Soil Nitrification In Rice Paddy Ecosystem And Lake Sediments In Response To Elevated CO₂

Human activities such as fossil fuel combustion and deforestation have continuously increased atmospheric carbon dioxide（CO₂）concentrations,posing a great challenge for holding global warming at well below 1.5°C relative to pre-industrial levels by the middle of this century.To this end,many countries including China,have taken the lead in achieving carbon neutrality by the middle of this century.Whether terrestrial ecosystems can act as carbon sinks and thus play an important role in carbon neutralization is worth further exploring.Wetland ecosystem,as one of the largest carbon pools on earth,has a profound impact on the global biogeochemical cycle.However,soil nitrogen（N）supply largely regulates the sustainability of wetland ecosystem productivity in response to increasing atmospheric CO₂ concentration.Nitrification,which is mediated by nitrifiers,has an important effect on the form and content of available nitrogen for plants by regulating the converting from soil ammonia to nitrate.However,the lack of research on the effects of elevated CO₂ concentration on nitrifiers in wetland ecosystems may affect our in-depth understanding and accurate prediction of the response of wetland ecosystems to future climate change.Therefore,in this study,we explored the response of nitrification in wetland ecosystems to elevated CO₂ by investigating paddy fields as typical constructed wetland ecosystems and freshwater lakes as typical natural wetland ecosystems.The main problems of this research include:（1）Nitrification activity and community composition of nitrifiers in paddy soils and lake sediments,and the effects of environmental factors governing the distribution and function of nitrifiers;（2）Effects of elevated CO₂ on nitrification in paddy soils and the related mechanisms;（3）Effects of elevated CO₂ on nitrification in lakes sediments and the related mechanisms.The main results of the study were as follows:1.Distribution characteristics of nitrifying microorganisms in paddy soils and lake sediments and main environmental factors.Soil samples were collected from a long-term Free-air CO₂ Enrichment（FACE）rice paddy platform and sediment samples were collected from Chaohu Lake and Taihu Lake for this study.The results showed that there were significant differences in the distribution of nitrifying microorganisms between paddy soils and lake sediments.Ammonia-oxidizing archaea（AOA）and complete ammonia oxidizers（Comammox）were the main drivers of nitrification in paddy soils and lake sediments respectively,accounting for 93.2%and 76.2%of total nitrifiers’abundance in average.The results of microcosm experiments showed that low ammonia addition increased the abundance of AOA and ammonia-oxidizing bacteria（AOB）in paddy soil,but high ammonia nitrogen inhibited all nitrifiers.And the increase of oxygen content only inhibited all nitrifiers under high nitrogen condition.For lake sediments,the ammonia addition inhibited both the abundance of AOA and comammox,but promoted the abundance of AOB.And the increase of oxygen content promoted the abundance of AOA and AOB in sediments.2.Effects and mechanisms of elevated CO2 on nitrification and nitrifiers in paddy fields.Based on the FACE platform,we used in situ determination,microcosm incubation,isotope tracer,metagenomic sequencing and 16S r RNA sequencing to determine the effects of elevated CO₂ on nitrification and nitrifiers in paddy soils.The results showed that elevated CO₂ significantly inhibited nitrification by 38.2%in average in paddy soils.The amo A gene abundance and activity of nitrifiers were also inhibited,and their community structures were shifted.Among nitrifiers,the abundances of AOA and AOB decreased by 15.6%and 27.7%under elevated CO₂,while comammox was not affected.We suggested that elevated CO₂ might inhibit nitrifying microorganisms by reducing the concentrations of available ammonia and oxygen in soil and increasing the competitive stress on nitrifying communities.3.Effects and mechanisms of elevated CO2 on nitrification and nitrifiers in lake sediments.We designed and constructed an enrichment device in which the Taihu Lake sediments were treated with low or high CO₂ concentrations and were set for a long-term enrichment culture.Combined with sediment properties monitoring,isotope tracer,16S r RNA sequencing and other technical means,the effects of elevated CO₂on nitrification and nitrification microbial community of lake sediments were explored.It was found that the ammonium consumption rate,nitrite production rate and nitrate production rate in high CO₂ group were significantly higher than those in low CO₂group.The total amo A gene abundance of ammonia-oxidizers was promoted by elevated CO₂ with the abundance of comammox clade A increased by 21.0%,and nitrifiers’community structure was also shifted.We inferred that in this system elevated CO₂ might promote nitrifying microorganisms through reducing high ammonia stress and increasing carbon source for nitrifiers.This study reveals the responses of nitrification processes and nitrifying microorganisms to elevated CO₂ concentrations in typical artificial and natural wetland ecosystems,and investigates possible mechanisms.Our work may contribute to the prediction of wetland nitrogen cycles changes under future climate change scenarios,thus enhancing the understanding of global biogeochemical cycles.It also provides theoretical basis for rational application of nitrogen fertilizer in paddy fields and prevention and control of lake eutrophication.