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Effect of Zero Tillage Practices and Nutritional Levels on Microbes, Enzymatic Activities in Soil and Productivity of Pigeonpea under Rainfed Situations

Conservation agriculture (CA) aims to conserve, improve and make more efficient use of natural resources through integrated management of available soil, water and biological resources combined with external inputs. It contributes to environmental conservation as well as to enhanced and sustained agricultural production. The retained surface crop residues increase the soil porosity and organic carbon.


Introduction
Conservation agriculture (CA) aims to conserve, improve and make more efficient use of natural resources through integrated management of available soil, water and biological resources combined with external inputs. It contributes to environmental conservation as well as to enhanced and sustained agricultural production. The retained surface crop residues increase the soil porosity and organic carbon.
spells. The concept of nitrogen synchrony is combining fertilizer with residues may serve to match the rate of soil N supply with the rate of plant N uptake, helps to increase the N use efficiency and reduce the N losses through leaching. Root colonization with arbuscular mycorrhizal fungi (AMF) can enhance the uptake of phosphorus. Thus fertilizer use or enhance the fertilizer use efficiency by minimum or zero tillage practices. All the modern practices like intensive tillage and high fertilizer use are energy intensive, as they use large share in global energy budget and these practices lead to emission of GHGs and which has led to global warming and climate change. Hence energy use efficiency in crop production is need of the hour.
The Roots of most plant species are usually colonized with AMF, the major function of AMF symbiosis for host plant helps to improve phosphorus nutrition, by enhancing the uptake of phosphorus by plant roots by providing larger surface area for absorbing the nutrients. Solubilisation of p is achieved by rhizospheric modification through the release of organic acids phosphatase enzymes and some specialized metabolites, like siderophores ( Shenoy and Kalagudi, 2005). AMF helps to ameliorate plant mineral nutrition, to enhance water stress tolerance, better soil aggregate formation, which helps for improving soil physical properties, these are the important factors for successful lowinput farming, which helps for sustainable agriculture production.
Enzymatic activities are considered to be good indicators of soil quality because they control the release of nutrients for plants and the growth of microorganisms. The activity of urease has also been widely used in the evaluation of soil quality changes due to soil management; example of nitrogen cycle process is characterized by urease activity. Significantly higher activity of urease and microbial biomass was measured using the minimum tillage system.
The highest dehydrogenase activity was measured during no tillage systems employed. (Mikanova et al., 2009) Therefore, in the present study, a polygonal approach was used to evaluate the sustainability of different conservation agricultural practices and inturn in reducing the nutrient requirement using biological, microbial and nutrient.

Materials and Methods
The experiment was conducted at Krishi Vigyan Kendra Farm, Kalburgi, University of Agricultural Sciences, Raichur, which is located at 16 0 2' North latitude, 76 0 42' East longitude. The soil was black soil of the order Vertisols. Zero tillage was practiced since last 3 years. Different nutrient doses were imposed as sub-plots during 2015 and 2016. Every year same crop pigeonpea was grown.
The rainfall received during 2015 was 601.1 mm, which was 20.86 per cent low as compared to average. Certified seeds of pigeonpea genotype TS-3R (12.5 kg ha -1 ) were sown with help of zero till machine. The prophylactic measures were undertaken against pests based on economic thresh hold level (ETL). The chlorophyll content was measured with help of SPAD chlorophyll meter. Plants from the net plot after threshing were dried and their weight was recorded from which stalk yield worked out and seeds were threshed and weighed.
All the Soil biological observations were analysed at 50 per cent flowering. Fresh and unsieved soil sample from rhizosphere zone were collected from the randomly selected seedlings in all plots at mid flowering stage and it was used for analysing the soil enzymes. The soil samples were used for determining dehydrogenase activity by the procedure described by Casida et al., (1964), Phosphatase activity by Evazi and Tabatabai (1979), Urease activity by Tabatabai and Bremner (1972), nodule number and nodule dry weight were recorded in five plants uprooted at mid flowering stage in gross plot area. Nodules were counted and expressed as number per plant. Nodules dry weight was expressed as mg per plant. Mycorrhizal root colonization (AMF) was estimated as per the procedure proposed by Philips and Hayman (1970).
The isolation and enumeration was done by using N-free malic acid semisolid medium for Azatobacter following MPN technique (Cochran, 1950), Pikovskaya's medium for phosphate solubilizers (Pikovskaya, 1948) and Kings B medium for Fluorescent pseudomonas. The number of colony forming units (CFU) was recorded. The counts were expressed per gram of soil.
The data collected from the experiment were analysed statistically following the procedure described by Gomez and Gomez (1984). The mean values of main plot, sub-plot and interaction were separately subjected to Duncan's multiple range test for analysis.

