Composition,Dynamic And Trait Interpretation Of Weed Soil Seedbank In Pudong Rice Field,Shanghai

Rice is one of the main food crops in Shanghai,long-term application of considerable chemical fertilizers and pesticides has led to many ecological environment problems,such as decreased soil fertility,expanded agricultural non-point source pollution,enhanced resistance to weeds.Although weed is blamed for affecting rice yield and quality,actually,as an important part of farmland ecosystem,it has its special ecological value in activating soil nutrients,maintaining farmland ecosystem function and etc.Weed soil seedbank,a potential weed community,is commonly known as a key to maintain weed species biodiversity.Studying its composition and dynamic changes is conducive to formulate weed control measures and diversity conservation measures.At present,Shanghai rice field actively proposes the "three-three system" plan,which includes reducing the rice-wheat rotation area,promoting green manure planting and implementing deep tillage on the fallow land.Based on this,field studies,microscopic examination and germination experiment were carried out to evaluate different effects of three rotation systems(green manure-rice rotation,GR;fallow-rice rotation,FR;wheat-rice rotation,WR)and two tillage depths(shallow tillage,ST;deep tillage,DT)on the characteristics and dynamics of weed soil seedbank.Finally,we introduced plant traits to explain the difference,including plant canopy height at maturity,specific leaf area and seed mass.The results of this study are helpful to select targeted agriculture measures and provide scientific basis for weed management under “three-three system”.The main results are as follows:(1)There were 14 families of 32 weed species under microscopic examination and 12 families of 31 species under germination experiment.They were mainly dominated by Poaceae(11 species)and Cyperaceae(6 species)and the Sorensen similarity coefficient with ground community was 0.64 and 0.69 respectively.Winter annual(84%)and broad-leaved weeds(58%)accounted for the main types of weed soil seedbank.The dominant species were Cardamine hirsuta(30%),Sclerochloa dura(21%),Capsella bursa-pastoris(11%)and Veronica peregrina(10%),amounted for 72% of the total seedbank.The results of species composition clustering classified GR+ST and FR+ST as one group,and in NMDS analysis,the distance between two was relatively close,which meant that the similarity was high.(2)The density of soil seedbank was significantly different under different experimental stages and agriculture measures.The microscopic density in the second year(2018)was significantly lower than that in the first year(2017),while the germination density in autumn was higher than that in spring.Compared with the original WR+ST mode,GR and FR mode substantially increased the microscopic density in the first year,but the increase in DT field(1.9 times)was much smaller than that in ST field(3.7 times);while the difference in the second year was not obvious(2.0 times,1.7 times).There was no significant difference of germination density under different agriculture measures.(3)With the progress of experiment,microscopic richness of soil seedbank decreased from 13 species to 10 species,while germination richness increased firstly and then decreased,among which,the highest was 7 species in autumn of 2017.There was no significant difference under different agriculture measures;relatively,the germination richness under GR+DT and FR+DT was higher than that under GR+ST and FR+ST.(4)Agriculture measures did well in explaining the species composition of weed soil seedbank,with the rate of above 50% both in microscopic examination and germination experiment.The abundance of Alopecurus aequalis and Alopecurus japonicus would increase when changing the summer stubble from WR to GR or FR,while the abundance of Veronica peregrina would decrease.The interaction between rotation systems and plant traits had no strong effect on the species composition of seedbank;while the interaction between tillage depths and seed mass strongly affected the species composition;compared with DT,the abundance of weeds with heavy seed mass was larger under ST.To sum up,when implementing the "three-three system" in Shanghai,it is suggested to adopt DT for GR and FR roatation systems,so as to reduce the density of weed soil seedbank without influencing its diversity,which is beneficial to maintain the ecological balance of weed community in rice field.