INTRODUCTION In India, rice is grown in an area of 43.66 M. ha annually with a production of 118.87 million tonnes and average productivity of 2722 kg ha-1 (Indiastat, 2020). Thus, it generates a huge amount of residue in the form of straw when rice crop is harvested. Rice was harvested manually by cutting method or using machinery. In cutting method, the harvested rice straw and grain were taken to a threshing yard to separate rice straw and grain. As a result, a small amount of straw was left infield itself. While in machine harvesting the total paddy straw was dumped in the field which was unutilized by the farmers. Based on the type of harvesting the amount of residue left over in the field is varying from small to huge amounts. The farmers were considering the left over paddy straw as a waste product and it was burned in field itself. Rice straw contains 0.57% N, 0.07% P2O5, 1.5% K2O, 0.1% (sulfur) S, and 5% silicon (Si) (Dobermann and Fairhurst 2002) and also reported that, at harvest stage the rice straw contains contain 0.5–0.8% N, 0.07–0.12% P2O5, 1.16–1.66% K2O, and 4–7% Si. The study conducted by Gupta et al. (2007) was proved that the in-situ incorporation of rice straw into the soil improves nutrient cycling, soil organic carbon (C), and yields of succeeding crops. The nutrient status of the paddy straw is well known but at the same time rice straw have the lower decomposition rate due to higher C: N ratio (50 to 60). In this condition, if sowing is taken up immediately after the rice straw incorporation, the establishment of the crop is affected due to this wider C: N ratio and poor microbial population (Udayasoorian et al., 1997). In this connection the present study was taken up with the combinations of different cutting heights of paddy straw, microbial consortia alone and also with the combination of cow dung and urea to know the decomposition rate and microbial population at different intervals. MATERIALS AND METHODS Field experiment was conducted at Agricultural College and Research Institute, Tamil Nadu Agricultural University, Madurai, Tamil Nadu during February 2020 to evaluate the impact of left-over rice straw after harvesting at different cutting heights (10 cm, 20 cm, and full rice straw) on microbial populations and decomposition rate. The experiment was laid out in split plot design and replicated thrice. The main plot contains different heights of rice straw i.e., 10 cm rice straw incorporation (M1), 20 cm rice straw incorporation (M2), full rice straw (M3) and sub plots contains different microbial consortia along with cow dung and Urea i.e., Bio mineralizer (2 kg/ tonne) (S1), Bio mineralizer (2 Kg/ tonne of residue) + Cow dung slurry (5%) + Urea (1%) (S2), Pleurotus (5 kg/ ha) (S3), Pleurotus (5kg/ ha) + Cow dung slurry (5%) + Urea (1%) (S4), Silica solubilizing bacteria (5kg/ha) (S5), Silica solubilizing bacteria (5kg/ha) + Cow dung slurry (5%) + Urea (1 %) (S6), Control (S7). Rice is harvested manually with sickle as per the treatments. For M1 and M2 treatment plots, rice straw was harvested by leaving 10 cm and 20 cm stubble heights from the ground level butin M3 treatment plot only economic parts are harvested by leaving the total rice straw in field itself. The left-over paddy straw was chopped by using tractor mounted shredder in their respective plots. Strengthen the already presented bunds and buffer channels all around to avoid seepage of water along with the nutrients. Drainage channels were provided all around the experimental field for effective drainage. The bio mineralizer, SSB were made in to slurry with water in 2: 40 proportion, cow dung slurry (5 %) and urea (1 %) were prepared and sprinkled on the rice straw as per the treatments schedule. Soil samples were collected from the experimental plots at 15 days and 30 days interval. The soil samples powdered, sieved through 2 mm sieve and used for chemical and biological analysis. The decomposition rate was estimated from C: N ratio of the soil whereas the chemical and biological analysis was done by the following methods. The analytical data of soil were subjected to statistical scrutiny as per the procedure given by Gomez and Gomez (1984). Wherever, the treatment