| Nitrogen-fixing cyanobacteria?NFC?are important microorganism resource in tropical and subtropical area.The present NFC strains were isolated and purified from paddy field in South China.And two excellent strains were screened based on growth status,growth rate,promoting ability for rice seedlings and adhesion ability on rice root.Moreover,their physiological and biochemical characteristic were determined.The mechanism of growth promotion of NFC to rice was clarified basically.In addition,the effect of excellent strains application on soil fertility,rice growth and yield as well as the mode of NFC inoculation were investigated by microzone control experiments.At last,the mode of rice-NFC cocultivation with excellent NFC strains was evaluated through field experiments and the economic benefits were analyzed.The main results are as follows:Twenty-four strains of filamentous and heterocystous NFC were separated and purified from the soil and water samples from paddy fields in Guangzhou,Zhongshan,and Haikou of South China.Rice seedling-NFC cocultivation experiments were employed to evaluate the adhesion ability of NFC on rice root and their growth promotion action on rice.The results showed that the adhesion ability of B,C,E,S and Z were better than the other strains.The root length of rice seedling treated with Z was 2.53-fold of that of 1/4N treatment.There was no significant effect on the seedling height of rice seedlings treated with all algal strains compared with CK?BG11 with 1/4 N?.There was no significant difference in the dry weight of rice seedlings when treated with strains A,C,O,T,and 1/4 Hoagland medium.However,when treated with strain A,it was significantly higher than that of other algal treatments.Both chlorophyll a content and specific growth rate of strain C was higher than other strains significantly.The biomass of strains C and Z were higher than strains B and E,significantly.Therefore,as excellent strains,C and Z were chosen for the subsequent study and application.Based on the results of the analysis of 16SrDNA sequence and the homologous analysis of the specific gene rbcLX,strains C and Z were identified as Nostoc piscinale and Anabaena variabilis,respectively.The ARA value?acetylene reduction activity?of strains C and Z were7.71 and 14.15 nmol·h-1·?g-1Chla,respectively.It was shown that strains Z and C might produce some plant hormones,such as gibberellins,cytokinins,abscisic acid,and auxins,based on detection results using UPLC-MS-MS.Among all hormones,the contents of IBA produced by strains C and Z were the highest ones.The contents of IBA in algal cells of C and Z were 5146.78 and 3468.60 ng/g DW,respectively.While the concentraions of IBA in filtrate of algal culture of C and Z were 105.03 and 143.11 ng/L,respectively.It was worth noting that the contents of TZR and SA in algal cells of strain C were 3791.72 and 4675.45ng/g DW which were higher than that of contents in NFC addressed in references.The production rates of algae strains C and Z were 1.455 and 0.892 g/L/d in wet weight,respectively.Cement pool experiments:In the first season,compared with CK?the conventional fertilization control?,mixed cyanobacteria treatment showed no significant effects on the physical and chemical properties of soil,rice organs of carbon and nitrogen,rice growth traits and yield.In the second season,it showed that the theoretical yield of 1/2N+C,C,Z,and1/2N+Z treatments were no significantly different compared with the conventional fertilization control.The yield of 1/2N+C and 1/2N+Z treatments were significantly higher than that of strain Z treatment.Replacing all or half chemical fertilizer with mixed NFC showed negative effects on individual physical and chemical properties of soil and no significant effects on rice growth characteristics and theoretical yield in the field experiments of Nanlang town.In the field experiments of Sanxiang town,when replacing all or half nitrogen with Z,it showed no adverse effects on the growth characteristics and physical and chemical properties of soil except soil effective P.However,the actual yield of 1/2N+Z treatment significantly higher than CK by 13.63%.In the field experiments of Xinhui city,replacing all nitrogen by NFC,the Z and C treatments had no adverse effects on the growth characteristics and physical and chemical properties of soil.However,the aboveground biomass?dry weight?of strain C treatment were significantly higher than CK by 37%.The actual yield of strain C and Z treatments increased by 17%and 22%,respectively compared with CK.This increase may be related with plant hormones produced by NFC.In conclusion,the application of nitrogen-fixing cyanobacteria in late rice to replace one half of the chemical nitrogen fertilizer will increase rice yield and promote the resistance to lodging and rice leaf blight,with not effects on soil fertility.According to the results of field experiments in two seasons,when replacing one half of chemical nitrogen fertilizer with NFC,the income gain was higher than that of treatment replacing all chemical nitrogen fertilizer.The income increased by 26.36%and 8.42%respectively in the experiments of Nanlang town,whileas it increased by 15.54%and 9.26%respectively in the experiments of Sanxiang town.The income of strain Z and C increased by 23.34%and 28.05%respectively in the experiment of Xinhui city compared with CK.It suggested that the replacement of one half of the chemical nitrogen fertilizer with NFC can achieve good economic benefits. |
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