Influence of Nutrient Mobilizers on Productivity and Nutrient Uptake in Blackgram (Vigna mungo L.) Crop under the Tarai Region of Uttarakhand

The experiment was conducted in the kharif season of 2016-17 and 2017-18 at the Norman E. Borlaug Crop Research Centre of Govind Ballabh Pant University of Agriculture and Technology, Pantnagar, Uttarakhand. The soil of the experimental site has been classified as series II silty clay loam under the order Mollisol (Deshpande et al., 1971). The soil is dark coloured, imperfectly drained with moderately high organic matter content developed in loamy alluvial sediments averaging 0.6 to 1.0 meter thick over loamy sand, sand or gravel. The soil has high cation exchange capacity and water holding capacity and also contains about 90 per cent saturation. The eleven  treatments were as follows: T₁ (Control), T₂ (Rh (Rhizobium) + RDF), T₃ (Rh + ZnSO₄ @ 12.5 kg/ha + RDF), T₄ (Rh + ZnSO₄ @ 25.0 kg/ha + RDF), T₅ (Rh + ZnO @ 14.0 kg/ha + RDF), T6 (Rh + ZnO @ 28.0 kg/ha + RDF), T₇ (Rh + Zn Nm (Zinc nutrient mobilizer) + PGPR + RDF), T₈ (Rh + Zn Nm + PGPR + ZnSO₄ @ 12.5 kg/ha + RDF), T₉ (Rh + Zn Nm + PGPR + ZnSO₄ @ 25.0 kg/ha + RDF), T₁₀ (Rh + Zn Nm + PGPR + ZnO @ 14.0 kg/ha + RDF) and T₁₁ (Rh + Zn Nm + PGPR + ZnO @ 28.0 kg/ha + RDF) with blackgram  (PU-31) crop having a plot size 5 m ´ 4 m. The treatments were replicated thrice and laid out under randomized block design (RBD). After thorough field preparation initial soil samples were taken to analyze the initial soil properties. The initial soil sample was analyzed for major nutrients viz. available N, P, K, S and Zn; organic carbon (OC), pH and EC. The pH of the experimental field was 7.21, EC 0.18 dSm^-1 and organic carbon was 0.69 per cent. The available N, P, K, S and Zn status of the experimental field was 148.25 kg ha^-1, 17.07 kg ha^-1, 168.70 kg ha^-1, 8.65 ppm and 0.79 mg ha^-1, respectively.
       
The observations on yield attributing characters were recorded on five randomly selected plants from each plot of each replication separately as per the standard method at the physiological maturity stage. The seed and straw yield was recorded from net plot area of each treatment.
       
For the assessment of nutrient content and uptake, the samples were collected for chemical analysis for nitrogen, phosphorus and zinc content in grain and straw. The nitrogen content was determined with Micro Kjeldhal’s method (Jackson, 1967). For phosphorous estimation, digestion of sample was done as per method No. 54 of 45 USDA, Handbook No. 60 using nitric acid and perchloric acid. Phosphorous was determined by vandomolybdo phosphoric acid yellow coloured method in HNO3 system as described by Jackson (1974).
       
Zinc was determined by using Atomic Absorption Spectrometer (AAS) method (Jackson, 1974). The data obtained by different treatments on various characters under study were analyzed by the method of analysis of variance as described by Gomez and Gomez (1984).

Yield and yield attributes
 
Among the different treatments, the average number of pods per plant from both the years ranged from 31.17 to 51.67 pods per plant (Table 1). Combined inoculation of Rhizobium, PGPR, zinc mobilizers and zinc fertilizers along with RDF recorded significantly more number of pods per plant ranging from 17.39 to 65.77 per cent over the uninoculated control. It can be clearly inferred from the table that the maximum number of pods per plant (51.67 pods plant^-1) has been counted from the treatment T₁₀ that was closely followed by T₉ (51.00 pods plant^-1). The longest pods (4.98 cm) have been measured in the treatment T₇ followed by T₁₀ (4.94 cm) which was at par with T₉ (4.93 cm) whereas, shortest pods (3.13 cm) have been noted in the control. A significant increase of 59.11 per cent has been observed over the control in terms of pod length.
 

       
The average number of seed per pod from both the years ranged from 3.74 to 6.00 seed pod^-1. The maximum number of seed per pod (6.00 seed pod^-1) has been found in the treatment T₁₁ which was non-significantly different from T₁₀ (5.85 seed pod^-1) and T₉ (5.51 seed pod^-1) while, minimum number of seed per pod (3.74 seed pod^-1) has been recorded in the control. Application of Rhizobium, PGPR, zinc mobilizers and zinc fertilizers along with RDF recorded significantly more number of seeds per pod ranging from 4.81 to 60.43 per cent over the uninoculated control. The average weight of the 1000-seeds from both the years ranged from 26.41 to 33.60 g. Data on 1000-seed weight revealed that mean maximum 1000-seed weight (33.60 g) among the different treatments has been found in T₁₁ while, minimum weight (26.41 g) was recorded from the control. A significant increase of 27.22 per cent has been observed over the control in terms of 1000-seed weight.
       
