Rhizosphere Effect of Kharif Crops on Phosphatases and Dehydrogenase Activities in a Typic Haplustert

In recent years phosphorus crisis is emerging at global level, every country in the world searching for alternative sources and management options for increasing its supply and use efficiency. Increasing cost of cultivation due to phosphatic fertilizers, attracting our mind towards it. In this experiment it was focused on effect of major kharif crops (maize, soybean, sorghum, pearl millet, finger millet and rice) on phosphatases and dehydrogenase activities in soil at different crop growth periods (30, 50, 75, 90 days after sowing and at crop harvest). Acid phosphatase activity varied crop wise and also at different crop growth periods. The maximum acid phosphatase activities were observed at 75 DAT in rice (0.206 mg PNP/g h), 90 DAS in sorghum (0.194 mg PNP/g h) and pearl millet (0.201 mg PNP/g h), 50 DAS in soybean (0.127 mg PNP/g h), and 30 DAS in maize (0.154 mg PNP/g h) and, 50 DAS as well as 75 DAS in finger millet (0.180 mg PNP/g h), respectively. But alkaline phosphatase activities were maximum at 75 days after sowing in all the crops i.e. sorghum (0.533 mg PNP/g h), soybean (0.465 mg PNP/g h), rice (0.440 mg PNP/g h), maize (0.538 mg PNP/g h), pearl millet (0.546 mg PNP/g h) and finger millet (0.530 mg PNP/g h), respectively. The dehydrogenase activities were also higher in 75 days after sowing in all the crops. These enzyme activities associated with higher availability and uptake of phosphorus.


Introduction
Phosphorus (P) is an important plant nutrient for crop growth and crop yield [1,2]. It maintains biochemical, metabolic functions in plants; maintains soil fertility and soil health [3]. But lower phosphorus use efficiency limits crop yield and economic return [4]. Phosphorus fertilizers are totally imported in India and indigenous rock phosphate having low P concentration, increasing crop production cost. Plants exudate various type organic substances [5], which convert immobilize P into more plant available P [6]. Different phosphatases enzyme activities speed up the conversion process of immobile P to plant available P in soil solution, and indirectly enhance plant P uptake [7]. In rhizosphere region many microorganisms play a crucial role to enhance enzyme activities and P solubilization process. But it varies accordingly crop wise, species wise and under different crop cultivation practices [8]. Bacteria are more effective in phosphorus solubilization than fungi [9]. Among the whole microbial population in soil, phosphate solubilizing bacteria constitute 1-50 %, while phosphorus solubilizing fungi are only 0.1-0.5 % in P solubilization potential [10]. Hence the effect of different crops on phosphatase enzyme activities and dehydrogenase activity (DHA) should be studied and understood. Keeping all the above in mind, the pot culture experiment was conducted to understand the rhizosphere effect of maize, soybean, sorghum, pearl millet, finger millet and rice on these enzyme activities.

Materials and Methods
A pot culture experiment was conducted at Indian Institute of Soil Science (IISS), Bhopal, India during kharif season of 2011. Geographically, Bhopal is situated at 23°18 0 N, 77°24 0 E and 485 m above mean sea level. It has semi-arid and sub-tropical dry summer and the cold winter climate with a mean annual air temperature of 25°C and annual rainfall of 1,208 mm. Soil for the pot experiment was collected from the field of IISS, Bhopal. The collected soil was air dried and ground, and passed through a 2 mm sieve. The experimental soil belongs to Vertisols i.e. Hypothermic family of Typic Haplusterts popularly known as ''black cotton soil''. The soil had the following properties : texture clay loam; pH(1:2.5) 8.06; EC (dSm -1 ) 0.57; organic carbon 0.44 %; available N 115 kg ha -1 ; available P 7.53 kg ha -1 , available K 185 kg ha -1 and available S 9.1 kg ha -1 . The processed soil was filled in 90 plastic pots @ 10 kg/pot. In this experiment six crops were grown viz. maize, soybean, sorghum, pearl millet, finger millet and rice; the details of crop varieties and their characteristic features were depicted in Table 1. The crops were grown and rice seedlings were transplanted and replicated thrice. Recommended dose of N, P 2 O 5 and K 2 O (kg/ha) were applied i.e. 100:60:60 for sorghum, rice, maize and pearl millet; 30:60:40 for soybean and 60:40:40 for finger millet through urea, diammonium phosphate and muriate of potash. Half dose (50 %) of N and full doses of P 2 O 5 and K 2 O were applied as basal at the time of sowing in all the crops but in soybean all the nutrients were applied at the time of sowing. The remaining N was applied as two equal splits at 30 and 60 DAS. Zinc sulphate was applied @ 10 kg/ha for rice and 7 kg/ha other crops at the time of sowing. Soil samples were collected during the crop growth period at different time interval (30, 50, 75, 90 days after sowing/ transplanting (DAS/DAT), and at crop maturity) and, the DHA [11], acid and alkaline phosphatase activities were measured [12]. Statistical analysis was done for complete randomized design using WASP 2 [13].

Results and Discussion
Acid Phosphatase Activity Data pertaining to acid phosphatase activities of soil was presented in Table 2. Variation in acid phosphatase enzyme activities was observed crop wise and at different crop growth periods. Among the crops, rice showed the highest mean acid phosphatase activity (0.173 mg PNP/g h) followed by finger millet (0.165 mg PNP/g h), pearl millet (0.154 mg PNP/g h), sorghum(0.140 mg PNP/g h), maize (0.140 mg PNP/g h) and soybean (0.107 mg PNP/g h). The acid phosphatase activity of soil varied in all the crops at different crop growth period. The maximum acid phosphatase activity was observed at 75 DAT for rice (0.206 mg PNP/g h), 90 DAS for sorghum (0.194 mg PNP/g h) and pearl millet (0.201 mg PNP/g h), 50 DAS for soybean (0.127 mg PNP/g h), and 30 DAS for maize (0.154 mg PNP/g h) and, 50 DAS as well as 75 DAS for finger millet (0.180 mg PNP/g h), respectively. Similar observations about the effect of different micro-organism on acid phosphates were observed in maize [9,14]. Different crop growth stages showed different amount of acid phosphatase activity, it might be due to secretion of root exudates and biochemical changes in plant system [15]. Rice crop produce higher amount of root exudation in initial stages of crop growth which enhanced microbial activity in crop and modify nutrient concentrations in soil solution [16].

Alkaline Phosphatase Activity
The alkaline phosphatase activity of soil determined at different crop growth periods was presented in Table 3.  . It might be due to vigorous growth of the crops at 75 DAS, and the root exudation also more in this period. More microbial activities increased the DHA in the rhizosphere, due to more availability of food materials for its growth [17]. Variation in DHA activity in the rhizosphere due to plant genetic as well microbial species associated with it [18].

Conclusion
It was concluded that the kharif crops play a vital role in Indian food grain production but the cost of production is increasing due to fertilizers especially phosphorus. It reduces the profit of farmers. Different crops having rhizosphere effect on nutrient availability. Experimental data showed that various crops rhizosphere produced a different level of acid phosphatase activity. The highest acid phosphatase activity was observed at different stages for different crops, but alkaline phosphatase and dehydrogenase activities were maximum at 75 DAS in all the crops. These enzyme activities associated with higher availability and uptake of nutrients particularly phosphorus in plants.
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