differences were found significant (F test), critical differences were worked out at 5 per cent probability level and the values were furnished in the respective table. The treatment differences that were not significant were denoted as “NS”. RESULTS AND DISCUSSION Effect of in situ rice straw decomposition on microbial populations. Microbial population i.e., bacteria, fungi and actinomycetes were examined. Rice straw with different cutting heights and microbial inoculants had a significant impact on bacteria, fungi and actinomycetes population (Table 1, 2, 3). With respect to in situ rice straw decomposition with different cutting heights higher microbial population of bacteria (at 15 days 36 cfu g-1 soil × 106 and at 30 days 67 cfu g-1 soil × 106), fungi (at 15 days 32 cfu g-1 soil × 104 and at 30 days 63 cfu g-1 soil × 104), actinomycetes (at 15 days 41cfu g-1 soil × 103and at 30 days 58cfu g-1 soil × 103) were recorded under the full rice straw decomposition treatment (M3) and the least microbial population of bacteria (at 15 days 33 cfu g-1 soil × 106 and at 30 days 60 cfu g-1 soil × 106), fungi (at 15 days 29 cfu g-1 soil × 104 and at 30 days 57 cfu g-1 soil × 104) and actinomycetes (at 15 days 37 cfu g-1 soil × 103 and at 30 days 52 cfu g-1 soil × 103) were recorded under 10 cm rice straw incorporation (M1). The data regarding to the microbial consortia, higher microbial population was observed under the application of bio mineralizer (2 Kg/ tonne of residue) + Cow dung slurry (5%) + Urea (1%) (S2) (bacterial count at 15 days was 39 cfu g-1 soil × 106 and at 30 days 73 cfu g-1 soil × 106, fungal count at 15 days 35 cfu g-1 soil × 104 and at 30 days 68 cfu g-1 soil × 104 and actinomycetes count at 15 days 45 cfu g-1 soil × 103 and at 30 days 62cfu g-1 soil × 103). This was on par with the application of silica solubilizing bacteria (5kg/ha) + Cow dung slurry (5%) + Urea (1 %) (S6) (bacterial count at 15 days was 39 cfu g-1 soil × 106 and at 30 days 71cfu g-1 soil × 106, fungal count at 15 days 34 cfu g-1 soil × 104 and at 30 days 67cfu g-1 soil × 104 and actinomycetes count at 15 days 44 cfu g-1 soil × 103 and at 30 days 61 cfu g-1 soil × 103). The lower microbial population of bacteria (at 15 days 27 cfu g-1 soil × 106 and at 30 days 51 cfu g-1 soil × 106), fungi (at 15 days 24 cfu g-1 soil × 104 and at 30 days 48cfu g-1 soil × 104) and actinomycetes (at 15 days 31 cfu g-1 soil × 103 and at 30 days 44 cfu g-1 soil × 103) were observed in the control plot i.e., without any microbial inoculant application. The interaction effect of the in-situ paddy straw incorporation at different cutting heights and microbial inoculant showed the significant effect. The higher amount of microbial population i.e., at 15 days 43 cfu g-1 soil × 106 and at 30 days 79 cfu g-1 soil × 106), fungi (at 15 days 38 cfu g-1 soil × 104 and at 30 days 73 cfu g-1 soil × 104) and actinomycetes (at 15 days 48 cfu g-1 soil × 103 and at 30 days 68cfu g-1 soil × 103) was observed under full rice straw along with the application of bio mineralizer (2 Kg/ tonne of residue) + Cow dung slurry (5%) + Urea (1%) (M3S2) recorded and it was on par with full rice straw incorporation along with the application of silica solubilizing bacteria (5kg/ha) + Cow dung slurry (5%) + Urea (1 %) (M3S6) (at 15 days 42 cfu g-1 soil × 106 and at 79 days 51 cfu g-1 soil × 106), fungi (at 15 days 37 cfu g-1 soil × 104 and at 30 days 73 cfu g-1 soil × 104) and actinomycetes (at 15 days 48 cfu g-1 soil × 103 and at 30 days 67 cfu g-1 soil × 103). The lowest microbial population (at 15 days 26 cfu g-1 soil × 106 and at 30 days 49 cfu g-1 soil × 106), fungi (at 15 days 23 cfu g-1 soil × 104 and at 30 days 46 cfu g-1 soil × 104) and actinomycetes (at 15 days 30 cfu g-1 soil × 103 and at 30 days 42 cfu g-1 soil × 103) was observed in full rice straw incorporation alone without any application of microbial consortia, cow dung and urea (M3S7). The combined application of chemical fertilizer and straw significantly increased soil microbial count over the control (Nie et al., 2007). According to Esther et al., (2013), in-situ incorporation of straw was showed maximum amount microbial count in soil and increased their content on soil