Seed bio-priming was carried out with micro organism Trichoderma asperellum that was cultured on PDA procured from the Department of Mycology and Plant Pathology, Institute of Agricultural Sciences, Banaras Hindu University. Spore count in suspension was maintained at 10⁸ CFU ml^-1, and 3-4 drops of tween-20 were added. Sunflower seeds were hydrated for 6 hr followed by surface sterilization with 1% sodium hypochlorite (NaOCl) solution that was placed in moisture chamber and then incubated for 48hr after drenching with spore suspension (Fig. 1).

Pot culture experiment was conducted for two seasons with sunflower (variety: PAC 334) at Department of Soil Science and Agricultural Chemistry, Institute of Agricultural Sciences, Banaras Hindu University, Varanasi (25°20′ N latitude and 83°01′ E longitudes). The experimental design was a completely randomized design with four replications. First season and second season experiments were conducted during February second fortnight to may second fortnight in 2018 and 2019, respectively. Treatments imposed were T₁: control, T₂: 70% RDF, T₃: 80% RDF, T₄: 90% RDF, T₅: 100% RDF (90-50-50), T₆: 70% RDF + Sees bio-priming, T₇: 80% RDF + Sees bio-priming, T₈: 90% RDF + Sees bio-priming. Initial properties of soil are given in table 1.

Soil reaction was determined by a combined glass electrode in 1:2.5 soil water suspension (Jackson, 1973).

EC of soil suspension was determined in the same sample used for soil reaction. Reading was taken by using an electrical conductivity bridge in supernatant solution after allowing the suspension to settle down (Jackson, 1973).

Soil samples were saturated with 1 N sodium acetate (pH 8.3). The excess sodium acetate was leached out by washing out with 95% ethanol. Then the adsorbed sodium was displaced with neutral normal ammonium acetate. The concentration of sodium in the leachate was estimated flame photometrically (Bower et al. 1952).

Available nitrogen content in soil samples was determined by the alkaline potassium permanganate method (Subbiah and Asija 1956). To the known weight of soil, KMnO₄ (oxidizing agent) was added. Nascent oxygen released in the presence of NaOH oxidizes organic matter, and the released ammonia absorbed in a boric acid of known concentration. The excess is known by the titration method by using a mixed indicator.

Available phosphorus in the soil samples was extracted with 0.5 M NaHCO₃ of pH 8.5, and the phosphorus in the extract was estimated colorimetrically by an ascorbic acid method using a spectrophotometer at 660 nm (Watanabe and Olsen 1954).

Available potassium in soil was extracted using neutral normal ammonium acetate, and potassium in the extract was determined to flame photometrically (Muhr et al. 1965).

Data analysis was done by using Statistical Package for Social Science (SPSS) software (Version 20). R studio (version 4.0.4) was used for the computation of the correlation matrix.

Pooled data showed that the soil reaction ranged from 7.2 to 7.5 (Fig. 1). During the first season, the highest soil pH was noticed in control and 100% RDF (7.4), which was slightly alkaline in reaction. Reduction in soil pH towards neutral was noticed in seed bio-priming treatments (7.1 to 7.3). Second season data showed that the highest pH was noticed in control, 70% RDF and RDF (7.5), while the lowest was in treatment supplied with 80% RDF along with bio-priming (7.2). Pooled revealed that the highest pH value was in control (7.5) while the minimum was noticed in T₇ (7.2). Soil physicochemical properties such as soil reaction, electrical conductivity, and cation exchange capacity play a vital role in the chemistry and fertility of the soil (Neina 2019). Soil reaction extensively influences soil physical, chemical, and biological properties that determine crop growth and yield (Minasny et al. 2012; Neina 2019). Similar results were found with Trichoderma inoculation in Lolium (Song et al. 2015), in Miscanthus, Salix, Phalaris and Panicum (Kacprzak et al. 2014), which might be due to the ability of Trichoderma to produce organic acids such as gluconic acid, fumaric acid and citric acid (Wang and Chen 2009; Song et al. 2015). In pearl millet and wheat crop, inoculation of cyanobacterial bio-fertilizer resulted in a decrease in soil pH (Nisha et al. 2007).

In the current experiment, pooled data showed that the range is between 0.34 dSm^-1 and 0.43 dSm¹ (Fig. 2). There is no specific trend in EC values with imposed treatments, however, in pooled data, maximum conductivity was observed in treatment supplied with 70% RDF (0.43 dSm^-1) while the minimum was noticed in treatment supplied with 70% RDF along with bio-priming (0.34 dSm^-1). Soil cation exchange capacity is the ability of soil to hold and exchange mineral elements. Soil chemical properties can be assessed by cation exchange capacity (Saidi 2012).

