Dynamics And Leaching Characteristics Of Soluble Organic Nitrogen Compositions In Paddy Soil Under Different Fertilization Treatments During Rice Growth Period

Soil soluble organic nitrogen(SON)is one of the most active components in soil nitrogen pool and plays an important role in the nitrogen cycle of farmland ecosystem.A 33-years long-term fertilization target area in the subtropics is selected for the study.The dynamics and leaching characteristics of SON compositions in yellow-muddy paddy fields under different fertilization during rice growth stages were investigated by modern analysis methods such as total organic carbon analyzer,flow injection analyzer,amino acid analyzer,Fourier infrared spectrometer,and high-throughput sequencer.The influencing factors were discussed and the scientific basis for clarifying the chemical nature and ecological functions of SON in paddy field was provided.Research indicates:(1)Long-term application of chemical fertilizer(NPK),chemical fertilizer combined with straw(NPKS)and chemical fertilizer combined with cow manure(NPKM)in the same growing period can significantly increase the soil SON content in paddy soil.The SON content in rice tillering stage and jointing stage are expressed as NPKM>NPKS>NPK>CK,but NPKS>NPKM>NPK>CK in seedling stage,flowering stage and maturity stage.SON content in different fertility stages in the same fertilization treatment is mature stage>tillering stage>flowering stage>jointing stage Period>Seedling period.(2)The contents of free amino acid nitrogen and soluble protein nitrogen were the important components of soil SON and accounting for 36.57%?68.94%of SON in the growth period of rice.Fertilization significantly increased the content of free amino acid nitrogen(FAA-N),soluble protein nitrogen(SP-N)and amide nitrogen in soil SON.The free amino acid composition of the different fertilization treatments was dominated by neutral amino acids(86.79%-97.42%of the total amino acid content)in rice growth period.The soil soluble nitrogen functional groups all had the infrared characteristic absorption peaks of amide compounds.(3)The grey slope correlation analysis showed that the contents of SON and FAA-N in paddy soil were mainly affected by microbial biomass and microbial diversity and soluble organic carbon(SOC),while SP-N was mainly affected by protease activity,followed by microbial diversity and SOC influences.The redundancy analysis results showed that the SON content and composition in different growth stages of rice were significantly different.The early growth stage of rice was mainly affected by Chloroflexi and Acidobacteria,and the latter was mainly influenced by Proteobacteria,Bacteroidetes and Ignavibacteriae.(4)Long-term application of fertilizers can increase the DON content of the flooded layer during rice growth period(NPK,NPKM,and NPKS treatments were 1.52 times,1.71 times,and 1.70 times higher than CK treatment,respectively),but had no significant effect on FAA-N content.The DON concentration in the flooded layer showed a trend of changing from high to low with the growth period of rice,while FAA-N/DON showed a trend of changing from low to high with the growth period of rice,indicating that FAA-N may be nitrogen at the later stage of rice growth,an important source of leaching.(5)The DON concentration of soil leachate in the seedling stage-jointing stage was 0-20 cm>20-40 cm>40-60 cm,but it was 20-40 cm>0-20 cm>40-60 cm during flowering stage and maturity stage,indicating that with the growth of rice,DON has a tendency to accumulate in the lower layer,and during the growth period of rice,the leachate FAA-N/DON shows 40-60 cm>20-40 cm>0-20 cm,indicating that FAA-N is the main component of DON downward migration.