The Leaching Of Rice(Oryza Sativa L.)Allelochemicals In Soil And Their Interactions With Microorganisms

Long-term and extensive use of synthetic herbicides brings many negative effects on weed management,food safety and environmental quality.China is a big rice(Oryza sativa L.)producer with wide sources of straws.Using allelochemicals released by rice crops as bioherbicide can effectively reduce the dependence on synthetic herbicide.Additionally,straw returning to field can improve soil fertility and optimize agricultural natural resources.Recognized allelopathic rice cultivar PI312777(PI)and non-allelopathic rice Lemont(LE)were employed in this study.To advance from simplistic laboratory demonstrations of bioherbicide to useful evidence of allelopathic weed suppression in fields,manipulative experiments were carried out to:(i)characterise the variation of allelochemicals and microbial communities in allelopathy encompasses;(ii)understand the microbe-by-residue interaction in allelopathic activities;and(iii)systematically evaluate allelopathy and nutrient availability of allelopathic rice straw returning to field.The main results are drawn as follows:(1)The allelochemicals produced by PI restrained the synthesis of soluble protein and chlorophyll.The allelopathic potential of the two rice straws were significantly different.The allelochemicals produced by PI restrained the synthesis of soluble protein in receptor plants,increased the dissociation state of free amino acids.The decrease of soluble protein directly affected the activeness of some essential enzymes in photosynthesis,reduced the chlorophyll content and blocked the photosynthesis,resulting in the suppression of plants growth.(2)The aromatic compounds,such as phenolic acids,flavonoids,benzoic acids and their derivatives,with phenolic hydroxyl active groups may be the key compounds for allelopathy.HPLC-MS/MS analysis detected 86 and 74 components in PI and LE straw,respectively.The amounts of aromatic compounds,such as phenolic acids,flavonoids,benzoic acids and their derivatives in the PI straw were more than that in the LE straw.Phenolic hydroxyl active groups of these chemicals turned out to be phytotoxicity.Moreover,the metabolites in PI straw,i.e.,polygonum and the interactions with soil microorganisms enhanced the allelopathic potential of PI.(3)The mobility of chemicals directly affected the actuating range of allelochemicals.Among the tested chemicals,the leachability of pretilachlor and imidacloprid were the best(L_f>0.8),followed by vanillin and coumarin(L_f>0.6).The leachability of daidzein,menthol,and m-tyrosine were medium(0.3<L_f<0.6).The lowest leachability has been observed on p-coumaric and p-hydroxybenzoic acids(L_f<0.3).The mixtures(1:1)of various allelochemicals had better leachability than chemicals applied alone,which was termed as joint action.The migration ability of chemical depends on the active groups,polarity and solubility of the molecule and is directly related to the actuating range of allelochemicals.(4)Leaching of allelochemicals affecting the microbial community and their interactions modulated the allelopathic potential.The leaching of allelochemicals in soil mediated the microbial communities and enriched certain bacteria and actinomycetes.In turn,soil microorganisms regulated the level of allelochemicals,resulting in different mobility factors and phytotoxic effects.The suppression of pretilachlor and imidacloprid on the germination of lettuce seeds function independentlywere characterized by concentration-dependent,while the inhibition of allelochemicals was more closely related to soil microorganisms,which was characterized by allelochemicals-microbial interaction.(5)The improved procedure revealed the mutable roles of microbes.We quantified the interference potential of microorganisms and three rice PI residue-derived fractions(extracts,straw and fresh residue)on germination using the procedure of Liebman and Sundberg with modifications.In order to compare different sets of treatments,we supposed the microbe and residues were additions to sterile soil receiving water-only.The interference of microbe-by-residue interaction showed mutable role:the inhibitory effects got stronger firstly but fell over time,then a feeble influence even a promotion in the later period,indicating the mutable role of soil microorganisms over time,indeed.(6)Four-stages characteristics were observed in the variation of soil microbial community.The variation of soil microbial community could be divided into four phases,during the incubation process of straw.Initially,there was a rapid development period of dominant bacteria(0-2 days),where the number and diversity of bacteria increased exponentially,and some dominant bacterial populations proliferated rapidly,i.e.,Firmicutes and Actinobacteria.Then a screening stage(2-8 days)with a decline of bacteria population and diversity followed.For example,Chitinophaga,Enterobacter and Pseudomonas were selective adaptive species at this time.There is a recovery period of soil microbial community(8-16 days)and a maturity period(16-32 days)in the later,characterized by recovery and stability,respectively.(7)PI straw returning to field created a“window”of seed suppression potential.Both PI and LE straw promoted soil fertility factors,such as soil organic matter(SOM),microbial biomass C and N(BC,BN),but there is a“window of the suppression potential”in soils amended with allelopathic rice PI straw(2-32 days).During this period,allelochemicals in soil and the coupling effect with microorganisms changed the physical and chemical properties of soil,inhibited the activeness of soil urease,reduced the level of available nitrogen(AN).These factors interact to create a micro-ecological environment of seed suppression potential.(8)There is a switch from allelopathy availability to nutrient availability after the window period.In the post-window period,the allelochemicals were degraded by microorganisms as carbon substrate,which expanded the carbon pool and improved soil fertility factors,i.e.SOM,BC and BN.There was no difference between the treatment groups and the blank control groups on day 32,showing the relieve of suppression potential.After 64days of straw incorporation,the germination rate was higher than that of the blank control group and showed a promoting effect,realizing a switch from allelopathy availability to nutrient availability.Thus,grasping the"window"and making full use of allelopathy and nutrient availability of straws is the key to achieve environmentally friendly weeding and soil fertility improvement.