Borah, NilayGogoi, Bhabesh2021-01-122021-01-122020-05https://krishikosh.egranth.ac.in/handle/1/5810159956The present work was carried out during 2016-2018which forms a part of the long-term Permanent Plot Experiment on Integrated Nutrient Supply System in Cereal Based Cropping Sequence laid out during 1987-1988 under All India Coordinated Research Project on Integrated Farming System at Assam Agricultural University (AAU), Jorhat. The experiment was laid out in a randomized block design at Instructional-cum- Research Farm, Assam Agricultural University, Jorhat replicating 3 times with 8 treatment combinations viz.,T 1 : no fertilizer, no organic manure (control), T 2 : 100% RDF (chemical), T 3 : 50% RDF (chemical) + FYM @ 2.5 t/ha for winter rice and 100% RDF (chemical) for autumn rice, T4 : 75% RDF (chemical) + FYM @ 1.25 t/ha for winter rice and 75% RDF (chemical) for autumn rice, T 5 : 50% RDF (chemical) + rice stubble @3.0 t/ha for winter rice and 100% RDF (chemical) for autumn rice, T 6 : 75% RDF (chemical) + rice stubble @1.5 t/ha for winter rice and 75% RDF (chemical) for autumn rice, T 7 : 50% RDF (chemical) + Azolla @ 0.5 t/ha for winter rice and 100% RDF (chemical) for autumn rice, T 8 :75% RDF (chemical) + Azolla @ 0.25 t/ha for winter rice and 75% RDF (chemical) for autumn rice. Results revealed that the application of 50% RDF (chemical) + Azolla @ 0.5 t ha-1 in case of winter rice and 100% RDF (chemical) in case of autumn rice (i.e. T7) showed the highest NH4-N, NO3-N and available N content in soil followed by the application of 50% RDF (chemical) + FYM @ 2.5 t ha-1 in winter rice and 100% RDF (chemical) in autumn rice (i.e. T3) in case of the rice-rice sequence after 32 cycles of the cropping. On the other hand, different fractions of P (viz., available P, Occluded P, Saloid P, Ca-bonded P and total P) and K (viz., water soluble K, available K, exchangeable K, non-exchangeable K, lattice K and total K) were found maximum in case of T3 followed by T5. Different fractions of C in rice soil were increased and varied significantly due to INM practices over unfertilized control (T1). The total organic carbon (TOC), total inorganic carbon (TIC) and total C was found to be highest in case of T3; whereas, the highest content of Walkley & Black C, less labile C and non-labile C in soils were recorded in case of T5. Yet again, T7 [50% RDF (chemical) + Azolla @0.5 t ha-1 in winter rice and 100% RDF (chemical) in autumn rice] was registered with the maximum content of water soluble C, microbial biomass C, very labile C and labile C in the soils underricerice system. In this study, all the fractions of NPK and C were found to be lowest in T1 (unfertilized control) treatment. The sensitivity index revealed that the microbial biomass C and water soluble C fractions were the most sensitive ones for different nutrient management practices as compared to other C fractions under study; whereas, the lowest sensitive fractions included non-labile C, less labile C, total inorganic C, total organic C andtotal C. Data on SOC stock due to INM practices varied significantly from 39.11 Mg ha-1 under T1 (unfertilized control) to 67.14 Mg ha-1 under T3(receiving FYM @2.5 t ha-1 + chemical fertilizers).The soil C sequestration ranged between (-)2.77 Mg ha-1 under T1 and 24.07 Mg ha-1 under T3. Over the control treatment (T1), 41.81 to 71.67% build up of C in the soils were recorded due to various INM treatments after 32 years of rice-rice cropping sequence. In this study, the highest bacterial population was recorded in case of T7(receiving Azolla @0.5 t ha-1 + chemical fertilizers); whereas, fungal population was found maximum in case of T3 (receiving FYM @2.5 t ha-1 + chemical fertilizers). Various 6 soil enzymes viz. dehydrogenase (DHD), phosphomonoeaterase (PMEase), fluorescein diacetate (FDA) and urease, involved in energy flow and nutrient cycling showed significantly higher activities