Effects Of Nitrogen Deposition On Soil Nitrogen Fractions,Nitrogen Mineralization Processes And The Underlying Mechanisms In The Typical Steppe

Soil nitrogen mineralization is the core process of nitrogen(N)cycling in grassland ecosystems,understanding the variation in soil organic nitrogen fractions under N deposition,and illustrating the effects and mechanisms of N deposition on soil N mineralization,which will be helpful to understand the N cycling processes,to forecast the variation of soil N supplying capacity,and to provide scientific basis for grassland management.At present,the impacts of N deposition on N mineralization processes are lack of knowledge,meanwhile,the effects of microbial variation resulted from N deposition on carbon source utilization patterns are still unclear.Our study was conducted in the typical grassland that is sensitive area for N deposition on the Loess Plateau.Four years simulated N deposition(0,1.15,2.3,4.6,9.2,13.8 gN.m-2y-1)combined with N mineralization experiments in field and in lad,the effects of N deposition on soil N mineralization and the underlying mechanisms were explored from the perspectives of soil physical and chemical properties,organic N fractions and microbial function diversity.The main results are as follow:1.Research of continuous 4 years showed that soil organic carbon(SOC)and total phosphorus(TP)were not affected by N treatment.Soil total nitrogen(TN)increased significantly with increasing concentrations of N addition at 10-20 cm layer after 4 years simulated N deposition,which indicated that the response of TN to N deposition had the time cumulative effect in the typical steppe on the Loess Plateau.In 20-40cm layer,high N treatment resulted in the increase in the concentrations of alkaline hydrolysis N,there were no significant differences in other layers.2.The results of N mineralization in lab incubation showed that the N mineralization potential(NO)increased linearly with increasing concentrations of N addition,but rate constants(KN)reached the highest level under N2.3 treatment.The results of field experiment showed that soil nitrification was the main N transformation process in this typical grassland.The seasonal patterns of N mineralization were not affected by N deposition.Soil N mineralization rate was significantly higher in summer than that in autumn in all treatments due to soil temperature and moisture.The trends of net N mineralization rate were similar with the results in lab,reaching the highest value under N2.3 treatment in summer.In autumn,net N mineralization rate were increased with increasing concentrations of N addition.The inter-annual variation of N mineralization was mainly influenced by water content.3.N addition for 4 consecutive years,soil organic N physical fractions were changed.Light fractions mass(LFOM),LFON and LFON/TN increased under N treatments in 0-10 cm and 10-20 cm layers,while LFOC/LFON decreased in 0-10 cm layer,which indicated that N deposition may accelerate the turnover rate of N.HFON increased,and HFOC and HFOC/HFON decreased under N treatments in 10-20 cm layer.Meanwhile,there was obviously negative correlation between concentration of N addition and HFOC/HFON,which showed that N deposition affected the stability and quality of HFON in this layer,and HFOM transformed from C-rich organic matter to N-rich organic matter.If high N deposition continue to occur,the stability of soil carbon may decline.4.N addition for 4 consecutive years,soil organic N chemical fractions were changed.Same as the changes of HFON and TN,non-hydrolysable N(NHN)significantly increased under N treatments in 10-20 cm soil layer.AN and AAN were increased with increasing concentrations of Naddtion,but HUN decreased.These results showed that N deposition may be retained in soil in the form of AN and AAN and accelerate the degradation of HUN.Soil N mineralization potential had positive correlations with AN,AAN and LFON,and negative correlations with LFOC/LFON and HUN,which illustrated that N mineralization depended on not only the capacity of soil easily mineralized N pool,but also the quality of organic matter(C/N ratio).The contribution rates of ANN and SOC to soil N mineralization potential were 65.4%and 15%,respectively.The mineral N was mainly derived from the mineralization of AAN.5.N deposition influenced soil nutrient supplying capacity,soil microbial community structure and functional diversity,and the activities of enzyme,which in turn affected substrate utilization patterns and soil N mineralization rate.Moderate N deposition(N2.3)increased bacteria/fungi ratio,bacterial and actinomycic number and Shannon-Wiener index,which,in turn,increased the soil microbial biomass and the activities of soil urease,invertase and alkaline phosphatase,stimulated the utilization of Amino acid and polymer by microbe,and led to the increase in N mineralization rate.The contribution rate of MBC on KN reached 42.6%.High N deposition stimulated the increase in fungi amount,and decreased the utilization ratio of carbohydrate and carboxylic acid.The results indicated that N deposition accelerated N transformation resulted from the variation of soil substrate.Soil N mineralization potential was positively correlated with MBP,which might reflect the increasing demand for phosphorus under high N deposition.6.Soil N leaching increased because of N deposition.The nitrate N in resin increased with increasing concentration of N addition.The amount of soil N Leaching accounted for 16.83%-32.14%of amount of nitrification N which indicated that the nitrate N leaching was one of the main way for N losing in this region.N2O emission was about 0.66-2.76?g.m-2.h-1.The emission of N2O in warm season was 2.29 times of that in cold season,which exhibited that the emission of N20 was mainly affected by soil temperature and moisture.The emission of N2O which was not influenced by N deposition accounted for about 0.11%of the amount of N nitrification.The results implied that N2O emission was not the main way for N losing in this region.