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Soil properties of rhizosphere under maize based cropping system

This field experiment was conducted for two consecutive years during 2014-15 and 2015-16 on loamy sand soil of Agronomy Main Research Farm, Department of Agronomy, Central Research Station of the Odisha University of Agriculture and Technology, Bhubaneswar. The treatments consisted of three legumes as main plot treatments taken up during the kharif season and two residue management practices as sub-plot treatments and four nitrogen levels as sub-sub plot treatments. It shows that soil microbial population increased linearly over the initial level by taking a legume-cereal sequence. Further the incorporation of legume residues improved the microbial population over no residue incorporation treatments. Decreased trend of the total microbes was seen after the harvest of maize. An increase in available N, P and K was noticed with increase in nitrogen levels during both the years of study. After the harvest, cowpea recorded the highest available soil nitrogen values of 278 and 284 kg ha in the first and second year respectively as well as highest total bacterial count of 228 x 10 g of soil was recorded with cowpea residue incorporation with nitrogen application at 180 kg ha.


Introduction
Food security, nutritional security, maintenance of soil health, enhancement of productivity and leaving rightful heritage for future generation are the main focus of our agricultural development. Among the various food crops, cereals have been the main focus of this progress and have been the keepings of the transformation of Indian agriculture from security to surplus. Among various cereal crops, maize (Zea mays L.) is one of the most predominant one. High amounts of chemical fertilizers have been used to increase output in monocropping systems. However, large-scale monoculture led to a decline in the biological diversity, destroyed the capability of self-adjustment of the farmland ecosystem and caused diseases. Intercropping could be a potential candidate used to develop a sustainable agriculture systems, which focused on low input and high output. It is growing of two or more crops in the same field during the growing season. Intercropping can effectively improve the mobilization and uptake of nitro-gen (N), phosphorus (P) (Li et al. 2014) [7] , potassium (K), and micronutrients through interspecific interactions in rhizosphere (Inal et al. 2007; Zuo and Zhang 2008) [4] ; efficient acquisition of nutrients (Li et al. 2014) [7] , establishment of soil microbial diversity (Lacombe et al. 2009) [6] and better utilization of resources. Soil microorganisms and soil enzyme activities play important roles in biochemical processes in the rhizosphere ecosystem including nutrient cycling, decomposition of organic matter and suppression of soil-borne pathogens (Li et al. 2012). The plant can affect its rhizosphere microbial community by release of root exudates (Kourtev et al. 2003) [5] . Changes in microbial community composition can influence the potential of soil enzymes. Alteration in microbial community can be observed due to quantity and qualitative differences in root exudation between intercropping and monocropping systems (Baudoin et al. 2003) [2] . The changes in soil microbial communities may be both a cause and a reflection of the better performance in the intercropping systems. Recently, many studies have focused on rhizospheric microbial community and biochemical processes between intercropping crops (Zhou et al. 2011;Bainard et al. 2013). Dai et al. (2013) [11,1,3] used phospho-lipid fatty acid (PLFA) analysis and proposed that peanut intercropped with medicine plant Atractylodes lancea could significantly increase the soil urease, invertase activities and soil Gram-negative bacteria (G-), which have the potential to reduce the accumulation of phenolic allelochemicals in soil. Peanut (Arachis hypogaea L.) is an important oil seed and cash crop, which has been used to intercrop with many plants. Many studies have suggested that maize intercropping was a successful crop management strategy, which could facilitate peanut acquisition of Fe and Zn and improve yield of both crops (Inal et al. 2007;Zuo and Zhang 2009) [4,12] . However, there is little information about the relationships among the microbial community, soil enzymes and soil nutrients in the intercropping system of maize. There has been an increase of interest in the rhizospheric biological processes and plant-microbe interactions.

Material and methods:
In this current study, the treatments consisted of three legumes, viz., groundnut (C 1 ), cowpea (C 2 ) and cluster bean (C 3 ) as main plot treatments taken up during the kharif season and two residue management practices viz., residue incorporation (I 1 ) and no residue incorporation (I 2 ) as sub-plot treatments and four nitrogen levels 75 % RDN (N 1 ), 100 % RDN (N 2 ), 125 % RDN (N 3 ) and 150 % RDN (N 4 ) as sub-sub plot treatments to maize as 120 kg N ha -1 being the recommended dose during rabi in Odisha and this corresponds to 90 kg ha -1 , 120 kg ha -1 , 150 kg ha -1 and 180 kg ha -1 respectively. The experiment was laid out in split -split plot design with three replications. During the field experimentation, a composite soil sample was collected from each experimental plot from 0-30 cm depth before sowing and after harvest from each treated pot. The collected soil samples were dried under shade, powdered using wooden pestle and mortar and passed through 2mm sieve and preserved in polythene bags for analysis of pH, EC, organic carbon and available nitrogen, phosphorus and potassium. For organic carbon analysis, the 2 mm sieved soil samples were subjected for further grinding and passed through 0.2 mm sieve.
Chemical properties and the methods employed for analysis of the soil
The enumeration of total fungi, bacteria, Azotobacter and Rhizobium in the soil samples collected from the experimental plots before sowing and after harvest of different legumes, after incorporation of the crop residues and after harvest of maize was estimated by following the standard dilution plate count technique by pour plate technique. Nutrient agar (NA) for bacteria, Martins Rose Bengal with streptomycin sulphate agar (MRBA) for fungi, Ashyb's agar for Azotobacter, Yeast extract Mannitol agar (YEMA) with congo red for Rhizobium were used for enumeration (Appendix-I). The petriplates were incubated after plating at 30 o C for two to four days and population was counted and expressed as number of cells per gram on dry weight basis for bacteria, Azotobacter and Rhizobium and cfug -1 of soil for fungi.
The objectives of this study were to determine the effects of intercropping on rhizosphere microbial community composition, soil properties and relationship between microbial community and soil nutrients.