Results and Discussion
The pooled data of pigeonpea indicated that, among the conservation agriculture practices zero till-raised bed with residues retention consistently produced higher seed and stalk yield (1,383 and 5,163 kg ha -1 , respectively) compared to conventional tillage practice followed by zero till-raised bed without residues retention (1,285 and 4,843 kg ha -1 , respectively). Sepat et al., (2015) observed that zero tillage with raised bed had lower traffic compaction especially at deeper soil layer and deep prolific roots of pigeonpea explored the deeper layer, helps nutrient recycling and seed yields were higher in raised bed than flat bed.
Among the nutrient doses, 100 per cent recommended dose of inorganic fertilizer recorded significantly higher seed and stalk yield (1,274 and 4,736 kg ha -1 , respectively), but it was on par with 75 per cent (1,229 and 4,642 kg ha -1 , respectively) recommended doses of inorganic fertilizer. Leaf litter fall in pigeonpea provide good scope for nutrient recycling as indicated by Ahlawat et al., (2005).
The interaction effect of conservation agricultural practices and nutrient doses varied significantly in pooled data and highest seed and stalk yield were recorded when 100 per cent recommended dose of inorganic fertilizer was applied in zero till-raised bed with crop residues retention (1,447 and 5,297 kg ha -1 , respectively) and it was on par with 75 per cent recommended doses of inorganic fertilizer in zero till raised bed with residues retention.
This was due to even reduced dose of fertilizer in pigeonpea can produce more pod by the virtue of higher branches per plant and retention of flower in ideal soil environment with good soil moisture content and translocation of accumulated photosynthates to sink by producing higher seed weight per plant. Sharma et al., (2012) also indicated the same results in pigeonpea that plant height, pods per plant and 100 seed weight were highest in 100 per cent RDF along with application of 5 tonnes FYM per hectare treatment. Pigeonpea yield recorded was more in 100 per cent RDF than 0 per cent RDF. Even under lower fertilizer dose, conservation agriculture practices help to build up of organic matter and improved recycling of inorganic inputs. Similar results were confirmed by Sainju et al., (2006).

Sustainable yield index
Significantly higher sustainable yield index was recorded in zero till-raised bed with residues retention (0.73) compared to other practices, lower was observed in conventional tillage (0.51). Among the nutrient doses, significantly higher sustainable yield index was recorded in 100 per cent recommended dose of inorganic fertilizer (0.64) than other doses.
Significantly higher sustainable yield index was observed in zero till-raised bed with residues retention along with application of 100 per cent recommended dose of inorganic fertilizer (0.74) compared to other combinations, but was on par with 75 and 50 per cent doses in zero till raised bed with residues retention. The nearness of the SYI to 1 implies the closeness to an ideal condition that can sustain maximum crop yields, whereas deviation from 1 indicates losses to sustainability (Reddy et al., 1999).

Chlorophyll content (SPAD values)
The pooled data indicated that effect of different conservation agricultural practices varied significantly with SPAD values at all the stages of crop. SPAD values recorded at mid flowering (42.10) was higher in zero tillage raised bed with crop residues retention than remaining treatments.
Higher SPAD values were recorded when 100 per cent recommended dose of inorganic fertilizer applied at mid flowering (41.73). Application of 100 per cent recommended levels of nutrients in zero till-raised bed with residues retention recorded higher SPAD values at mid flowering (44.00). Higher quantity of fertilizer has increased plant chlorophyll content and plant biomass growth thereby increasing SPAD values (Govaerts et al., 2006). This was due to balanced nutrient helps to more chlorophyll development in crop plant, which helped in production of higher plant dry matter (Kumar et al., 2014).

