Soil Microoganism And Activities Of Soil Enzymes In Tobacco Cultivated Soil Under Continuous Cropping Conditions

Flue-Cured tobacco,an important cash crop in China,belongs to Nicotians,Solanaceae, Tubiflorae,Dicotyledoneae and distributes widely in Southwest China,which accounts for over 70%of total cultivation area grown in China.However,continuous tobacco cropping is quite common in this region,which produces a series of cultivation problems such as growth inhibition, soil toxicant accumulation,serious pest and disease occurrence,soil nutrient unbalance and biochemical block,and poor yields and qualities under long-term continuous cropping conditions. These negative multiple effects could be unbeneficial to sustainable tobacco production in this area.Much research focuses on changes in soil nutrients under continuous cropping conditions but little on organism and biochemistry in soils including specie groups,components,roles of microorganisms and enzymes in soils.Due to their importance for soil nutrient transformation and supplies,it is significant to study soil microorganisms and enzymes in different tobacco grown soils and areas in order to realize their function.In our present experiments,we selected typical and representative tobacco cultivated soils in Southwest China to investigate soil microorganisms(including their numbers,groups and components)and soil enzyme activities in relation to nutrient availabilities in soils through pot and field experiments as well as long term field experiment.The main objectives are(â…°)to realize obstructive reasons produced by continuous tobacco cropping through microorganism activities, biochemistry process,and nutrient supplies in soils;(â…±)to manage reasonably tobacco nutrient resources during cultivation;and(â…²)to diagnose correctly the obstructive factors of continuous tobacco cropping and to prevent ecological environment.Following results were obtained.1 Pot Experiment1.1 Continuous cropping practice inhibited significantly tobacco growth.The negative effects got more serious as years prolonged and varied greatly with soil types.Poorer growth was observed in yellow clay soil formed in quaternary period parent(CS)than yellow soil derived from lime stones (LS),suggesting that the later was more suitable for continuous tobacco cropping than the former being consistent with cultivation practices. 1.2 The number of microorganisms in tobacco rhizosphere varied in the sequence:bacteria＞actinomycetes＞fungi.Bacteria in LS were much more than CS in same continuous cropping season,which decreased firstly,then increased and finally decreased again during the period of continuous tobacco cropping.Actinomycetes decreased constantly in LS but showed similar pattern as bacteria in CS during the period of continuous tobacco cropping.In addition,No obvious influence of continuous tobacco cropping was found on fungi in soils,indicating relative stability of fungi in soils.Consequently,the influence of continuous tobacco cropping on soil microorganisms varied with soils.1.3 Continuous tobacco cropping acidified soils,particularly LS.However,pH was much lower in CS than LS in same Continuous cropping Season.1.4 The contents of ammonium nitrogen and nitrate nitrogen in two soils tended to increase with continuous tobacco cropping prolonged.NH₄⁺-N was higher in CS than LS in general and NO₃^- -N behaved otherwise at the same tobacco cropping season.1.5 Continuous tobacco cropping increased generally ammonification intensity in LS in contrast to nitrification which initiated reduction in the 4th season of continuous cropping till constant status. The intensities of ammonification and nitrification in CS were lower than those in LS and changed little,if any,during whole period of continuous tobacco cropping.Therefore,both processes in LS suitable for continuous tobacco cropping kept strong,which were beneficial to nitrogen supplies, although they varied greatly in continuous tobacco cropping.1.6 The activities of acid phosphatase,urease and saccharase in soils decreased,in general,but the catalase activities which increased gradually under continuous cropping condition.After long time of continuous tobacco cropping,the activities of former three enzymes kept in quite low levels in soils.Besides,soil enzyme activities in LS changes less compared to those in CS,indicating the stable biochemistry reactions related to these enzymes in the soil suitable for continuous tobacco cropping.2 Long Term Field Experiment2.1 Continuous tobacco cropping elevated activities of acid phosphatase in soil in general and those treated with farmyard manure and without fertilizer(blank control)in particular,which were much higher than the treatments of chemical fertilizer and organic complex fertilizer.2.2 Highest urase activities in soil were found in farmyard manure treatment in thesame continuous cropping season.Similarly,the activities of catalase and saccharase were also highest in the treatments of blank,chemical fertilizer and organic complex fertilizer than any other treatments in the same case.The results suggest the benefit of organic fertilizer to soil biochemistry reactions under continuous tobacco cropping conditions. 3 Field Experiment3.1 In tobacco growing area of Yunnan,activities of catalase,urease and saccharase increased in general except acid phosphatase which decreased slightly in soil with tobacco cropped continually. Activities of urease and acid phosphatase were higher than any other enzymes in soils,suggesting active transformation of N and P in soils under continuous tobacco cropping conditions.3.2 In tobacco growing area of Chongqing,activities of urease and acid phosphatase decreased significantly after continuous tobacco cropping for more than 10 years,indicating transformation inhibition of N and P and possibility of low nutrient availability in soils.3.3 In tobacco growing area of Guizhou,activities of soil enzymes decreased in general with the years of continuous tobacco cropping prolonged.Activities of acid phosphatase and saccharase were higher in yellow soils than the others while urease and catalase higher in phaeozem and spodosol.3.4 Comparison of enzyme activities in soils of Yunnan,Guizhou and Chongqing indicates that enzyme activities were higher in the soils of Yunnan than Guizhou and Chongqing.In details, higher activities of urease and catalase in the soils of Guizhou were found than Chongqing but acid phosphatase and saccharase behaved otherwise in continuous tobacco cropping for less than five but longer than ten years.Under continuous cropping conditions for five to ten years,however, activities of urease and saccharase were observed higher in the soils of Chongqing than Guizhou in contrast to catalase and acid phosphatase which showed stronger activities in the soils of Guizhou than Chongqing.