CO2 emission fluxes and bacterial community structures characteristics in the water bodies of the Lingqu and the Darongjiang Rivers

LUO Ting; JIN Zhenjiang; YUAN Wu; YANG Chengxi; LI Jia; WANG Shixuan; FAN Chen; CHEN Guilun; ZHANG Xiaowen

doi:10.11932/karst20250509

Abstract：Rivers connect two major carbon pools-terrestrial ecosystems and marine ecosystems-and play a crucial role in the global carbon cycle.Microorganisms are the main agents driving the carbon cycling processes within river ecosystems.To some extent,microorganisms and the physicochemical characteristics of rivers can affect the production of CO2 in rivers and are the key factors in the riverine carbon cycle.Therefore,studying the interrelationships among CO2 emission fluxes,microorganisms and physicochemical properties at the water-air interface of rivers is essential for understanding the underlying mechanisms of riverine carbon emissions.In South China,carbonates are widely distributed,and karst processes are highly active,serving as significant drivers of the river carbon cycle.However,the differences in CO2 emission characteristics between rivers in Karst Areas(KA)and Non-Karst Areas(NKA)as well as their driving mechanisms,remain unclear.To investigate CO2 emission fluxes and their driving factors in river sections in KA,the Lingqu section(KA)and the Darongjiang River section(NKA)in the upper reaches of the Lijiang River were selected as the sampling points.During the wet season(August 2023),dry season(January 2024),and normal season(March 2024),CO2 emission fluxes were measured over 48-hour periods at both the Lingqu River section and the Darongjiang River section by the static chamber method.The bacterial community structures of the rivers were characterized by high-throughput sequencing technology.Correlations between CO2 emission fluxes,river physicochemical properties,and the microbial communities were analyzed.The main results are as follows. (1)Within 48 hours during the three hydrological periods,the CO2 emission fluxes at the Lingqu River section were 14.54 to 352.88 mg·(m2·h)-1,0.85 to 10.47 mg·(m2·h)-1,and 1.05 to 13.83 mg·(m2·h)-1,respectively.The total emissions were 25.82 kg·hm-2,1.38 kg·hm-2,and 2.29 kghm-2,respectively.At the Darongjiang River section,CO2 emission fluxes were 3.70 to 399.90 mg·(m2·h)-1,1.25 to 8.24 mg·(m2·h)-1,and 4.14 to 36.09 mg·(m2·h)-1,respectively,with total emissions of 40.46 kg·hm-2,1.88 kg·hm-2,and 4.36 kg·hm-2,respectively.The CO2 emission fluxes at the water-air interface in both KA and NKA were positive,indicating that the study area is a source of CO2.The CO2 emission followed the pattern:flood season＞normal season＞dry season.(2)The CO2 emission flux at the Lingqu River section was significantly negatively correlated with pH(P＜0.01)and Total Nitrogen(TN)(P＜0.05),while the CO2 emission flux at the Darongjiang River was significantly positively correlated with Dissolved Organic Carbon(DOC)(P＜0.05)and TN(P＜0.01).When the CO2 emission flux and physicochemical properties of both the Lingqu River and Darongjiang River sections were analyzed,the CO2 emission flux showed a significant positive correlation with DOC(P＜0.05)and a significant negative correlation with pH(P＜0.01).(3)At the bacterial genus level,twelve dominant bacterial genera were identified in the Lingqujiang River section,including Limnohabitans(1.20％to 26.58％),Pseudomonas(0.24％to 22.00％),and unclassified Micrococcaceae(0.45％to 21.07％).In contrast,nine dominant bacterial genera were found in the Darongjiang River section,including Acinetobacter(0.23％to 23.79％),unclassified Sporichthyaceae(1.64％to 13.54％),and the CL500-29 marine group(3.74％to 12.02％).Heat map analysis showed that the CO2 emission fluxes at both the Lingqu and Darongjiang River sections were significantly positively correlated with the relative abundances of the CL500-29 marine group(P＜0.01)and the hgcI clade(P＜0.01).Conversely,emissions were significantly negatively correlated with the relative abundance of Pseudomonas(P＜0.01).(4)Ecological network analysis shows that the number of nodes,the total number of edges,and the modularity index of the Lingqu River section network are all lower than those of the Darongjiang River section.This indicates that the interactions among bacteria in the KA are weaker,the community structure is unstable,and the ecosystem is more fragile.Therefore,both biological factors(such as the CL500-29 marine group,hgcI clade,and Pseudomonas)and abiotic factors(such as pH,DOC,and TN)jointly regulate the CO2 emission process of the Lijiang River.To maintain a low CO2 emission at the water-air interface,interference to the river ecosystem in the KA should be minimized as much as possible. This study analyzes the CO2 emissions and microbial communities at the water-air interface of the river in the KA(the Lingqu River section)and the NKA(the Darongjiang River section)of the upper reaches of the Lijiang River.The research shows that the CO2 emission flux in the KA basin is relatively low and reveals differences in the community structure and stability of microorganisms between these two areas.