Decomposition Process And Microbial Diversity Of Straw In A Fluvo-aquic Soil Under Different Nitrogen Application Rates

Crop straw is rich in nitrogen,phosphorus and potassium,which is an important nutrient resource in agricultural production.The total annual production of straw in China is large,but the utilization rate is not high.Nitrogen application is a key measure to accelerate straw decomposition and nutrient release.However,the dynamic change process of straw residues and its microbiological mechanism under different nitrogen application rates are still unclear.This study focus on a fluvo-aquic soil in North China Plain.The field straw bags experiment and the greenhouse pot experiment were used to determine:1)wheat straw decomposition rate and the changes in the structure of straw residue,2)microbial community succession and its functional diversity of key microorganisms,3)the flow and distribution of maize straw-derived C and N in soil-crop system,and 4)the effects of straw returning on assembly of rhizosphere microbial community under different nitrogen fertilizer applications by using solid-state nuclear magnetic resonance(¹³C-NMR),isotope tracer,stable isotope nucleic acid probe(DNA-SIP),PCR-sequencing,metagenomic analysis,and other molecular ecological methods.The main progress of this paper is as follows:1.Straw decomposition rate and structure under different nitrogen application rates.The decomposition of wheat straw was fast in the early stage and slow in the later stage.The first two weeks were the rapid decomposition period.The average decomposition rate of straw in this stage was 46%,and that of the whole maize season(100 days)was 71%.High N nutrient level had no significant effect on the decomposition rate of straw in the first two weeks,but significantly accelerated the decomposition of straw after the second week.The decomposition rates in the SN1 treatment(180 kg N/hm²)and SN2 treatment(240 kg N/hm²)were on average6 percentage points higher than that of SN0 treatment(no N fertilizer).However,there was no significant difference between SN1 and SN2 treatments.The trend of C release rate was consistent with the straw mass loss rate.The order of final release rate of straw N,P and K nutrients after187 days was K(96%?97%)>N(52%?86%)>P(29%?45%),in which K was released completely in the first two weeks,while the release rate of N and P occasionally occurred negative increase in the later period.The decomposition rate of cellulose and hemicellulose was consistent with the decomposition rate of straw,showing a faster decomposition rate in the early stage and a slower one in the late stage.However,lignin remained a high decomposition rate in the middle and late stage.Scanning electron microscopy showed that the structure of wheat straw was gradually destroyed,with the surface increasing rough,faults and cavities,and fiber bundles becoming loose.The straw residues finally formed a network structure.The damage degree of the surface structure of wheat straw under high N rate was more apparent than that under no N treatment.The results of nuclear magnetic resonance showed that the relative contents of Alkyl-C,Methoxyl-C/N-Alkyl-C,Carbonyl-C and Phenol-C were significantly increased with straw decomposition,while O-Alkyl-C significantly decreased.2.Microbial community succession and function of key microorganisms in straw decomposition under different nitrogen application rates.For bacterial community structure,decomposition time was the first factor,followed by nitrogen fertilizer.On the contrary,nitrogen fertilizer was the first factor affecting the change of fungal community structure,followed by decomposition time.During the whole decomposition process,eutrophic bacteria such as Proteobacteria,Firmicutes and Bacteroidetes were transferred to oligotrophic bacteria such as Actinobacteria,Chloroflexi and Acidobacteria.The bacterial community in straw residues was gradually succeed to the soil bacterial community over time.The change trend of fungal community of wheat straw showed that the abundance of Sordariomycetes increased and Dothideomycetes decreased.For both bacteria and fungi,the effect of nitrogen application on community composition and function was more severe in the first two weeks of straw decomposition.Metagenomic results showed that the microorganisms associated with straw decomposition were mainly involved in carbohydrate transport,amino acid transport and metabolism.Nitrogen application increased the abundance of amino acid biosynthesis,carbon metabolism,ribosome,pyruvate metabolism,glycolysis/gluconeogenesis pathway genes.With the extension of decomposition time,the metabolic activity of microbial community decreased gradually.3.Distributions of straw carbon and nitrogen in the soil-crop system.The proportion of straw-derived C distributed to total wheat plant carbon(TC),soil soluble organic carbon(DOC),soil microbial biomass carbon(MBC),soil residual carbon and lost or uncounted parts were 0.04%?0.43%,0.32%?0.97%,3.67%?13.49%,31.67%?57.79%and 35.25%?62.81%,respectively.The proportion of carbon distributed to soil residual carbon decreased over the cultivation time,while the straw carbon loss rate increased.The application of nitrogen fertilizer increased the conversion proportion of straw carbon to MBC and DOC at the early stage.Nitrogen fertilizer significantly affected the allocation of straw nitrogen in soil-plant system.The proportions of straw nitrogen allocated to wheat plant total nitrogen(TN),soil mineral nitrogen(Nmin),soil microbial biomass nitrogen(MBN),soil soluble organic nitrogen(DON),soil residual nitrogen and lost or uncounted parts were 1.97%?14.55%,0.07%?2.55%,5.14%?14.78%,0.70%?4.56%and 34.32%?53.03%and 31.11%?42.66%,respectively.Nitrogen application significantly increased the utilization efficiency of straw nitrogen by average47.81%.After 60 days of wheat growth,42.69%of straw nitrogen still remained in the soil,which was an important potential nitrogen source.4.Effects of straw addition on rhizosphere microbial community assembly under nitrogen application.In both rhizosphere and bulk soils,the abundance of bacteria and fungi in the combined application of straw and nitrogen fertilizer treatment(S1N1)was higher than that in the other treatments at different growth stages,reaching the peak value on the 14th day.This indicated that nitrogen application could promote the growth and activity of microorganisms under straw returning.Some groups of soil bacteria and fungi were directly or indirectly involved in straw decomposition.Actinobacteria,Proteobacteria and Firmicutes were the main groups involved in straw decomposition in rhizosphere soil.Among them,Micrococcales(Micrococcaceae),Propionibacteriales(Nocardioidaceae and Intrasporangiaceae)play a key role.Fungal groups mainly include Sordariales(Lasiosphaeriaceae and Rhizophlyctidaceae)and Pezizales.Network analysis showed that the microbial co-occurrence networks in both rhizosphere and bulk soil tended to be simplified over time,while the root endophytic microbial co-occurrence networks tended to be complex.