Nitrogen fixation is one of the most important sources of new nitrogen in the ocean and thus profoundly affects the nitrogen and carbon biogeochemical processes.The distribution,controlling factors,and flux of N2 fixa...Nitrogen fixation is one of the most important sources of new nitrogen in the ocean and thus profoundly affects the nitrogen and carbon biogeochemical processes.The distribution,controlling factors,and flux of N2 fixation in the global ocean remain uncertain,partly because of the lack of methodological uniformity.The^(15)N_(2)tracer assay(the original bubble method→the^(15)N_(2)-enriched seawater method→the modified bubble method)is the mainstream method for field measurements of N2 fixation rates(NFRs),among which the original bubble method is the most frequently used.However,accumulating evidence has suggested an underestimation of NFRs when using this method.To improve the availability of previous data,we compared NFRs measured by three^(15)N_(2)tracer assays in the South China Sea.Our results indicate that the relationship between NFRs measured by the original bubble method and the^(15)N_(2)-enriched seawater method varies obviously with area and season,which may be influenced by incubation time,diazotrophic composition,and environmental factors.In comparison,the relationship between NFRs measured by the original bubble method and the modified bubble method is more stable,indicating that the N2 fixation rates based on the original bubble methods may be underestimated by approximately 50%.Based on this result,we revised the flux of N2 fixation in the South China Sea to 40 mmol/(m2·a).Our results improve the availability and comparability of literature NFR data in the South China Sea.The comparison of the^(15)N_(2)tracer assay for NFRs measurements on a larger scale is urgently necessary over the global ocean for a more robust understanding of the role of N2 fixation in the marine nitrogen cycle.展开更多
Soil heterotrophic respiration during decomposition of carbon(C)-rich organic matter plays a vital role in sustaining soil fertility.However,it remains poorly understood whether dinitrogen(N_(2))fixation occurs in sup...Soil heterotrophic respiration during decomposition of carbon(C)-rich organic matter plays a vital role in sustaining soil fertility.However,it remains poorly understood whether dinitrogen(N_(2))fixation occurs in support of soil heterotrophic respiration.In this study,^(15)N_(2)-tracing indicated that strong N_(2)fixation occurred during heterotrophic respiration of carbon-rich glucose.Soil organic ^(15)N increased from 0.37 atom%to 2.50 atom%under aerobic conditions and to 4.23 atom%under anaerobic conditions,while the concomitant CO_(2)flux increased by 12.0-fold under aerobic conditions and 5.18-fold under anaerobic conditions.Soil N_(2)fixation was completely absent in soils replete with inorganic N,although soil N bioavailability did not alter soil respiration.High-throughput sequencing of the 16S rRNA gene further indicated that:i)under aerobic conditions,only 15.2%of soil microbiome responded positively to glucose addition,and these responses were significantly associated with soil respiration and N_(2)fixation and ii)under anaerobic conditions,the percentage of responses was even lower at 5.70%.Intriguingly,more than 95%of these responses were originally rare with<0.5%relative abundance in background soils,including typical N_(2)-fixing heterotrophs such as Azotobacter and Clostridium and well-recognized non-N_(2)-fixing heterotrophs such as Sporosarcina,Agromyces,and Sedimentibacter.These results suggest that only a small portion of the soil microbiome could respond quickly to the amendment of readily accessible organic C in a fluvo-aquic soil and highlighted that rare phylotypes might have played more important roles than previously appreciated in catalyzing soil C and nitrogen turnovers.Our study indicates that N_(2)fixation could be closely associated with microbial turnover of soil organic C when available in excess.展开更多
基金The National Natural Science Foundation of China under contract Nos 42076042 and 41721005the Fund of Ministry of Science and Technology of China under contract No.2017FY201403the Fund of China Ocean Mineral Resources R&D Association under contract No.DY135-13-E2-03.
文摘Nitrogen fixation is one of the most important sources of new nitrogen in the ocean and thus profoundly affects the nitrogen and carbon biogeochemical processes.The distribution,controlling factors,and flux of N2 fixation in the global ocean remain uncertain,partly because of the lack of methodological uniformity.The^(15)N_(2)tracer assay(the original bubble method→the^(15)N_(2)-enriched seawater method→the modified bubble method)is the mainstream method for field measurements of N2 fixation rates(NFRs),among which the original bubble method is the most frequently used.However,accumulating evidence has suggested an underestimation of NFRs when using this method.To improve the availability of previous data,we compared NFRs measured by three^(15)N_(2)tracer assays in the South China Sea.Our results indicate that the relationship between NFRs measured by the original bubble method and the^(15)N_(2)-enriched seawater method varies obviously with area and season,which may be influenced by incubation time,diazotrophic composition,and environmental factors.In comparison,the relationship between NFRs measured by the original bubble method and the modified bubble method is more stable,indicating that the N2 fixation rates based on the original bubble methods may be underestimated by approximately 50%.Based on this result,we revised the flux of N2 fixation in the South China Sea to 40 mmol/(m2·a).Our results improve the availability and comparability of literature NFR data in the South China Sea.The comparison of the^(15)N_(2)tracer assay for NFRs measurements on a larger scale is urgently necessary over the global ocean for a more robust understanding of the role of N2 fixation in the marine nitrogen cycle.
基金financially supported by the National Science Foundation of China(Nos.91751204,41530857,and 41471205)the National Basic Research Program of China(No.2015CB150501)the Strategic Priority Research Program of Chinese Academy of Sciences(CAS)(No.XDB15040000)。
文摘Soil heterotrophic respiration during decomposition of carbon(C)-rich organic matter plays a vital role in sustaining soil fertility.However,it remains poorly understood whether dinitrogen(N_(2))fixation occurs in support of soil heterotrophic respiration.In this study,^(15)N_(2)-tracing indicated that strong N_(2)fixation occurred during heterotrophic respiration of carbon-rich glucose.Soil organic ^(15)N increased from 0.37 atom%to 2.50 atom%under aerobic conditions and to 4.23 atom%under anaerobic conditions,while the concomitant CO_(2)flux increased by 12.0-fold under aerobic conditions and 5.18-fold under anaerobic conditions.Soil N_(2)fixation was completely absent in soils replete with inorganic N,although soil N bioavailability did not alter soil respiration.High-throughput sequencing of the 16S rRNA gene further indicated that:i)under aerobic conditions,only 15.2%of soil microbiome responded positively to glucose addition,and these responses were significantly associated with soil respiration and N_(2)fixation and ii)under anaerobic conditions,the percentage of responses was even lower at 5.70%.Intriguingly,more than 95%of these responses were originally rare with<0.5%relative abundance in background soils,including typical N_(2)-fixing heterotrophs such as Azotobacter and Clostridium and well-recognized non-N_(2)-fixing heterotrophs such as Sporosarcina,Agromyces,and Sedimentibacter.These results suggest that only a small portion of the soil microbiome could respond quickly to the amendment of readily accessible organic C in a fluvo-aquic soil and highlighted that rare phylotypes might have played more important roles than previously appreciated in catalyzing soil C and nitrogen turnovers.Our study indicates that N_(2)fixation could be closely associated with microbial turnover of soil organic C when available in excess.