Significant differences have been reported among the treatments regarding grain and straw yield. Maximum grain yield (1219.4 kg ha^-1), straw yield (1337.5 kg ha^-1) and harvest index (47.7 per cent) was also recorded in the treatment T₁₁ which gave significantly 31.44, 17.27 and 6.2 per cent increase over control, respectively (Table 2 and Fig 1).
 

 

       
Inoculation with nutrient mobilizers viz. Rhizobium and PGPR has found to increase the yield attributing characters in urdbean. Mishra et al., (2010) in field pea and Hosseini et al., (2014) in green gram noted a positive effect of dual inoculation of Rhizobium and PGPR and observed significant results on yield attributing characters. PGPR leads to the production of plant hormones (phytohormone) i.e., auxins, gibberellins, cytokinins, abscisic acid and ethylene (Gopalakrishnan et al., 2015). These hormones when applied to the plants helps in their better growth and increase in the yield attributing characters (Frankenberger and Arshad, 1995). Also, these helps in the fixation of atmospheric nitrogen by enhancing root nodule number or mass, hence, better growth and development of the plants. The favourable effect of PGPR and Rhizobium may be attributed to increase in the activity of microorganisms due to synergistic association between them (Bhardwaj et al. 2014). They colonize the plant roots completely and enhance the plant’s growth by different mechanisms such as phosphate solubilization (Ahemad and Khan, 2012), formation of indole acetic acid (IAA), production of siderophores (Jahanian et al., 2012), 1-amino-cyclopropane-1-carboxylate (ACC) deaminase and hydrogen cyanate (Liu et al., 2016); suppressed deleterious rhizobacteria (Kloepper and Schroth, 1981) and antibiotics or lytic enzymes (Xie et al., 2016).  Inoculation of the field with bacterial isolates (PGPR and Rhizobium) improved the photosynthetic efficiency maybe by accelerating the water and nutrients absorption that led to the production of more assimilate, thus, improving plant growth (Naseri and Mirzaei, 2010).
 
Nutrient content and uptake
 
Nitrogen content in grain and straw
 
From the Table 3, it can be concluded that not much significant difference has been found in the nitrogen content in the grain. The average numerical value ranged between 3.59 to 4.35 per cent in both the year which gave 8.00 to 60.41 per cent increase as compared to control. Maximum N content (4.35 per cent) has been recorded in the treatment T₁₁ which was non-significantly different from treatment T₁₀ (4.32 per cent) whereas, mean minimum was noticed in the control (3.59 per cent). Similarly, the maximum nitrogen content in the straw (2.03 per cent) has also been recorded in the treatment T₁₁ followed by T₁₀ (2.00 per cent). A significant increase of 40 per cent has been observed by the combined application of Rhizobium, PGPR, zinc mobilizers and zinc fertilizers along with RDF over the control.
 

 
Nitrogen uptake in grain and straw
 
The average numerical value for N uptake in grain and straw ranged between 33.24 to 53.32 kg ha^-1 and 16.48 to 27.16 kg ha^-1, respectively (Table 3). Treatment T₁₁ resulted in maximum nitrogen uptake in the grain (53.32 kg ha^-1) and straw (27.16 kg ha^-1) followed by treatment T₁₀ (49.41 and 26.46 kg ha^-1) whereas, minimum was noted from the control. A significant increase of 60.14 and 64.81 per cent has been observed over the control due to combined application of Rhizobium, PGPR, zinc mobilizers and zinc fertilizers along with RDF in grain and straw, respectively.
       
The effects of PGPR and Rhizobium on nitrogen contents in urdbean when compared with uninoculated control are conclusive and strongly indicate a possible role of these inoculants in providing nitrogen. Co-inoculation of PGPR and Rhizobium has led to the maximum nitrogen content (4.61 and 2.49 per cent) and uptake (165 and 568 mg plant^-1)   in grain and straw than the control (2.92 and 1.76 per cent; 41 and 320 mg plant^-1) in mungbean (Raza et al., 2004).  Similar results have been illustrated by Verma et al., (2010a) in chickpea, who observed increased content of nitrogen in seed (31.75 and 39.67 per cent) and straw (55.17 and 48.38 per cent) for two consecutive years. In a study by Zafar et al., (2012), inoculation of lentil with PGPR increased the nitrogen content in the seed and plant by an average of 25 and 29 per cent, respectively. They also recorded significant increase in nitrogen uptake in plants (range between 60 and 105 mg plant^-1) with PGPR application, demonstrating a 28 to 123 per cent increase over the control (47 mg plant^-1). The increase in nitrogen content consequently increased nitrogen uptake, which was certainly due to the nitrogen fixation by the PGPR. The increase in nitrogen content and nitrogen uptake by plants due to inoculation were in agreement with some previous studies (Abbasi et al., 2011). Increase in total nitrogen content and plant uptake could be due to nitrogen fixation and nitrate reductase activities of PGPR, or to the uptake of NH⁴⁺ and amino acids produced by plant growth promoting rhizobacteria (Osman et al., 2010).
 
