The Impact Of Different Fumigation On Dissolved N Composition And Nitrous Oxide Emission In Greenhouse Soil And Its Microbiological Mechanisms

Greenhouse cultivation and continuous cropping provide a suitable environment for occurrenceand development of soil borne diseases and pest. Soil borne disease and root knot nematodes havebecome a serious restriction upon greenhouse production. Soil fumigation is highly direct and effectivetechnique to control continuous cropping disease. However, fumigants are a class of pesticide withbroad biocidal activity and affect many non-target soil organisms. Soil nitrogen mineralization,nitrification and denitrification are the most important processes in soil nitrogen cycle and these threeprocesses are the main biological pathways which provided soil available nitrogen uptake by plants.Fumigation cause significant impact on soil nitrogen microorganisms would directly affect the dynamicchange of soil available nitrogen. One objective of this study was to quantify the dynamic effects offumigation on nitrogen mineralization and nitrification in laboratory incubation and field studies.Another objective of the study was to identify the soil biotic group responsible for the production ofnitrous oxide following fumigation as well as to investigate the potential mechanisms of this nitrousoxide formation. The main conclusions are as follows:1. The dynamics and biological characteristics on soil nitrification process in different fumigatedsoils were evaluated in order to quantify the effects of fumigation on mineral nitrogen in laboratoryincubation and field studies. The results indicated all the fumigation treatments depressed nitrificationtemporarily, although the treatments exhibited significant differences in the duration of nitrificationinhibition. In both studies, for a limited period of time, Pic showed a stronger inhibitory effect onnitrification compared to other fumigant treatments. The time of maximum nitrification (t_max) in DMDSand MS treatments were0.97week and1.03week, which is similar to the untreated control (t_max=1.02week). While Pic has the longest effect on nitrifying bacteria, nitrification appears to restart at a latertime (t_max=14.37week). Dose-response relationship between the inhibition effect of nitrification andPic and1,3-D was observed, fumigant concentration and t_maxare significantly positive correlated（P<0.05）. IC50of Pic and1,3-D were65.73mg kg^-1and25.74mg kg^-1respectively. No obvious imapcton nitrogen mineralization in laboratory fumigation study was observed. But in field condition, soilfumigation was shown to increase soil ammonium and short-term nitrogen mineralization rates.Nitrification in fumigated soil in field condition has a faster recovery rate compared with laboratoryincubation.2. Fumigation was significantly increased soil ammonium nitrogen and dissolved amino acidscontent, but changes of mineral nitrogen in fumigated soil have some differences. Soil ammonium afterfumigation in sandy loam soil recovered much faster, but ammonium in acidic soil needed a longerrecovery time after fumigation. Nitrification was significant inhibited in all5fumigated soils and forlong-term incubation nitrification tended to recover. Nitrification rate reached up to90%in allunfumigated soils at3WAF, which were higher than fumigated soils. Inhibition rates of nitrificationwere all over70%in5different fumigated soils at1WAF. In addition to the acidic soil from Guangxiinhibition rate was decreased below10%after18WAF. The nitrification dynamic results from4 different areas showed t_maxin all unfumigated soils were all between2-3wk and nitrification appearedto restart at a later time in all fumigated soils. Fumigation caused the longest inhibition effect ofnitrification in soil from Shanxi and shortest in soil from Beijing.3. Selective inhibition method was used in order to identify the soil biotic group responsible for theproduction of nitrous oxide （N2O） in fumigated soil as well as to investigate the potential mechanismsof nitrous oxide formation. The results indicated higher N2O productions were found in Pic and DZfumigated soils and the production rates of N2O in Pic and DZ fumigated soils were significant higherthan DMDS and1,3-D treatments. Nitrification potential was significantly suppressed in4fumigatedtreatments, but the significant increase of potential denitrification rate indicated great stimulation on soildenitrification after Pic and DZ fumigation. Besides fumigated soils had drastic increase in ammoniumand DAA content and decrease in MBN and MBC. Based on the results from selective inhibitionincubation studies we hypothesize that the production of N2O following Pic fumigation ispredominantly due to aerobic bacteria denitrification process and fungal denitrification process could bethe potential source contribute to N2O production in DZ fumigation. Degradation products of Pic andDZ in soil as nitrogen source caused a great stimulation on soil denitrification which resulted in asignificant increase of N2O production.