Effects Of Soil Fertility On Nitrogen Use Efficiency Of Rice And Nitrogen-cycling Microorganisms

Rice is the main cereal crop and staple food for more than 60%population in China.Improving varieties,increasing nitrogen(N)fertilizer and improving cultivation and management techniques are the basic ways to improve production.However,the large application of chemical fertilizers could cause serious problems,such as environmental pollution,soil fertility decline,which will not only increase agricultural costs,but also lead to unsustainable agricultural development.Therefore,it is important to study the difference of N use efficiency(NUE)and its fate in different fertility soils to improve NUE,reduce environmental pollution and improve rice yield.In this paper,15N isotope tracer technique was used to study the N use and its fate of rice in three different types(lake alluvial,river alluvial and quaternary red clay)and fertility soils in Jiangxi province,and the biological characteristics and nutrient contents were also studied.Additionally,N cycle functional gene abundance and diversity were also studied.The main results are as follows:The higest biomass,grain yield,N uptake,effective panicle number and grain number per panicle of rice were obtained in high fertility soil and the application of N fertilizer.However,the yield increasing rate was the lowest.The grain and dry matter production efficiency as well as agronomic and physiological efficiency of N fertilizer were lowest in high fertility soil,while the apparent and partial productivity efficiency was higghest.The rice N derived from soil and fertilizer of high feirtility soil was the most,while the ratio of N fertilizer to total N uptake was the least.NUE and residual rate was highest in high fertility soil,while the loss rate lowest.The correlation analysis also showed that soil fertility was significantly or positively correlated with NUE and residual rate(P<0.05 or<0.01),but negatively correlated with N loss rate(P<0.001).The activity of soil urease,sucrase,dehydrogenase,protease,nitrate reductase and nitrite reductase was highest in high fertility soil and the application of N fertilizer,while the hydroxylamine reductase was lowest.MBC and MBN increased significantly in high soil fertility and N application.High soil fertility and N fertilizer could increase the ammonia oxidation potential,microbial activity and basal respiratory strength,nitrification,ammonization and denitrification,but could reduce the respiratory quotient,and N fertilizer would reduce soil nitrification and denitrification,while increase ammonization.Soil soluble organic carbon and N,total organic N,ammonium N,particulate organic N and light fraction organic N increased in high soil fertility and the application of N fertilizer.The correlation analysis showed that soil fertility was positively correlated with soil nutrient content(P<0.05).No matter with or without N fertilization in the second year,the proportion of nitrogen fertilizers from the first year absorbed by rice was very low,only 1%-2%,and the remaining 98%-99%derived from the soil or nitrogen fertilizer of the second year.The absorded proportion of the first seasonal nitrogen fertilizer decreased with the increasing soil fertility,and it would decrease with application of nitrogen fertilizer in the second year.Regardless of nitrogen fertilizer application or not in the second year,the residual nitrogen utilization rate increased with increasing soil fertility,and it was higher than no nitrogen treatment.In the second year,6%-8%of the first year nitrogen remained in the soil after harvest,and the residual rate increased with increasing soil fertility.The application of nitrogen fertilizer was slightly lower than that of the no nitrogen fertilizer treatment.The results of quantitative PCR showed that gene copy numbers of AOA-amoA,AOB-amoA,nifH and nirS were 1.98×105-5.04×105,1.22×103-4.54×103,1.92×108-12.7×108 and 1.09×106-3.00×106,respectively,and all of the functional gene copy numbers was highest in high soil fertility,while N fertilizer would reduce the copy number AO A-amoA,nifH and nirS.As the copy number ratio of AO A to AOB was 81-274,AOA might be the main microorganism that plays the role of ammoxidation.Soil N fixation was stronger in high soil fertility,and less N loss,which leading to higher NUE.The correlation analysis also showed that the copies number of N cycle functional genes was significantly or positively correlated with soil fertility under N application.The soil fertility significantly affected the diversity and abundance of AOA-amoA gene.The abundance,evenness and diversity of AOA-amoA gene decreased with increasing soil fertility.Soil fertility also had a significant effect on the composition of AOA-amoA community.Thaumarchaeote appeared only in low fertility soils,while Crenarchaeota