Cation exchange capacity during the first season experiment ranged between 22.73 and 24.11 cmol (p+) kg^-1. Minimum value was noticed in control, wherethe CEC of treatments supplied with 70% RDF + bio-priming and 80% RDF + seed bio-priming were 5.71 and 6.07% higher, respectively. In second season experiment treatments supplied with 80% RDF + bio-priming, 80% RDF + bio-priming and 90% RDF + seed bio-priming were 3.7, 1 and 3.02% higher values were noticed (Fig. 3). An increase in soil CEC with Azotobacter was noticed in Solanum (Demir, 2020) whereas in pearl millet and wheat crop, inoculation of cyanobacterial bio-fertilizer showed a slight increase in soil CEC (Nisha et al. 2007). According to a study on Miscanthus, Salix, Phalaris, and Panicum, there is no effect of Trichoderma on soil CEC (Kacprzak et al. 2014).

Soil available nitrogen content varied significantly with different imposed treatments in both the seasons (Fig. 4) and ranged from 205 to 244 kg ha^-1 on pooled bases. The highest value was recorded in treatment supplied with 80% RDF + bio-priming with Trichoderma asperellum, which was significantly superior to control (205 kg ha^-1), 70% RDF (225 kg ha^-1), and significantly superior to the rest of the treatments.

The highest available soil nitrogen in both the seasons (2018 and 2019) was recorded in the treatment T₇ (80% RDF + bio-priming with Trichoderma asperellum), i.e., 240 and 249 kg ha^-1 respectively that was at par with T₃ – T₆ and T₈ treatments. Enhance in soil available nutrients was due to the ability of Trichoderma to solubilize soil nutrients (Yadav et al. 2009; Kapri and Tewari 2010). Inoculation of Trichoderma spp. enhanced soil available nitrogen in Pinus (Halifu et al. 2019), in pepper (Liu et al. 2020), soil available phosphorus in maize (Mercl et al. 2020), sugarcane (Yadav et al. 2009), in Lolium perenne (Song et al. 2014), in alfalfa (Zhang et al., 2019), cassava (Hridya and Byju 2014), sugarcane (Thakur et al. 2010).

Available soil phosphorus content was ranged between 15.5 and 24.07 kg ha^-1 in pooled analysis (Fig. 5). Pots received 80%, 90% RDF along with bio-priming showed maximum available phosphorus content (23.15, 23.32 kg ha^-1) that was comparable to 100% RDF treatment (22.58 kg ha^-1) and significantly superior to the rest of the treatments in the first season.

The highest available P at harvest of sunflower in the second season experiment was noticed in T₈ (90% RDF + bio-priming with Trichoderma asperellum) (24.82 kg ha^-1) that was found at par with the treatments T₃ to T₇ (21.33 to 24.14 kg ha^-1) and significantly superior to control (16.62 kg ha^-1) and T₂ (20.45 kg ha^-1). Phosphorus solubilization of a substrate (calcium phosphate) by Trichoderma spp. was ranged between 51.7 % and 90.3 % after 10 days incubation (Bononi et al. 2020). Inoculation of Trichoderma spp. enhanced soil available phosphorus in Pinus (Halifu et al. 2019), in pepper (Liu et al. 2020), soil available phosphorus in maize (Mercl et al. 2020), sugarcane (Yadav et al. 2009), in Lolium perenne (Song et al. 2014), in alfalfa (Zhang et al. 2019), cassava (Hridya and Byju 2014), sugarcane (Thakur et al. 2010).

Available soil potassium content at harvest was ranged from 212 to 247 kg ha^-1 according to pooled data (Fig. 6) in which the highest was found in the treatment supplied with 90% RDF + bio-priming with Trichoderma asperellum (247 kg ha^-1) comparable to the treatments T₂ to T₇ and all fertilized treatments except T₂ were significantly superior to control. The highest available potassium content in both first and second season crop was found in T₈ (246 and 248 kg ha^-1 respectively) that was comparable to the treatments T₂ to T₇ (239 to 247 kg ha^-1 and 233 to 246 kg ha^-1 respectively) and significantly superior to control. Enhance in soil available nutrients was due to the ability of Trichoderma to solubilize soil nutrients (Yadav et al., 2009, Kapri and Tewari, 2010). Available soil potassium with Trichoderma inoculation was recorded in alfalfa (Zhang et al., 2019), cassava (Hridya and Byju, 2014), sugarcane (Thakur et al. 2010).

Correlation among soil pH, EC, CEC, available N, P, and K was presented in Fig. 7.

Normal curve of each parameter is depicted. The correlation coefficient ranged from−0.611 to 0.971. Soil reaction and electrical conductivity showed a negative correlation with all studied parameters viz., CEC, available nitrogen, phosphorus, and potassium ranged from−0.611 to−0.025 with pH and−0.359 to−0.148 with EC. Cation Exchange Capacity was positively correlated with Soil available nitrogen (0.779), phosphorus (0.777) and potassium (0.803). Soil nitrogen strongly correlated with soil phosphorus (0.959) and potassium (0.971), while phosphorus and potassium were strongly correlated with each other (0.949).