Research On Functional Gene Abundance Of Nitrogen Cycling In Both Profiles And Rhizosphere Of Subtropical Forest Red Soil

Soil nitrogen?N?cycling plays a key role in soil nutrient cycling in ecosystems.Nitrogen cycling microorganisms drive the soil nitrogen cycling and affect the entire soil biogeochemical processes.Soil nitrogen cycling functional genes,including mineralization?chi A?,nitrogen fixation?nif H?,nitrification rate limiting step ammonia oxidation?AOB,AOA?,and denitrification?nir S,nir K?,encode key functional microorganisms for key processes of soil nitrogen cycling.Soil nitrogen cycling functional gene abundances can present potentials of soil nitrogen mineralization,nitrogen fixation,nitrification,and denitrification processes.The study of nitrogen cycling functional genes is of great significance for nitrogen nutrient supply,nitrate leaching and N2 O emissions of greenhouse gases.In this study,the red soil was selected in Qianyanzhou subtropical forest in Jiangxi Province.The Pinus massoniana,mixed coniferous and broad-leaved mixed forests and naturally restored coniferous and broad-leaved mixed forests were selected.The soil profiles from the surfaces to the bed rocks were excavated.Each soil profile was sectioned into 7 layers,totally 21 soil samples were obtained.Five trees were selected from each plots in Cunninghamia lanceolata?AM?forest,Pinus massoniana forest and pinus elliottii?ECM?forest.The rhizosphere soils and first order roots were collected,totally 15 rhizosphere soil samples were obtained.The real-time fluorescence quantitative PCR method was used to determine the abundance of nitrogen cycling functional genes in the profile and rhizosphere soil samples.The soil physical and chemical properties were analyzed,and the mycorrhizal colonization rates of the the first order roots were determined by microscopic examination for the three tree species.Redundancy and pearson's correlation analyses were used to study the correlations between nitrogen cycling functional gene abundance and soil environmental factors.The distribution characteristics of nitrogen cycling functional genes along the red soil profiles in subtropical forests and in the hotspots of rhizosphere were analyzed.The result showed that:Along the soil profiles from the surfaces to the bed rocks,1.Except for AOA and AOB,the abundance of nitrogen cycling functional gene abundance at 0-40 cm and 40-bed rocks were not significantly different.2.The specific nitrogen cycling functional gene abundance which was normalized by organic carbon and DNA contents was gradually increased with increasing depth from soil surfaces to bed rocks,while the special abundance of nitrogen cycling functional genes was the lowest in the bed rocks;3.The abundance of mineralized functional genes?chi A?dominated the nitrogen cycling functional genes,accounting for about 27-68% of the measured functional gene abundances.The nif H,AOA,AOB,nir K and nir S accounted for different percents of the total measured functional gene abundance in different soil layers,indicating that the distribution patterns of the nitrogen cycling microorganisms were different in the soil profiles.4.Redundancy analysis showed that the nitrogen cycling microorganism abundance mainly depended on the contents of carbon,nitrogen and available phosphorus in the soil profiles.For the abundance of nitrogen cycling functional genes in rhizosphere soils,1.the chi A was not significantly different among the three tree species?p>0.05?;the nif H was lower in Pinus massoniana than the other two tree species,but not significantly different between Cunninghamia lanceolata and Pinus elliottii?p>0.05?.The nir S gene was higher in Pinus elliottii than the other two tree species,and there was no significant differences between Cunninghamia lanceolata and Pinus massoniana?p>0.05?.The AOA and AOB genes were higher in Cunninghamia lanceolate than the other two tree species,and there was no significant differences between Pinus massoniana and Pinus elliottii?p>0.05?;2.The abundance of specific nitrogen cycling functional genes normalized by organic carbon contents had the similar pattens with the non-normalized abundance.The specific abundance of chi A,nif H,nir K,and nir S normalized by DNA contents was higher in Pinus elliottii than the other two tree species,and the specific abundance of AOA and AOB genes was higher in Cunninghamia lanceolata than the other two tree species,and there was no significant differences between the other two tree species.3.There were no significant differences in the relative abundance of nitrogen cycling function genes in the rhizosphere soils of the three tree species?p>0.05?.4.Pearson's correlation analyses showed that the abundance of nitrogen cycling functional genes in the rhizosphere of Cunninghamia lanceolata,Pinus massoniana and pinus elliottii were mainly affected by mycorrhizal colonization rates,dissolved organic carbon?DOC?,ammonium nitrogen?NH₄⁺-N?,nitrate nitrogen?NO₃^--N?,organic carbon?SOC?and total nitrogen?TN?contents.This study indicated that N transformations mainly occured in topsoil in the red soil profiles.Compared with the bedrock layers,the nitrogen cycling microorganisms have higher activity and efficiency in the soil layers.The contents of soil organic carbon,nitrogen and available phosphorus drove the nitrogen cycling functional microorganisms to occupy different niches in different soil layers.Nitrogen transformation ability,cycling microbial activity,and efficiency were different in rhizosphere soil among Cunninghamia lanceolata,Pinus massoniana and Pinus elliottii.The four studied nitrogen cycling functional microorganisms were affected by mycorrhizal colonization rates and soil nutrient contents?DOC,NH₄⁺-N,NO₃^--N,SIN,SOC and TN?among different tree species.The results of this study provide a new understanding for the microbiological mechanism of soil nitrogen cycling in red soil.