The Effects Of Long-term Different Fertilization And Extracellular Polysaccharide Producing Strains On Soil Aggregate Structure And Biological Activity

Soil quality degradation caused by excessive application of chemical fertilizers has become one of the urgent problems,so the soil fertility improvement will be the strategic measures in the future.Soil aggregate is an important indicator of soil quality and it can be affected by fertilization by direct effect and indirect ways.The direct effect is that the fertilizer itself takes part in the formation of soil aggregate,while the indirect effect is that it impacts the biological,chemical,physical properties of soil,which in turn impacts the formation of aggregates.In addition,soil polysaccharide,especially microbial extracellular polysaccharide(EPS)is also one of the important factors in the formation of soil aggregates.Based on six different fertilization treatments(CK,N,NPK,NPKS,NPKM,M)at Qiyang long-term fertilization experimental station in Hunan province,the research explored the effect of long-term fertilization on soil aggregates.At the same time,highly producing EPS bacterial strains were isolated and applied in the soil culture experience to study their roles in the formation of soil aggregates.The main results were as follows:1.The effects of long term different fertilizers on the distribution and stability of soil aggregates were great.The application of organic fertilizer could significantly promote the formation of large aggregates from micro aggregates,while single application of inorganic fertilizer easily lead to decompose of large aggregates into small aggregates.There were significant difference between each treatment in geometric mean diameter(GMD),mean weight diameter(MWD)and the content of soil aggregates larger than 0.25mm(WSA),and all of them varied as follows:M>NPKM>NPK>NPKS>CK>N.The GMD,MWD and WSA of soil aggregates in treatment M reached 1.85%,3.90%,82.9%respectively,and increased 324%,485%,107%compared with that in CK.What's more,there was also significant difference in the content of soil organic matter among treatments,with the following rules:M>NPKM>NPKS>NPK>CK>N.The organic matter content decreased with the decrease of soil aggregates size.The content of organic matter in treatment M,which was the highest among all treatments,reached 2.4%,2.2%,2.0%respectively and increased by 2.4,2.2,2 times compared with that in CK in three particle size ranges(5-2,2-1,1-0.25mm).2.Long term fertilization had significant effects on soil microbial community diversity,microbial abundance and soil enzyme activities.The treatment M had the highest bacterial and fungal community diversity index with Shannon index 3.295 and 2.545 respectively,followed by treatment NPKM.The abundances of bacteria,fungi,nitrogen fixing bacteria,ammonia oxidizing bacteria and ammonia oxidizing archaea in treatment M and NPKM were significantly higher than other treatments.But the microbial abundance was not significantly different among different soil aggregate size in same treatment.Different fertilization treatments also showed different effects on the soil invertase,urease,catalase,cellulase activity.Organic fertilizer treatment had the highest soil enzyme activity,the soil invertase and urease activity increased with the aggregate particle size.3.A strain producing extracellular polysaccharide(EPS)was obtained and identified as Rhizobium sp.according to its morphological feature and 16S rDNA sequence(KP751382),and named Rhizobium.sp NJAU003.Strain NAU003 can reach maximum growth and EPS yield at 30 0C,pH 7.1%mannitol as carbon source,0.2%yeast powder as nitrogen source.Under the optimum conditions,the strain could produce the highest yield of EPS which reached 1.667 g·L-1 in 48 h.4.The research studied the effect of fertilization combined with inoculation of EPS-producing bacterial strain NAU003 and EPS produced by strain NAU003 on soil aggregates.The results showed that fertilization combined with inoculation of bacteria and EPS could significantly improve the content of polysaccharide and organic matter in soil,and inoculation of EPS-producing bacterial strain NAU003 and EPS could have a significant influence on the formation of aggregates.