Research On Soil Nitrogen Balance And Main Nitrogen-metabolism Microbial Communities Under Different Cropping Systems

As an environment-friendly agricultural model,organic agriculture improves the condition of soil nutrient balance.Soil microorganisms play key roles on nutrient cycling,which governs the formation and maintenance of soil fertility.Based on a long-term field trial of organic(ORG),low-input(LOW)and conventional(CON)greenhouse vegetable cropping systems which were started in 2002,a two-year systematically study on nitrogen dynamics and balances was conducted.Quantitative real-time PCR was used to measure the abundances of nitrogen fixing bacteria,ammonia oxdizing bacteria(AOB),ammonia oxidizing archaea(AOA)and denitrifying bacteria which involve in nitrogen cycling.A metagenomic approach of Illumina MiSeq high-throughput sequencing was firstly applied to monitor the structures of the above bacteria.The main conclusions were as follows:1.ORG changed spatial distribution pattern of soil inorganic nitrogen in soil profile and increased nitrogen supply and maintenance.In ORG,NO3--N accumulation of 0-100 cm soil layer in 0-200 cm soil layer was 3.4%and 12.4%higher than that in LOW and CON,respectively;while the proportion of 100-200 cm soil layer was 3.4%and 12.4%lower than that in LOW and CON,respectively.NH4+-N accumulation of 0-100 cm soil layer in 0-200 cm soil layer in ORG was 9.7%and 7.7%higher than that in LOW and CON,respectively;while the proportion of 100-200 cm soil layer was 9.7%and 7.7%lower than that in LOW and CON,respectively.The average total nitrogen(TN)content in 0-20 cm soil layer in ORG was 43.9%and 96.8%higher than that of LOW and CON,respectively.Moreover,ORG reduced non-point source pollution of the environment.Cumulative leaching TN of 0-100 cm soil profile in ORG was 2.9%and 30.8%lower than that in LOW and CON,respectively,while the values for cumulative leaching NO3--N were 8.0%and 43.5%,respectively,and values for cumulative leaching NH4+-N were 18.2%and 42.7%,respectively.Cumulative N2O emission in ORG was 29.1%and 35.3%lower than that in LOW and CON,respectively.Furthermore,vegetable yield in ORG was 15.5%and 22.5%higher than that in LOW and CON,respectively;nitrogen uptake by vegetable was 6.7%and 16.3%higher than that of LOW and CON,respectively;nitrogen surplus in ORG was 12.4%lower than that in CON.2.In ORG,abundances of soil bacterial(16S rRNA gene)and AOA(amoA gene)were significantly higher than that in CON(p<0.05),and abundances of nitrogen fixing bacteria(nifH)and denitrifying bacteria(nirS,nosZ genes)were also promoted,while abundances of AOB(amoA gene)were inhibited(p<0.05).These results indicated that the nitrogen fixation and denitrification in 0-20 cm soil in ORG may be improved,while ammonia oxidation may be inhibited.3.The population structures of bacteria,AOB and denitrifying bacteria in ORG were significantly different from that of CON.Evenness of AOB was increased and abundances of Nitrosomonas which was able to adapt to the environment with rich organic matter were significantly improved in ORG Compared with denitrifying bacteria,population structures of AOB in ORG were significantly different from other systems in all nineteen samplings,showing they could be used to indicate different cropping systems stably.Community compositions of nirK-,nirS-and nosZ-denitrifying bacteria in ORG and CON were different in most sampling times(12-13/19),and diversities in ORG were increased,showing that the denitrifying bacteria were selectively enriched in ORGThe effects of different cropping systems on abundances,diversities,communities of soil bacteria and nitrogen transformation key microorganisms,soil fertility and nitrogen leaching were further analyzed by PLSPM model.The results showed that in ORG,soil fertility was directly improved and indirectly promoted by increasing the abundance of nitrogen-fixing bacteria and regulation of communities of AOB.In short,this study revealed the effects of long term organic vegetable system on soil fertility,nitrogen leaching,nitrogen balance,and population structures,diversities,and abundances of nitrogen transformation key microorganisms,and potential regulatory mechanisms.