Effects Of Carbon And Nitrogen Addition On Carbon And Nitrogen Mineralization Of Soil Fractions With Different Particle Sizes

Soil SOM is not homogeneous,and can be divided into two types:particulate SOM（POM:53-250μm,250-2000μm,>2000μm）and mineral-associated SOM（MAOM:<2μm,2-20μm,20-53μm,<53μm）.The turnover rate and stabilization mechanism of SOM composed of different fractions can be evaluated by studying the decomposition and biodegradability of organic matter in different particle size fractions,such as mineralization.Soil carbon and nitrogen transformation is the most important soil process.The difference in carbon and nitrogen mineralization of soil particle with different sizes,the effect of carbon and nitrogen on different particle sizes,and the roles of enzyme activity and microbial community structure in them are still unclear.The knowledge of the difference in soil composition can provide a basic theoretical basis for understanding the overall behavior of bulk soil.Therefore,subtropical broad-leaved forest soil was selected for this study.Soil physical classification for particulate SOM（POM:53-250μm,250-2000μm,>2000μm）and mineral-associated SOM（MAOM:<2μm,2-20μm,20-53μm,<53μm）were collected by combination of ultrasonic technology,wet screening and repeated centrifugation method.Deionized water（CK）,C₆H₁₂O₆(500 mg·kg^-1,C)and NH₄Cl(30 mg·kg^-1,N)were respectively added to different size fractions for incubation experiment.This study was designed to explore the dynamics and differences in carbon and nitrogen mineralization of different soil fractions,and to reveal the physical,chemical and biological factors that affect carbon and nitrogen mineralization of different soil fractions.The main results were as follows:（1）The dynamics of carbon mineralization and nitrogen transformation for soil particles with different sizes were significantly different due to their specific matrix quality and micro environment after physical fractionation.The small particles and unstable carbon in larger soil particles were released and the carbon was exposed to air after physical fractionation.The soil fractions with small particle size showed higher carbon mineralization（emission of CO₂）,especially for<2μm.Carbon and nitrogen addition had contrasting effects on carbon mineralization for different soil particle fractions.Carbon addition increased the cumulative emission of CO₂for each fraction,while nitrogen addition inhibited it.（2）The net ammoniation and mineralization rate of MAOM was significantly higher than that of POM under CK.Carbon addition decreased net ammoniation rate of each soil fractions,while nitrogen addition promoted it and and the promotion effect on MAOM was significantly higher than POM.The net nitrification rate of each soil fractions has a significant negative correlation with the net ammonification rate under CK(except for<53μm fraction,which was 1.37 mg·kg·d^-1).The carbon addition promoted the net nitrification rate of POM,but inhibited the net nitrification rate of MAOM.Nitrogen addition reduced the net nitrification rate of all soil fractions,and the net nitrification rate of 53-250μm fractions decreased by 73.2%.（3）The differences in the properties of different soil fractions,such as the total pore volume,specific surface area,labile carbon and nitrogen,and the decreasing and then increasing of soil microbial biomass carbon（MBC）and nitrogen（MBN）with the decrease of particle size,may be the main physical and chemical factors to impact carbon and nitrogen mineralization of different soil fractions.（4）β-1,4-glucosidase（βG）,cellulose hydrolase（CBH）,phenol oxidase（PHO）and peroxidase（PEO）had high activity in small particles under CK,carbon addition and nitrogen addition treatments.The addition of N promoted theβG in small soil particles with higher substrate utilization.In addition,PEO and PHO were found high in 20-53μm fractions under CK,carbon addition and nitrogen addition treatments.The activities ofβ-1,4-N-acetylglucosaminidase（NAG）and leucine aminopeptidase（LAP）increased with the increase of soil particle size.（5）The contents of Gram-negative bacteria（GN）,Gram-positive bacteria（GP）,actinomycetes（ACT）,arbuscular mycorrhizal fungi（AMF）,fungi（F）and total PLFAs in different soil fractions decreased at first and then increased with the decrease of particle size under CK treatment.GN,GP,F and total PLFAs increased with the decrease of particle size under carbon addition and nitrogen addition treatment.Fungi are the main microbial factors that dominate the transformation of soil carbon and nitrogen.Because of the variability of the environment,microbial community in the large particle fraction was significantly reduced due to carbon addition and nitrogen addition treatment,which resulting in the microbial community prefer to survive in the mineral-associated soil fraction.（6）The results of redundancy analysis showed that biological factors had a great influence on the carbon and nitrogen mineralization for different soil fractions,and the carbon and nitrogen cycle in different soil fractions was affected by the differences of enzyme activity and microbial community structure.The most critical factors affecting soil carbon and nitrogen mineralization of<2μm,2-20μm,20-53μm,<53μm,53-250μm,250-2000μm and>2000μm wereβG,fungi,MBC,AMF,MBC,LAP and MBN,respectively.