Phosphorous content in grain and straw
 
A significant difference regarding the phosphorous content in the grain has been found among the different treatments (Table 4). The maximum phosphorous content in the grain (1.20 per cent) and straw (0.44 per cent) has been recorded in the treatment T₁₁ while, the minimum (0.67 and 0.23 per cent) has been noted from the control (uninoculated). Due to the use of different nutrient mobilizers, an increase of 79.10 and 91.30 per cent has been noted in the grain and straw, respectively when compared to control.
 

 
Phosphorous uptake in grain and straw
 
From the data presented in Table 4, it can be observed that the phosphorous uptake in the grain differed significantly among the treatments. The average phosphorous uptake in grain and straw ranged from 6.15 to 14.74 kg ha^-1 and 2.62 to 5.93 kg ha^-1 which gave 21.46 to 139.67 per cent and 15.65 to 126.34 per cent increase over the uninoculated (control). The maximum phosphorous uptake in the grain (14.74 kg ha^-1) and straw (5.93 kg ha^-1) has been recorded in the treatment T₁₁.
       
The present results were in accordance with Verma et al., (2010a) who observed increased content of phosphorous in grain (52.78 and 34.21 per cent) and straw (36.04 and 33.04 per cent) for two consecutive years in chickpea. Similarly, Zafar et al., (2012) also observed increased phosphorous content in plant (0.42 to 0.71 per cent), grain (0.52 to 0.83 per cent) and uptake (17 to 35 mg/plant) by plants with the application of PGPR as compared to control (13 mg/plant). The increases in phosphorous content and uptake may partially be attributed to the production of a variety of organic acids by the PGPRs, which helps in decreasing soil pH, leading to the conversion of non-available phosphorous into available phosphorous. Soil microorganisms can also make phosphorous available to the plants by producing chelating substances, which leads to solubilization of phosphates (Osman et al., 2010). Cong et al., (2009) explained that the higher nutrient uptake was due to inoculation with Rhizobium and PGPR that might attribute to morphological changes in the plant roots, especially increase in root number, length and its thickness. Co-inoculation of Rhizobium and PGPR has been reported more significant because Rhizobium is an effective nitrogen fixer while, PGPR stimulates plant growth by direct (production of phytohormone (IAA), improved nutrient acquisition increasing uptake of phosphorous from phosphate mineral solubilizing bacteria) and indirect mechanisms (suppression of plant diseases like wilt and root rot) (Verma et al., 2010b).
 
Zinc content in grain and straw
 
As observed from the Table 5, it can be opined that the zinc content in the grain differed significantly among all the treatments giving 6.77 to 68.26 per cent increase over the control. It was found that the maximum zinc content in the grain (34.56 ppm) and straw (15.02 ppm) has been recorded in the treatment T₁₁ while the minimum content (20.54 and 9.82 ppm) has been noted from the control (uninoculated), respectively.
 

 
 
Zinc uptake in grain and straw
 
As observed from the Table 5, it can be observed that the zinc uptake in the grain and straw differed significantly among the treatments and gave 11.53 to 121.17 per cent and 7.06 to 79.45 increases over the uninoculated (control). The maximum zinc uptake in the grain (42.20 g ha^-1) and straw (20.08 g ha^-1) has been recorded in the treatment T₁₁ while, minimum was noted from the control.
       
It was also found that bacterial inoculation along with zinc application has led to an increased nutrient content and uptake in seed and straw (Ahmad et al., 2013).  Significantly maximum total zinc content (41.48 ppm) and uptake (654.66 g ha^-1) has been recorded by Tiwari et al., (2018) with the inoculation of Rhizobium along with ZnSO₄ and RDF in lentil. This increase may be due to the beneficial effect of micronutrients on bacterial intensification. It was reported that soil inoculation with beneficial microorganisms has the tendency of mobilizing unavailable forms of nutrient elements to available forms. This property has been successfully exploited for increasing the quality grain production (Ibrahim et al., 2010). This may also be due to the production of gluconic acid by bacterial isolates which enables solubilization of insoluble zinc and make it available to the plants (Vaid et al., 2014).

As the population is expanding day by day, there is a need to boost up the pulse production so as to fulfill the demands of the people. Experiment results revealed that urdbean responds significantly towards the incorporation of nutrient mobilizers in the soil and improves the productivity. On the basis of the experimental findings from field, it could be concluded that the treatment T₁₁ which came up with the best in almost all the yield attributes may be followed for efficient nutrient management and more grain yields of urdbean during the Kharif season. However, the treatments T₁₀ and T₉ not showing much deviation from treatment T₁₁ in terms of yield and yield attributes could also be followed for better economics and for realizing higher yield levels in kharif urdbean under the Tarai conditions of Uttarakhand. The success of this experiment could be very helpful in narrowing the gap between nutrient availability and increased nutrient content and uptake in the pulses especially phosphorous and zinc in urdbean on nutrient deficient soils.