over the control treatment. In field experiment, these results were in agreement with the findings of Mandal et al., (2004) who reported that microbial activity was higher in residue incorporated into the soil and the higher microbial biomass could be obtained through residue incorporation than their removal or burning. Effect of in situ rice straw decomposition on C: N ratio. C:N ratio of paddy straw was analysed at 15 and 30 Days and the ratio was declined with time. The C:N ratio was ranged from 37.40: 1 to 55.30: 1 at 15 days and progressively it declined to 20.10: 1 to 48.49: 1 at 30 days. The different cutting heights of rice straw incorporation with microbial inoculants, cow dung and urea cause the significant variation in C: N ratio between the treatments (Table 4). At 15 days. The lower C: N ratio was recorded under the rice straw decomposition with the cutting of 10 cm (M1) (41: 1) and this was followed by the rice straw with the cutting height of 20 cm (M2) (48: 1). The higher C: N ratio was recorded in the treatment with the full rice straw decomposition (M3) (53: 1). With respect to microbial inoculations, at 15 days the least C: N ratio was observed under the treatment bio mineralizer (2 Kg/ tonne of residue) + Cow dung slurry (5%) + Urea (1%) (S2) (45: 1). The highest C: N ratio was recorded under Control treatment without microbial, cow dung (5%) and urea (1%) application (S7) (50: 1). Interaction effect showed that least C: N ratio was found in full rice straw decomposition with bio mineralizer (2 kg/ tonne of residue) + cow dung slurry (5 %)+ urea (1 %) (M3S2) (52: 1) which was on par with full rice straw decomposition with Silica solubilizing bacteria (5kg/ha) + Cow dung slurry (5%) + Urea (1 %) (M3S6) (54: 1). At 30 days. The lower C: N ratio was found in 10 cm rice straw incorporation (M1) (22: 1), 20 cm rice straw incorporation (M2) (27: 1) and the higher C: N ratio was recorded infull rice straw decomposition (M3) (45: 1). The least C: N ratio was observed in bio mineralizer (2 Kg/ tonne of residue) + Cow dung slurry (5%) + Urea (1%) (S2) (30: 1) and the higher C: N ratio was noted in Control (S7) (35: 1). Interaction effect at 30 days showed that least C: N ratio was found in full rice straw decomposition with bio mineralizer (2 Kg/ tonne of residue) + cow dung slurry (5%) + Urea (1%) (M3S2) (20: 1) and it was on par with 10 cm rice straw decomposition with bio mineralizer (2 Kg/tonne of residue) + cow dung slurry (5%) + Urea (1%) (M1S2) (20: 1) and 20 cm rice straw incorporation bio mineralizer (2 Kg/tonne of residue) + cow dung slurry (5%) + urea (1%) (M2S2) (20.32: 1) and the lower C: N ratio was registered in full rice straw decomposition without microbial consortia, cow dung and urea (M3S7) (45: 1). The different rice straw management practices helped to bring the C: N ratio by varying degree. Among the residue management practices, the rice straw decomposition with the cutting height of 10 cm, 20 cm and full rice straw along with the application of (2 kg/ tonne of residue) + cow dung (5 %) + urea (1%) (M1S2) decreased the C: N ratio. Gaur et al., (1987) was reported that effectiveness of inoculation with a suitable microorganism will speed up the bio degradation process and reduction in C: N ratio. The effective biodegradation of residue was confirmed with microbial inoculants to bring down the C: N ratio to an arrow level (Esther et al., 2013). Rice residue (straw and stubbles) contains high amount of silica, cellulose and lignin, these major chemical constituents were easily degraded by using microbial inoculants (bio-mineralizer and cow dung slurry), it favours the gradual release of nutrient and 25 kg additional N enhanced the initial crop growth. These results are in line with the findings of Singh et al. (2005); Kumari et al. (2008); Singh et al. (2008). The C: N ratio was reduced in all the treatments with passage of time but there was wide variation in the treatments this was due to the microbial development during the time gap which increased the degradation process and released the available nutrients present in the added rice straw to bring down the C: N ratio. This was in agreement with the findings of Azmal et al., (1997).