20.28%), sugars (14-15%), minerals (5-7%), vitamins, calcium (1450 mg/100 g) and phosphorus (570 mg/100 g), with micronutrients following the order Fe > Cu > Zn > Mn (Suresh et al., 2013) [2] , and provides 635 calories per 100 g (Kumar and Goel, 1994), making it a valuable substitute for ghee. Sesame cake, the oil-extraction residue, contains 6.0-6.2% N, 2.0-2.2% P and 1.0-1.2% K (Yadav et al., 2008) [3] and is valuable both as cattle feed and organic manure. India ranks second in the world in sesame production, with the crop cultivated on 15.24 lakh hectares, producing 9.34 lakh tonnes at an average productivity of only 613 kg ha -1 (MoA&FW, GOI 2025-26) [4] well below its genetic potential. This is largely attributable to cultivation on marginal, sandy soils with low organic matter, nitrogen and sulphur status, especially in the semi-arid regions of Rajasthan. Sulphur requirement in plants is nearly equal to that of phosphorus (Scherer, 2001) [5] and is essential for synthesis of the sulphur-containing amino acids methionine (21%), cysteine (26%) and cystine (27%), which together constitute about 90% of plant sulphur. Sulphur is also required for co-enzyme A, biotin, thiamine and glutathione synthesis, and plays a central role in chlorophyll formation, glucoside and glucosinolate (oil) biosynthesis, enzyme activation, and formation of disulphide (-S-S-) linkages responsible for drought and cold tolerance in oilseeds. In sesame, sulphur promotes floral primordial initiation, resulting in more capsules per plant, more seeds per capsule, and higher seed oil content (Hassan et al., 2007) [6] . In soil, sulphur exists mainly as sulphate-S, organic-S, total-S and non-sulphate-S (Pasricha et al., 2012) [7] , with sulphate (SO 4

2-

) being the dominant plant-available form (Zhou et al., 2005) [8] ; its release depends on soil environmental conditions, source characteristics and microbial activity. Biochar is a porous, carbon-rich material produced by heating organic matter to 300-600 °C under limited or no oxygen (Verheijen et al., 2010; Zheng et al., 2010) [9,10] . Meta-analyses show biochar is most beneficial in acidic, degraded and coarse-textured soils (Crane-Droesch et al., 2013) [11] typical of the sandy loam soils of semi-arid Rajasthan. Its high surface area and porosity improve water and nutrient retention, while its stable carbon structure contributes to long-term soil carbon sequestration and is regarded as one of the most effective strategies for enhancing soil biodiversity (Brussaard et al., 2007) [12] . The porous structure provides habitat for soil microorganisms (Sohi et al., 2009) [13] , promoting microbial colonisation, reducing nitrogen losses and improving overall nutrient availability and plant productivity (Winsley, 2007) [14] . Biochar and other carbon-rich amendments can influence soil carbon dynamics by modifying carbon quality, microbial activity and the stabilization of labile and recalcitrant carbon pools (Singhal et al., 2026) [15] Prosopis juliflora (babool) is a widely spreading invasive shrub across arid and semi-arid India with limited direct economic use; converting its biomass into biochar offers a dual-purpose strategy of weed management and soil amendment production. While the individual effects of sulphur nutrition and biochar amendment on soil fertility are documented separately, information on their combined application particularly using biochar derived from P. Juliflora on soil physico-chemical and biological properties in loamy sand soil is limited. The present investigation was therefore undertaken to study the individual and interactive effects of sulphur and babool biochar on soil properties of sesame grown in loamy sand soil.

Material and Methods

The field experiment was conducted at the Instructional Farm, S.K.N. College of Agriculture, Jobner, Rajasthan (26°06′ N latitude, 75°28′ E longitude; 427 m above mean sea level) during the kharif season of 2025. The experimental site falls under Agro-climatic Zone III-A (Semi-arid Eastern Plain Zone) of Rajasthan. The experiment consisted of four levels of sulphur (S), viz., 0, 20, 40 and 60 kg ha -1 , applied in the form of elemental sulphur containing 99.5% S, and four levels of babool biochar, viz., 0, 3, 6 and 9 t ha -1 , resulting in 16 treatment combinations. The treatments were evaluated in a factorial randomized block design (FRBD) with three replications, comprising a total of 48 experimental plots. Each plot had a gross size of 4.0 × 3.0 m and a net plot size of 3.0 × 2.4 m. Babool stalks were subjected to pyrolysis under low-oxygen conditions to prepare the biochar, which was uniformly surface-spread and incorporated into the soil at the time of sowing. The chemical and physical characteristics of the prepared babool biochar had a pH of 8.14, electrical conductivity of 0.55 dS m -1 , bulk density of 0.48 Mg m -3 , particle density of 1.37 Mg m -3 and porosity of 68%. It contained 1.41% total N, 0.74% total P, 1.46% total K, 0.37% total S and 70.23% total carbon with ash and moisture contents of 30.21 and 7.92%, respectively. After harvest, soil samples were collected from the 0-15 cm soil depth from each exper-imental plot and analyzed for important physical, chemical and biological properties. The soil samples were analyzed for pH, electrical conductivity (EC), organic carbon, bulk density, particle density, porosity, available N, P, K and S, DTPA-extractable Fe, Mn, Cu and Zn, microbial biomass carbon, dehydrogenase activity and alkaline phosphatase activity using the standard analytical procedures presented in Table 1. Data were analysed by analysis of variance for factorial RBD following Panse et al. (1985) [16] ; treatment means were compared using critical difference (CD) at P = 0.05.