Nodule number, nodule weight and arbuscular mycorrhizal fungi (AMF) root colonization
Significantly higher nodules, nodule weight and Arbuscular mycorrhizal fungi (AMF) root colonization were observed in zero tillage raised bed with residues retention (19.92, 124.59 mg plant -1 and 21.04 %) and lowest was found in conventional tillage practices (15.58, 81.68 mg plant -1 and 14.87 %). Significantly higher nodule number recorded with the treatment received no fertilizer (18.73, 106.41 mg plant -1 and 22.17 %) and number decreased with increase in the dose of inorganic fertilizer and lowest number was found in 100 per cent recommended inorganic fertilizers (17.00, 99.82 mg plant -1 and 14.74 %).
Interaction effect of zero nutrient in zero tillage raised bed with residues retention (21.33, 128.92 mg plant -1 and 25.56 %) was higher compared to all other combinations. Higher number of nodules per plant and higher biologically fixed nitrogen in soybean grown in CA than conventional (Muchabi et al., 2014). Nodulation and nodule dry weight on flat bed and conventional tillage was less due to water stagnation due to reduced root growth, nodule fresh mass, root mass density in conventional tillage and on flat beds by inhibiting aerobic respiration (Singh et al., 2010).
Higher root nodule at lower dose of fertilizer was due to the fact that the mineral nitrogen reduces nodule formation and thereby affecting symbiotic N fixation, smaller starter dose stimulate nodule formation. Rhizobium population was increased at lower pH towards neutral pH (Basu et al., 2008).
These results were due to lower C:N ratio material like legume avoids initial immobilization. It helps to build up higher soil organic carbon which increases the microbial activity. Different doses of recommended inorganic fertilizer had non-significant effect on dehydrogenase and urease activity, but phosphatase activity was significant with different nutrient doses.
Enzymatic activity was increased with reduction in recommended dose of inorganic fertilizer, the higher dehydrogenase enzyme activity (14.44 g TPF g -1 day -1 ), phosphatase enzyme activity (28.62 g p-NP g -1 h -1 ) and urease enzyme activity (2.49 μg NH 4 -N g -1 h -1 ) were recorded with the treatment received zero fertilizer.
Interaction effect of different conservation agricultural practices and fertilizer doses was varied significantly. Significantly the higher dehydrogenase enzyme activity (18.53g TPF g -1 day -1 ), phosphatase enzyme activity (32.55 g p-NP g -1 h -1 ) and urease enzyme activity (3.60 μg NH 4 -N g -1 h -1 ) were found in treatment where no fertilizer was applied in zero till-raised bed with crop residues retention. The no-tillage practices increased the availability of soil enzymes like acid phosphatase, amylase, cellulose etc.   The increased microbial activity improved the nutrient availability and circulation of minerals (Sharma et al., 2011). Mina et al., (2008) found that increased enzyme activity was due to higher level of intercellular and or extracellular enzymes, immobilized by recalcitrant humic moieties.

Population of Psolubilisers, free living N 2 fixers and fluorescent pseudomonas
The data on population of microbial enumeration in rhizosphere soil collected at mid flowering stage were varied significantly with different zero tillage practice.
Application of 100 per cent recommended levels of inorganic fertilizer dose recorded lower P-solubilizers (16.37 × 10 4 cfu g -1 ), free living N 2 fixers (13.20 × 10 6 cfu g -1 ) and Fluorescent pseudomonas (9.83 × 10 4 cfu g -1 ) count. The population of microbial enumeration count was increased with reduction of nutrient doses. The highest population have enumerated with no fertilizer (22.30 × 10 4 cfu g -1 , 19.03 × 10 6 cfu g -1 and 14.10 × 10 4 cfu g -1 , respectively) The populations of microbial enumeration count were influenced significantly by interactive effect of zero tillage practice and nutrient levels. The higher P-solubilizers (29.50 × 10 4 cfu g -1 ), free living N 2 fixers (25.67 × 10 6 cfu g -1 ) and Fluorescent pseudomonas (17.50 × 10 4 cfu g -1 ) count were recorded in zero till-raised bed with residues retention along with zero fertilizer treatment compared to all other combinations. These results were due to the favourable effect of zero tillage with residues retention on soil microbial population was mainly due to increased soil aeration, cooler and wetter conditions along with higher soil organic carbon content. Plant residues on surface contribute to suppression of soil-borne pathogens in minimum tillage systems due to microbial antagonists.
The yield, yield parameters and all microbial parameters were higher in zero till raised bed with residue retention. The effect of nutrient doses shown increase in nutrient application increases the yield and yield parameters, but all the microbial parameters like Nodule number, nodule weight and Arbuscular mycorrhizal fungi (AMF) root colonization, dehydrogenase, phosphatase and urease enzyme activity were higher in treatment where no fertilizer applied. Finally, it can be concluded that reduced dose of nutrient helps in sustaining the crop yield in conservation agriculture practices. In conservation agriculture practice of zero till raised bed with residue retention, there was saving of 50 per cent inorganic fertilizer.