Mahanta, KaberiParit, Rajat Kumar2020-06-122020-06-122019-07http://krishikosh.egranth.ac.in/handle/1/5810147555The study on the “Effect of Conservation Tillage and Herbicide Application on Soil Microbial Activity in Rice-Mustard sequence”in the year 2018-2019 which forms a part of the long term trial under AICRP on Weed Management that was established during 2016 at Instructional-cum-Research (ICR) farm of Assam Agricultural University. The experiment was laid out in randomized block design replicating three times with five treatments in winter rice and mustard crop separately. The treatments of winter rice was comprised of treatments viz., T1- CT (Conventional tillage)+ Transplanting, T2- CT+Transplanting+ Pretilachlor, T3- CT+ Direct Seeded +Pretilachlor, T4- MT (Minimum tillage)+ Direct Seeded +Pretilachlor and T5- MT+ Direct Seeded+ Residue +Pretilachlor. Similarly, the treatments of mustard crop was comprised of treatments viz., T1-CT, T2- MT+ Pendimethalin, T3- CT+ Pendimethalin, T4- MT+ Residues+ Pendimethalin, T5- MT+ Residues+ Pendimethalin.The physical and chemical properties viz., bulk density, porosity, water holding capacity, pH, CEC and available nutrients viz; nitrogen, phosphorus, potassium, calcium, magnesium and soil organic carbon were determined from surface soil samples collected after the harvest of both winter rice and mustard crop. Conservation tillage i.e. MT along with residue has improved soil physical properties like bulk density, porosity, water holding capacity and CEC except the pH of the soil did not vary significantly. The soil organic carbon content increased significantly due to MT, herbicide application and residues. Available nitrogen, phosphorus, potassium, calcium and magnesium content of the soil was recorded in the medium range with highest available nitrogen, phosphorus, potassium, calcium and magnesium content recorded in the treatment T5 receiving MT+ Residues+ Herbicide Application in both winter rice and mustard crop. Biological parameters were analysed from surface soils collected periodically at 0, 3, 7, 15, 30, 45, 60 DAA of the herbicide in both winter rice and mustard crop. After the study, MT was found to improve the major enzymatic activities, soil microbial biomass carbon, physicochemical properties, available nutrients and soil organic carbon content as compared to conventional tillage (CT). Application of pretilachlor in winter rice showed a decline in β-glucosidase and urease activity for first 15 DAA and then increased till harvesting. However, a stimulatory effect was observed on activity of acid phosphatase, dehydrogenase and microbial biomass carbon for short period of time which later on decreased. Application of pendimethalin in mustard caused a decrease in dehydrogenase and β-glucosidase in initial stage while it increased the activity of acid phosphatse, urease and microbial biomass carbon in initial stage. Regarding the effect of herbicide application on available nutrients and soil organic carbon content it showed a stimulatory effect on these parameters. Significantly higher enzyme activities were recorded in treatments T5 which was comprised of MT+ Residues+ Herbicide Application.enThesis