was the most in medium fertility soil with relative abundance of 45%-50%,and the average abundance in low fertility soil and high fertility soil was only 13%and 11%,respectively.At the level of OTU,the community structure was also very different.Soil fertility was the most important factor affecting the structure of different communities,while N fertilizer did not have much effect.With the increasing of soil fertility,the number of AOA that increasing NUE and residual rate increased significantly,while that increasing nitrogen loss rate significantly decreased.Evolutionary analysis showed that AOA derived mainly from Soil Cluster.High soil fertility and N fertilizer had a significant improving effect on the abundance of AOB,while high soil fertility reduced the evenness and diversity of AOB.At the phylum level,AOB was mainly dominated by Proteobacteria,which was more than 92%,and Nitrosospira was the dominating species which accounted for 73%-93%.Compared with AOA,the soil fertility had little effect on the AOB community structure,and the highest abundance OTUs was OTU112,which reached 70%-91%in the three soils,while other species were very small.With the increasing of soil fertility,the number of AOB that increasing NUE and residual rate increased significantly,while that increasing nitrogen loss rate significantly decreased.Evolutionary analysis showed that AOB was predominantly by Nitrosospira-related species,whose ratio reached 81%and the remaining OTUs belonged to Nitrosomonas.The number of nifH OTUs in different soils was much higher than that of ammonia-oxidizing microorganisms,and the number of OTUs was 1520-1946.The soil fertility had significant effect on the diversity,evenness and abundance of OTUs species.With increasing soil fertility,the OTUs richness improved significantly,while the diversity and evenness reduced significantly.Proteobacteria was the dominant species with the highest number of OTUs which accounted for 76%-89%,and were affected by soil fertility.The relative abundance of cyanobacteria was only 0.3%-5.8%,and N fertilizer significantly inhibited their growth.PCA and db-RDA on OTU level showed that soil fertility was the main reason for the difference of community structure of N-fixing bacteria.With the increasing of soil fertility,the abundance of nitrogen-fixing bacteria that promoting the utilization of nitrogen and residual increased significantly,while that promoting nitrogen loss decreased significantly.The results of nirS-containing denitrifying bacteria showed that OTUs number was 775-951,and soil fertility had a significant effect on the abundance and diversity of OTUs.With increasing soil fertility,the abundance of OTUs increased,while the evenness and diversity decreased,and N fertilizer had the greatest impact on the evenness.On the phylum level,the community composition of different treatment was the same.The abundance of Proteobacteria increased with increasing soil fertility and the application of N fertilizer,and the community composition on OTU level was different.PCoA and db-RDA showed that soil fertility was the main factor to distinguish different community structure of nirS-containing denitrifying bacteria and N fertilizer also affected the community structure of nirS,but far less than soil fertility.With the increasing of soil fertility,the abundance of OTUs promoting nitrogen use efficiency and residual rate increased significantly,while that increasing nitrogen loss decreased significantly.Evolutionary analysis showed that the representative sequence mainly belonged to?-Proteobacteria,which accounted for 62%.127-211 OTUs were detected in npr gene,and the N fertilization could decrease the richness of the npr-OTUs.The effect of soil fertility on OTUs abundance was different.The effect of N fertilizer on OTUs abundance was very small,and the diversity and evenness were the lowest,while it went higher in low and high fertility soils.At the level of OTU,the composition of npr community is very different,especially in different fertility soils.In general,soil fertility could separate different npr-containing communities.With the increasing of soil fertility,the abundance of OTUs promoting nitrogen use efficiency and residual rate increased significantly,while that increasing nitrogen loss decreased significantly.Evolutionary analysis showed that 38%of the OTUs belonged to Bacillus,followed by new genus of Fervidicella,which produced M4-family peptidase,accounted for 33%.In summary,high fertility soil could improve soil nutrient supply,increase soil microbial biomass and enzyme activity,promoting microbial activity and biochemical strength,increase the abundance of N-cycle microorganisms,improve the community structure of N-cycle microorganisms,thereby increasing rice yields,fertilizer N uptake and residue,while reducing fertilizer N losses.