Effect Of Elevated Carbon Dioxide Concentration On Nitrification And Denitrification In Agricultural Soil

Elevated atmospheric CO₂ concentration is one of the major factors affecting nutrient cycling in global terrestrial ecosystem.Soil is the largest nitrogen pool in terrestrial ecosystem,and the increase of atmospheric CO₂ concentration will affect soil nitrogen cycle process by changing the interaction of plant-soil-microorganism.However,the results of previous studies are inconsistent due to different experimental conditions,management practices,environmental factors and plant species.On the other hand,meta-analysis on the effects of elevated atmospheric CO₂ concentration on nitrogen cycling and functional microorganisms in agricultural ecosystems are rarely reported.Based on this,this study performed a meta-analysis to evaluate the effects of elevated CO₂ on soil nitrification and denitrification rates,N₂O emissions,and nitrogen cycling functional microorganisms in agricultural ecosystems;further carried out a pot experiment to study the effects of elevated CO₂ on nitrification and denitrification processes in maize rhizosphere soil and underlying microbiological mechanism.The results can provide a theoretical basis for scientific evaluation of soil nitrogen cycle in agricultural ecosystems under future elevated CO₂ scenarios.The main results are summarized as follows:（1）Based on the meta-analysis of 147 observation groups in 34 literatures,elevated CO₂ significantly increased potential nitrification rate and ammonia-oxidizing bacteria（AOB）abundance in agricultural soil.Climate,experimental conditions,soil properties and agricultural practices were important factors in regulating the responses of soil nitrification to elevated CO₂.The responses of nitrification potential were positively correlated with AOB abundance,while no evidence was found for ammonia-oxidizing archaea（AOA）abundance.The elevated CO₂ response of AOB abundance was more closely related to the responses of soil nitrification than a wide range of environmental and experimental factors,as well as management practices.Furthermore,various effects of elevated CO₂ on AOB abundance were determined by soil type and magnitude of elevated CO₂.Collectively,elevated CO₂strengthens soil nitrification mainly by increasing AOB populations in agroecosystems.（2）Based on the meta-analysis of 782 observation data in 50 literatures,elevated CO₂ significantly stimulated denitrification and N₂O emissions in agricultural soil,and the experimental conditions,soil properties and agricultural practices were importantly regulatory factors affecting N₂O emissions under elevated CO₂.Importantly,elevated CO₂ significantly increased soil dissolved organic carbon（DOC,11%）,and nitrate nitrogen（13%）,but significantly reduced soil pH.Moreover,the corresponding soil biological characteristics showed that elevated CO₂ significantly increased the content of soil microbial biomass carbon（MBC,28%）and soil microbial biomass nitrogen（MBN,7%）.Additionally,elevated CO₂ also significantly increased the abundance of nirK（15%）and nir S（15%）,but showed no effect on the abundance of nosZ.Theses findings indicate that elevated CO₂ substantially stimulates denitrification and N₂O emissions through increasing soil labile substrates（DOC,MBC and MBN）,decreasing soil pH and thus increasing the abundance of functional genes nirK and nirS in agricultural soil.（3）Pot experiment was conducted to further verify the effects of elevated CO₂on soil nitrification and denitrification.The results showed that elevated CO₂ can promote the growth of maize plant in all studied soils,while the soil nitrification and denitrification have various responses.In HEB,QHD and HZ soil,elevated CO₂did not change soil mineral nitrogen availability（ammonium nitrogen,nitrate nitrogen）and labile carbon availability（DOC,MBC）,so the soil potential nitrification and denitrification rates did not change significantly;In JA soil,elevated CO₂ increased potential denitrificaiton rate by increasing labile organic carbon such as DOC and MBC,and increasing the abundance of AOB,nirK and nirS genes.Therefore,combined with the results of meta-analysis previously,it can be inferred that the response of nitrification and denitrification in acidic soil with low organic matter to elevated CO₂ is more obvious.