Results and Discussion

Bulk density, particle density and porosity: Sulphur had no significant effect on any of these parameters (Table 2). Biochar significantly reduced bulk density from 1.53 (BC 0 ) to 1.48 Mg m -3 (BC₃) and increased porosity from 41.24% to 44.91%, reflecting the inherently low bulk density (0.48 Mg m -3 ) and high porosity (68%) of the biochar, which physically loosens the sandy matrix. Particle density remained unaffected by either treatment. Biochar particles can also promote aggregation and improve pore-size distribution, thereby decreasing soil compaction and increasing total porosity. A global metaanalysis by (Blanco-Canqui, 2017) [17] reported that biochar application generally decreases soil bulk density and increases soil porosity, particularly in coarse-textured and degraded soils. Similarly, (Verheijen et al., 2010) [9] reported that the low density and porous structure of biochar can modify soil physical properties following incorporation. Greater carbon accumulation within soil aggregates and enhanced enzyme-mediated nutrient cycling (Singhal et al., 2024). Soil pH and electrical conductivity: Sulphur application significantly and progressively reduced soil pH from 8.38 (S 0 ) to 8.01 (S 3 ) and EC from 0.426 to 0.414 dS m -1 (Table 2). This decline can be attributed to microbial oxidation of elemental sulphur to sulphuric acid, releasing H + ions into the soil solution a mechanism also reported by (Pasricha et al., 2012) [7] . Biochar application caused a slight, statistically non-significant increase in pH (8.10 to 8.29) and EC (0.417 to 0.426 dS m -1 ) with increasing rate, consistent with the mildly alkaline reaction of the biochar itself (pH 8.14, Table 2). Biochar effects on soil pH are highly dependent on the initial soil condition and biochar characteristics, rather than producing a uniform increase in all soils. The slight increase in EC with biochar application may be attributed to the release of soluble mineral salts and basic cations from the ash fraction of biochar (Su et al., 2024) [18] . Organic carbon: Sulphur had no significant effect on organic carbon (0.224-0.230%). Biochar significantly and progressively increased organic carbon from 0.202% (BC 0 ) to 0.252% (BC 3 ) a 24.75% increase attributable to the recalcitrant aromatic carbon in biochar (70.23% total C, Table 3) which resists rapid microbial decomposition, a finding consistent with (Zheng et al., 2010) [19] . Available macronutrients: Sulphur increased available N, P and K numerically, but not significantly (Table 6), while significantly increasing available S from 13.28 to 16.68 kg ha -1 , with S 3 at par with S 2 . Biochar significantly increased available N (+17.12%), P 2 O 5 (+16.05%), K 2 O (+15.97%) and S (+11.59%) at 9 t ha -1 over control, likely due to both direct nutrient contribution from the biochar (Table 2) and improved retention of nutrients on its charged surfaces in this coarse-textured soil, consistent with (Sohi et al., 2009) [13] . DTPA-extractable micronutrients: Sulphur had no significant effect on Fe, Mn, Cu or Zn (Table 2). Biochar significantly increased all four: Fe (+7.18%), Mn (+14.90%), Cu (+15.09%) and Zn (+26.09%) at 9 t ha -1 , possibly due to release of micronutrients from the ash fraction of biochar (30.21%, Table 3) and improved moisture retention favouring micronutrient solubility. The positive response observed in the present study may therefore be related to the particular characteristics of babool biochar, its relatively high ash content and the properties of the experimental soil. In addition, the porous structure of biochar can improve soil water retention, particularly in coarse-textured soils, which may facilitate nutrient diffusion and maintain micronutrients in plant-available forms (Schmidt et al., 2021) [20] . Soil biological properties: Sulphur did not significantly affect microbial biomass carbon (MBC), dehydrogenase (DHA) or alkaline phos-phatase activity (Table 2), though all three increased numerically with sulphur rate. Biochar significantly enhanced all three: MBC (+21.94%), DHA (+25.27%) and alkaline phosphatase activity (+13.12%) at 9 t ha -1 , supporting the view that the porous structure of biochar provides a favourable habitat for microbial colonisation while its labile carbon fraction supplies an energy source for microbial growth (Winsley, 2007; Sohi et al., 2009) [14,13] . Enhancing carbon stabilization and strengthening the association between soil organic carbon and enzyme activities, particularly in degraded semi-arid soils (Singhal et al., 2025).

Conclusion

Under loamy sand soil conditions of semi-arid Rajasthan, sulphur application (40-60 kg ha -1 ) significantly improved soil sulphur status and reduced pH/EC, while babool biochar (particularly 9 t ha -1 ) produced broad improvements in soil physical structure, nutrient availability and microbial/enzymatic activity. Both inputs independently and significantly increased sesame seed yield, and their integrated application is recommended for enhancing sesame productivity while simultaneously improving soil health and valorising invasive P. juliflora biomass in semi-arid sandy soils.

Acknowledgement

AcknowledgementDepartment of Soil Science and Agricultural Chemistry, S.K.N. Agriculture University, Jobner

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