under INM treatments. Significantly highest activity of DHD and urease was found in T7, while PMEase and FDA hydrolysis activities were found to be maximum in T3.There was a decrease in all the enzymatic activities over initial in the unfertilized control treatment (T1) after 32 years of rice-rice cropping. The pattern of CO2, CH4 and N2O emissions under rice-rice cropping system varied significantly with the stages of rice growth as well as by the different INM treatments under study. The CO2 and CH4 emissions peaked at 60 days after transplanting (DAT) of winter rice (cv. Ranjit) and 45 DAT of autumn rice (cv. Disang). On the other hand, N2O emission peaked first at 30 DAT and secondly at 60 DAT of winter rice (cv. Ranjit) in case of all the treatments except unfertilized control. However, only one N2O emission peak was observed at 45 DAT in case of autumn rice (cv. Disang) under study. The highest emissions of CO2 and CH4 during winter crop (cv. Ranjit) were observed in case of T5 receiving rice stubbles @3.0 t ha-1 + chemical fertilizers. In contrast, N2O emission during winter crop cv. Ranjit initially (up to 45 DAT) was found to be highest in case of the T2 (100% RDF, chemical); and afterwards, highest N2O emission was observed in case of T7 receiving Azolla @ 0.5 t ha-1 + chemical fertilizers. In case of autumn rice (cv. Disang), the maximum emissions of CO2, CH4 and N2O were recorded in T5(receiving rice stubbles@3.0 t ha-1 + chemical fertilizers). The lowest CO2, CH4 and N2O emissions were recorded in T1. It was evident in this study that the GHG emissions for the control (T1) and for Azolla cover + chemical fertilizer treatments (i.e. T7 and T8) were relatively low and similar during the initial stages of winter rice cv. Ranjit (up to 60 DAT) and autumn rice cv. Disang (up to 30 DAT). Among all the organic sources, supplementation of Azolla along chemical fertilizers resulted maximum reductionin GHG emissions from rice-rice system over FYM and ricestubbles. Pearson correlation matrix between the GHGs indicated that the emission of CO2 had a positive and significant correlation with CH4 (r=0.874**)and N2O (r=0.748*)emissions from the rice-rice cropping system. However, the correlation between the CH4 and N2O emission was positive and non-significant (r=0.623NS)in this study. Significant and positive correlation of CO2 and CH4 emissions from rice-rice cropping system were recorded with different fractions of C viz., WSC, WBC, MBC, VLC, LLC, LC, NLC, TOC and TC. The correlations of N2O emission with NH4-N, NO3-N and available N were found to be significant and positive; whereas, it was positive but nonsignificant with total N in soil. Likewise, microbial activities, enzymatic activities in soil and yield and yield attributing characteristic of rice crop were positively correlated with the emissions of CO2, CH4 and N2O from the rice-rice system of cropping. Yet again, in this study, GHGs were found to have not significant correlation with the plant height of rice crop. Overall, the findings of the present study lead to the conclusion that application of 50% RDF (chemical) + rice stubbles @ 3.0 t ha-1 in winter rice (cv. Ranjit) followed by 100% RDF (chemical) in autumn rice (cv. Disang) i.e. T5 could be considered as the best nutrient management practice for the rice-rice sequence in terms of highest yield (7.27 Mg ha-1), gross return (67.72 ×103 Rs. ha-1) andnet return(39.79 ×103 Rs. ha-1)with a B:C ratio of 2.42 in one way, enhancing the soil health under long run condition, in other. However, so far as the issue of GHG emission and global warming is concerned, application of 50% RDF (chemical) + FYM @ 2.5 t ha-1 in winter rice and 100% RDF (chemical) in autumn rice (2nd best treatment in terms of soil properties and yield with the B:C ratio 2.41) may be considered as better option for rice-rice cropping system under the prevailing climatic condition of Assam.EnglishThesis