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Suppressed N fixation and diazotrophs after four decades of fertilization.

ABSTRACT: BACKGROUND:N fixation is one of the most important microbially driven ecosystem processes on Earth, allowing N to enter the soil from the atmosphere, and regulating plant productivity. A question that remains to be answered is whether such a fundamental process would still be that important in an over-fertilized world, as the long-term effects of fertilization on N fixation and associated diazotrophic communities remain to be tested. Here, we used a 35-year fertilization experiment, and investigated the changes in N fixation rates and the diazotrophic community in response to long-term inorganic and organic fertilization. RESULTS:It was found that N fixation was drastically reduced (dropped by 50%) after almost four decades of fertilization. Our results further indicated that functionality losses were associated with reductions in the relative abundance of keystone and phylogenetically clustered N fixers such as Geobacter spp. CONCLUSIONS:Our work suggests that long-term fertilization might have selected against N fixation and specific groups of N fixers. Our study provides solid evidence that N fixation and certain groups of diazotrophic taxa will be largely suppressed in a more and more fertilized world, with implications for soil biodiversity and ecosystem functions.

SUBMITTER: Fan K

PROVIDER: S-EPMC6824023 | biostudies-literature | 2019 Oct

REPOSITORIES: biostudies-literature

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Suppressed N fixation and diazotrophs after four decades of fertilization.

Fan Kunkun K Delgado-Baquerizo Manuel M Guo Xisheng X Wang Daozhong D Wu Yanying Y Zhu Mo M Yu Wei W Yao Huaiying H Zhu Yong-Guan YG Chu Haiyan H

Microbiome 20191031 1

<h4>Background</h4>N fixation is one of the most important microbially driven ecosystem processes on Earth, allowing N to enter the soil from the atmosphere, and regulating plant productivity. A question that remains to be answered is whether such a fundamental process would still be that important in an over-fertilized world, as the long-term effects of fertilization on N fixation and associated diazotrophic communities remain to be tested. Here, we used a 35-year fertilization experiment, and ...[more]

PMID: 31672173

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Project description:ObjectiveAs the number of female medical students and surgical residents increases, the increasing number of female academic surgeons has been disproportionate. The purpose of this brief report is to evaluate the AAMC data from 1969 to 2018 to compare the level of female academic faculty representation for surgical specialties over the past four decades.DesignThe number of women as a percentage of the total surgeons per year were recorded for each year from 1969-2018, the most recent year available. Descriptive statistics were performed. Poisson regression examined the percentage of women in each field as the outcome of interest with the year and specialty (using general surgery as a reference) as two predictor variables.SettingData from the American Association of Medical Colleges (AAMC).ParticipantsAll full-time academic faculty physicians in the specialties of obstetrics and gynecology (OB/GYN), general surgery, ophthalmology, otolaryngology (ENT), plastic surgery, plastic surgery, urology, neurosurgery, orthopaedic surgery and cardiothoracic surgery as per AAMC records.ResultsThe percentage of women in surgery for all specialties evaluated increased from 1969 to 2018 (OR 1.04, p<0.001). Compared with general surgery, the rate of yearly percentage change increased more slowly in neurosurgery (OR 0.84; P = .004), orthopaedic surgery (OR 0.82; P = .002), urology (OR 0.59; P < .001), and cardiothoracic surgery (OR 0.38; P < .001). There was no significant difference in the rate of yearly percentage change for plastic surgery (OR 1.01; P = .840). The rate of yearly percentage change increased more rapidly in OB/GYN (OR 2.86; P < .001), ophthalmology (OR 1.79; P < .001) and ENT (OR 1.70; P < .001).ConclusionsRepresentation of women in academic surgery is increasing overall but is increasing more slowly in orthopaedic surgery, neurosurgery, cardiothoracic surgery and urology compared with that in general surgery. These data may be used to inform and further the discussion of how mentorship and sponsorship of female students and trainees interested in surgical careers may improve gender equity in the future.

Project description:Biological dinitrogen (N2) fixation is one mechanism by which specific microorganisms (diazotrophs) can ameliorate nitrogen (N) limitation. Historically, rates of N2 fixation were believed to be limited outside of the low nutrient tropical and subtropical open ocean; however, emerging evidence suggests that N2 fixation is also a significant process within temperate coastal waters. Using a combination of amplicon sequencing, targeting the nitrogenase reductase gene (nifH), quantitative nifH PCR, and 15N2 stable isotope tracer experiments, we investigated spatial patterns of diazotroph assemblage structure and N2 fixation rates within the temperate coastal waters of southern Australia during Austral autumn and summer. Relative to previous studies in open ocean environments, including tropical northern Australia, and tropical and temperate estuaries, our results indicate that high rates of N2 fixation (10-64 nmol L-1 d-1) can occur within the large inverse estuary Spencer Gulf, while comparatively low rates of N2 fixation (2 nmol L-1 d-1) were observed in the adjacent continental shelf waters. Across the dataset, low concentrations of NO3/NO2 were significantly correlated with the highest N2 fixation rates, suggesting that N2 fixation could be an important source of new N in the region as dissolved inorganic N concentrations are typically limiting. Overall, the underlying diazotrophic community was dominated by nifH sequences from Cluster 1 unicellular cyanobacteria of the UCYN-A clade, as well as non-cyanobacterial diazotrophs related to Pseudomonas stutzeri, and Cluster 3 sulfate-reducing deltaproteobacteria. Diazotroph community composition was significantly influenced by salinity and SiO4 concentrations, reflecting the transition from UCYN-A-dominated assemblages in the continental shelf waters, to Cluster 3-dominated assemblages in the hypersaline waters of the inverse estuary. Diverse, transitional diazotrophic communities, comprised of a mixture of UCYN-A and putative heterotrophic bacteria, were observed at the mouth and southern edge of Spencer Gulf, where the highest N2 fixation rates were observed. In contrast to observations in other environments, no seasonal patterns in N2 fixation rates and diazotroph community structure were apparent. Collectively, our findings are consistent with the emerging view that N2 fixation within temperate coastal waters is a previously overlooked dynamic and potentially important component of the marine N cycle.

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Suppressed N fixation and diazotrophs after four decades of fertilization.

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Suppressed N fixation and diazotrophs after four decades of fertilization.

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