Mechanisms Underlying The Effects Of Simulated Atmospheric Nitrogen Deposition On Soil Nitrous Oxide Emission In A Subtropical Karst Forest

Since the industrial revolution,the production and use of nitrogen（N）-containing chemical fertilizers and the combustion of fossil fuels have led to the rapid increase of reactive nitrogen emitted by human activities to the environment,resulting in the rapid increase of atmospheric nitrogen deposition,which has affected the structure,function and process of ecosystems.Nitrous oxide（N₂O）is the main greenhouse gas in the troposphere and the main ozone destroying substance in the stratosphere.Soil nitrogen transformation processes in forests is a main source of atmospheric N₂O.Therefore,the study of forest soil N₂O emission and its responses to atmospheric nitrogen deposition will not only help to deeply understand the process of soil nitrogen transformation,but also help to reveal the response mechanisms of soil N₂O emission under the background of increased nitrogen deposition,and provide scientific and technological support for improving the simulation and prediction ability of soil N₂O emission of Earth system models.Although a large number of studies have been conducted about soil N₂O emissions from forests worldwide,these studies are mainly carried out in non-karst forests.Due to a series of special properties of soil biotic and abiotic factors,the emission and production process of soil N₂O in subtropical karst forest may be different from those in non-karst forests at the same latitude,so targeted research is needed.A typical karst forest in northwest Guangxi was selected in the current study in order to investigate the impacts of nitrogen addition on soil N₂O emission and its production mechanisms in the valley and on the slope.A completely randomized block design was adopted and three nitrogen addition treatments were applied at each topographic position,including control（no additional nitrogen addition）,medium nitrogen addition(nitrogen addition level of 50 kg N ha^-1 yr^-1)and high nitrogen addition(nitrogen addition level of 100 kg N ha^–1 yr^–1).Firstly,soil N₂O emission fluxes were measured by a static chamber method under field conditions for three consecutive years.On this basis,in order to reveal the relative contribution of different pathways to soil N₂O production,we used ¹⁵N-¹⁸O isotope double labeling technology to determine the relative contributions of autotrophic nitrification,heterotrophic nitrification,nitrifying bacteria denitrification,nitrification coupled denitrification,heterotrophic denitrification and combined denitrification to soil N₂O production.Finally,because ammonia oxidation process was the main pathway of N₂O production,we further investigated the relative contributions of ammonia oxidizing bacteria（AOB）and archaea（AOA）to N₂O production.Enzyme stoichiometry,soil physicochemical properties and nitrogen transformation functional gene abundances were used to explore main controlling factors of soil N₂O production,and the mechanisms underlying the differential responses of soil N₂O emission to N addition between two topographic positions.The main results are presented below:（1）Nitrogen addition consistently increased N₂O emission in the valley,but only high N addition significantly increased N₂O emission on the slope in 2017.The cumulative N₂O fluxes across the three years were 1.16±0.24 kg N ha^–1 in the valley and 1.50±0.06 kg N ha^–1 on the slope under the control.Nitrogen addition stimulated N₂O emission by 88.7～113.3%in the valley due to increased ammonium,nitrate and dissolved organic carbon availabilities and ammonia–oxidizing bacteria（AOB）amo A abundance.High N addition stimulated N₂O emission by 84.3%on the slope owing to increased nitrate and carbon availabilities,AOB amo A and nir K abundances.The stimulation of N₂O emission by moderate N addition was more pronounced in the valley than on the slope largely owing to the lower N status in the valley.This work highlights the importance of N status in regulating the responses of soil N₂O emission to elevated N deposition.（2）Autotrophic nitrification pathways（ammonia oxidization,nitrifier denitrification and nitrification coupled denitrification）accounted for>70%of total N₂O production,but denitrification pathways（heterotrophic denitrification and co–denitrification）were the minor source of N₂O at both topographic positions.In the valley,chronic N addition stimulated ammonia oxidization–derived N₂O production paralleled by increased ammonia oxidizing archaea（AOA）amo A gene expression,but inhibited nitrifier denitrification and nitrification coupled denitrification derived N₂O production along with suppressed ammonia oxidizing bacteria（AOB）amo A gene expression and nir S,nos Z??gene expression.On the slope,chronic N addition enhanced nitrifier denitrification–derived N₂O production congruent with increased AOB amo A and decreased nir K gene expressions,but suppressed heterotrophic denitrification derived N₂O production along with increased nos Z??gene expression.Our results indicate that the effects of N deposition on the contribution of soil N₂O production pathways to N₂O emissions are topography–dependent.（3）AOB was primarily responsible for soil N₂O production under the control regardless of topographic position.For the organic horizon,N addition stimulated AOA derived N₂O in the valley because of increased N mineralization due to alleviated microbial carbon limitation;but suppressed AOA and AOB derived N₂O on the slope because of decreased N mineralization attributed to aggravated microbial phosphorus limitation.For the mineral horizon,N addition enhanced AOB derived N₂O in the valley because of increased ammonia availability,but stimulated AOA and AOB derived N₂O on the slope because of increased N mineralization and ammonia availability owing to aggravated microbial C limitation and alleviated phosphorus limitation.Our results indicate that the mechanisms underlying the impacts of N deposition on soil N₂O production by ammonia oxidizers are topography dependent,so that topography specific niche specialization between AOA and AOB should be integrated into Earth system models in order to better predict soil N₂O production under elevated atmospheric N deposition.