Study On The Nutrient Release Pattern And Allelopathic Mechanism Of Faba Bean Green Manure In ChongQing

Vicia faba L.belongs to the family Leguminosae,Papilionoideae,and Vicieae.It is an annual or biennial crop.Faba bean has a long history as green manure of legume,but its use as green manure has been gradually ignored with the emergence of vegetable faba bean.In Chongqing area,faba bean has more advantages than common green manure.Firstly,Faba bean is a cold season crop,not competing for land with local summer crops,and the southwest region mainly harvested fresh pods,combined with subsequent summer crops,can be used as good stubble before spring maize planting next year to achieve the purpose of land conservation.Secondly,the Rhizobium of Vicia faba has strong nitrogen-fixing capacity and high nutrient content of fresh leaves.In order to make better promote the utilization of broad bean resources,it is necessary for us to understand the effect of soil improvement and its possible adverse effects after returning to the field.In view of this,we used nylon bag burying method to analyze the nutrient release rules of faba bean roots,stems,leaves,aboveground parts?including stems and leaves?and whole plant in soil,as well as the allelopathic effect of faba bean straw extracts on subsequent crop maize and sorghum.Meanwhile we preliminarily identified the allelochemicals substances of faba bean.The main results are as follows:1.After returning to the field,Faba bean residues showed a quick decomposition rate in the first 35 days,and the decomposition rate was relatively slow in the next 35days to 245 days.The amount of N released in soil ranged from 52 kg N/ha in root to 60kg N/ha from aboveground parts during 0-5 weeks,benefiting the subsequent crop.The C remaining?%?was negatively correlated with the N concentrations and positively correlated with their C/N ratios in faba bean plants?data from leaf were not included?,suggesting that higher N concentration in plant residue would result in high amount of C release in plant residue.Among the five plant parts,root and stem had the highest remaining and less decomposed when compared with other plant parts,which suggested that the plant quality influenced the decomposition rate.Therefore,when faba bean as green manure to improve soil quality and crop yield,different management strategies should be adopted to match nitrogen and carbon release patterns with plant residues.2.Compared with stem,leaf had higher alkane concentrations,average chain length,carbon preference index,and even higher proportions of alkane released into soil.The relative abundance of the low chain length compounds of alkanes such as C₂₃,C₂₄,C₂₅5 and C₂₆6 increased with the time after residue placement;whereas the relative abundance of C₂₉,C₃₁1 and C₃₃3 showed reduced trend,resulting in varied average chain length and carbon preference index.Those suggested that plant species with varying n-alkane distribution pattern also differed in their decomposition rates in soil.3.With the increase of concentration of water extracts from faba bean aboveground and root,MBC in sorghum soil showed an upward trend,while MBN and MBC/MBN showed a downward trend.At the same time,the dehydrogenase activity of sorghum soil was inhibited and the urease activity of sorghum soil was promoted.The direct return of whole plant and stem of broad bean to the field reduced MBC and MBC/MBN in Maize soil,inhibited urease activity,while the direct return of whole plant and aboveground of broad bean to the field increased MBN in Maize soil.4.Alkanes,alcohols,esters,ketones and amides were identified in the faba bean extract.As the concentration of the aboveground root and root water extract of faba bean increases,the germination potential and germination rate of sorghum and maize seeds showed a downward trend,and their radicle and germ growth were significantly inhibited.Comprehensive analysis showed that broad bean can be used as a dual-purpose green manure crop.After being buried in soil,more nutrients such as nitrogen?N?,phosphorus?P?,potassium?K?can be released in the early stage,which can partly meet the needs of subsequent crops.However,there are allelopathic substances in broad bean straw which have allelopathic effects on seed germination and early growth of subsequent crop,and we should pay more attention to the early growth of subsequent crop while it is widely used in production.