Effect Of Functional Traits Of Tree Species On Nitrification And Soil Nitrifiers In Rhizosphere And Bulk Soils

The subtropical forest plantations represent the significant part in Chinese forests.To improve the ecological services of these forest plantations and increase the diversity of timber supply,more tree species are planted in south China than ever.The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmosphere,terrestrial,and marine ecosystems.Long-term nitrogen deposition increases in the past decades in south China and has improved the soil fertility.On the other hand,the increased atmospheric nitrogen deposition might result in the deterioration of the environment through the eutrophication and acidification of groundwater and releases of greenhouse gas N₂O to atmosphere.In this proposal,the impact of tree functional traits on soil physi-chemical properties and nitrogen transformation rate will be investigated in long-term common garden tree species plots.Roles of N cycling-related microbial communities in autotrophic nitrification will be analyzed in both bulk and rhizosphere soils under these forest plantations of different species.Using field sampling,laboratory incubation,soil net nitrogen mineralization,potential nitrification rates will be measured in soils under these forest plantations of different species based on related models.The aim of this study is to link tree functional traits,soil physic-chemical properties,soil microbial functional genes to nitrogen-cycling processes in soils under these forest plantations.We hope the proposed studies can help local farmers to choose proper tree species so that more nitrogen can be retained in both soils and above-ground forest biomass.The main results of this study are as follows:1)In rhizosphere soil,the tree species with small root carbon nitrogen ratio,large specific root area and high root carbon and nitrogen content were classified as acquisitive tree species.The content of mineral nitrogen in rhizosphere soil of the acquisitive tree species was higher than that of the conservative,indicating that there was a fierce competition between plants and microorganisms in the rhizosphere soil for soil available nitrogen.（2）In rhizosphere soil,soil nitrifier are affected by root function and soil physi-chemical properties.For the acquisitive species with small root carbon and nitrogen ratio and large specific root area,the nitrifier（such as Ammonia oxidizing archaea（AOA）,Ammonia oxidizing bacteria（AOB）,Complete ammonia oxidizing bacteria（Comammox））in rhizosphere soil are low.In rhizosphere soil,the content of tannin and soluble phenolics in roots of conservative tree species with high root carbon and nitrogen ratio were higher,and the presence of these root chemicals would inhibit the abundance of nitrifier.In addition to the characteristics of plant nutrient acquisition（specific root area,root carbon nitrogen ratio,etc.）,wood density is also an important factor affecting the measurement of Comammox clade A in rhizosphere soil.（3）In rhizosphere soil,the mineralization rate of soil net nitrogen was mainly net nitrification rate.The results showed that root carbon and nitrogen ratio,root tannin content and soil NH₄⁺content were significantly negatively correlated with rhizosphere soil net nitrogen mineralization rate.The potential nitrification of rhizosphere soil was mainly affected by the abundance of ammonia oxidizing microorganisms（AOA,AOB）,soil nitrate nitrogen and root nitrogen content.（4）In bulk soil,the abundance of Comammox was much higher than AOB and AOA,and the abundance of AOB was higher than AOA.The abundance of AOA and Comammox clade B of conservative tree species was significantly higher than that of acquisitive tree species,but the Comammox clade A abundance of conservative tree species were significantly lower than acquisitive tree species.（5）In bulk soil,the content of litter tannin and wood density had a negative impact on the abundance of AOA,while the content of clay and nitrate nitrogen in bulk soil had a positive impact on the abundance of AOA.At the same time,the water holding capacity of the leaves litter and the soluble phenolics content of the leaves had a significant positive impact on the abundance of AOB,while the p H value of the litter leaves and the wood density had a significant negative impact on the abundance of AOB.The litter with high tannin concentration could directly inhibit the abundance and nitrification rate of AOA.Tree species with low wood density can significantly improve the abundance and activity of nitrifier in the process of soil ammonia oxidation.The litter with high p H value has more difficult to decompose organic acids（such as tannin）,so it can significantly reduce the abundance and activity of relevant microorganisms in the process of soil ammonia oxidation.The linear model was used to analyze the effects of soil and tree properties on the abundance of AOB.It was found that the abundance of AOB was mainly affected by the soil properties of the sample plot,and the plant functional properties could not explain the abundance of Comammox in the soil of different tree species,indicating that the effects of tree species on total ammonia oxidizing bacteria were not significant.（6）In bulk soil,this study found that soil microbial biomass carbon and nitrogen ratio,the p H of the litter leaves of different tree species and wood density had negative effects on the net nitrogen mineralization rate.The high ratio of microbial biomass carbon and nitrogen in soil indicates that the content of bacteria involved in nitrification is low,which leads to the decrease of soil nitrification rate and mineralization rate.Plants with high p H value of litter and high wood density can inhibit the mineralization rate of soil net nitrogen through the nutrient regression of litter and the nutrient acquisition mechanism of plants.In bulk soil,the carbon content of fallen leaves and microbial biomass nitrogen had a negative effect on nitrification potential,and the content of soluble phenolics in leaves had a significant stimulating effect on nitrification potential.The tree species with high carbon content in their litter leaves are not easy to decompose,but their soil nitrogen availability is high,which inhibits soil nitrification.