Results and Discussion Soil microbial population
The population of soil microbes was assessed at four stages viz., initial, after the harvest of kharif legumes as well as after the incorporation of legumes and after the harvest of rabi maize. The data presented in Table 2. revealed that there was an increase in microbial population compared to initial population during both the years of experimentation. Improvement in soil microbial population viz., total bacterial, Rhizobium, Azotobacter, Actinomycetes and fungi was observed over the initial population when legumes were grown during kharif. Among the legumes, maximum number of soil microbes was found after the harvest of cowpea followed by cluster bean and groundnut. During both the years of experimentation, the micro flora count was more in the second year compared to first year of experimentation. Soil incorporation of legume crop residues has further improved the micro flora population during both the years.
The maximum bacterial population of 175.3 x 10 4 g -1 of soil and 218.4 x 10 4 g -1 of soil were found with incorporation of cowpea residues followed by cluster bean during 2014-15 and 2015-16, respectively. The least population of microbes was observed after the soil incorporation of groundnut residues during both the years of experimentation ( Table 2). Perusal of data presented in Table 3. revealed that there was a decreasing trend in the population of entire soil microbes after the harvest of rabi maize during both the years and maintained similar trend as that was observed in the initial as well as after the harvest of kharif legumes. Further it was observed that the microbial population increases with increased level of nitrogen application. The highest total bacterial count of 177 x 10 4 g -1 of soil and 228 x 10 4 g -1 of soil was recorded with cowpea residue incorporation with nitrogen application at 180 kg ha -1 during 2014-15 and 2015-16, respectively, while the lowest total bacterial count of 67 x 10 4 g -1 of soil and 102 x 10 4 g -1 of soil were recorded in groundnut without residue incorporation and with nitrogen application at 90 kg ha -1 .

Soil bulk density
The bulk density of soil was estimated at three phases viz., initial, after the harvest of legumes and after the incorporation of legume residues during both the years of experimentation. It was observed that the soil bulk density decreased over the initial value with all the legumes cultivation. The lowest bulk density of 1.49 and 1.44 g cc -1 were recorded with cowpea during 2014-15 and 2015-16, respectively. Similarly maximum bulk density of 1.51 and 1.46 g cc -1 were obtained with groundnut for the corresponding period. Legume residue incorporation had shown further decrease in soil bulk density over no residue incorporation. The lowest bulk density values of 1.47 and 1.4 g cc -1 were recorded with cowpea residue incorporation in the first and second year, respectively, while the highest value (1.48 and 1.43 g cc -1 ) was obtained with groundnut residue incorporation in both the years (Table 1).

Soil characteristics after harvest of legumes
The analytical data on different soil characters after the harvest of different legumes was presented in Table 3 for both the years of study. The data revealed that, there was not much variation in soil pH, EC and organic carbon content after the harvest of legumes during 2014-15 and 2015-16.The available N, P and K in the soil were found to be higher in plots after the harvest of cowpea during the first and second year of study as compared to groundnut and cluster bean plots.

Post harvest status of available N, P and K in the soil after the harvest of maize
Residual fertility of the soil is dependent on the initial nutrient reserve of the soil, addition of nutrients to the crop and utilization by the crop. The available N, P and K in the soil were the highest in plots of groundnut and cluster bean compared to cowpea after the harvest of maize. The residual values of available N, P and K were recorded more or less equal than the initial values which indicated that soil fertility was maintained with different management practices. Further, the available status of N, P and K was more in plots where residues were incorporated compared to their removal. The differential behavior of a legume in influencing the soil fertility largely depended on the growth of the legume. Also, the higher yield of cereals might have removed large amount of nutrients leaving the soil under nutrient stress. These results are in confirmity with the findings of Tripathi and Hazra (2002), Shrikant Chital et al. (2003) and Tarfa et al. (2006) [10,8,9] .

Conclusion
Soil fertility was maintained at the end of crop sequence. After the legume harvest, the available N, P and K in the soil was more in cowpea followed by cluster bean and groundnut.