Effects of long-term fertilization on soil microorganisms in tobacco fields

【Objectives】Microbes are important components of soils because they participate in nutrient turnover and supplies. A 12-year field experiment was used to investigate the effects of different fertilization on the soil cultural microorganisms, microbial biomass carbon(C) and nitrogen(N) contents, labeled phospholipid fatty acids (PFLAs) contents, microbial groups and beneficial microorganisms in tobacco cultivated fields. 【Methods】The soils from the three treatments were used: no fertilizer (control), single chemical fertilizer (CF) and combined chemical fertilizer and manure (CFM). Cultivated soils (0-20 cm) were sampled at the fast growing period of tobacco. Microbial biomass carbon and nitrogen, labeled phospholipid fatty acids of microbes were determined. The character values of microbial community including diversity, evenness and dominant indexes were calculated according to the corresponding content of PLFAs, the number of microorganisms were measured, the azotobacteria, phosphobacteria and potassium bacteria were indentified by microscope together with physiological and biochemical reactions. 【Results】The results showed more cultural microorganisms existed in the soil treated with CFM than with CF. Compared to CK, CFM increased the bacteria population by 6.14 times, fungi by 2.30 times and actinomycetes by 1.56 times, respectively, CF increased those obvious less than CFM. Fertilization also increased microbial biomass C and N contents. The microbial biomass C with CFM was increased by 2.46 times and 71.8% with CF than that with CK. The ratio of microbial biomass C to N changed with different treatments. CK showed the highest ratio (14.8), followed by CF (13.3) and the lowest with CFM (11.2). The total PFLAs in the soil with CFM is higher than soils with CF and CK. Compared with CK soil, the bacterial and the fungal PFLAs were increased by 25.41% and 15.59%, but the actinomycetal PFLAs was decreased by 24.63% in CF soil; all the bacterial, fungal and actinomycetal PFLAs were increased by 87.66%, 39.24% and 83.86% respectively in CFM soil. The phosphobacteria in the CFM soil was 1.15 times and potassium bacteria 1.02 times higher than those in CK soil, while the azotobacteria in CF soil was decreased by 56.69% and phosphobacteria by 41.30% than those in CK soil. There are totally 20 genera of azotobacteria, phosphobacteria and potassium bacteria in CFM soil, 19 genera in CK soil and 16 genera in CF soil. 【Conclusions】 Combined use of chemical and organic fertilizer in long-term promotes the microbial growth, reproduction and their biodiversity, resulting high fertilizer use efficiency and healthy soil with strong resistance to the potential decrease in microbial groups after a long-term application of chemical fertilizers. The chemical fertilizer plus manures in the tobacco cultivation should be advocated.