Abstract
BackgroundN 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.ResultsIt 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.ConclusionsOur 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.
Highlights
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
N fixation and N fixers under long-term fertilization scenarios Our results indicated that N fixation rates were significantly suppressed by a wide range of fertilizers after almost four decades of fertilization (Fig. 1a)
We found that N fixation rates dropped by 50%, which was more noticeable in bulk soils than in the rhizosphere (Additional file 1: Table S6)
Summary
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. N fixation and its associated microbial communities have been largely challenged by the industrial Haber process, and later by inorganic and organic fertilization [4], which provides 32 Tg N year−1 to global croplands [5] Such a large amount of fertilization might relegate N fixers to a second place [6, 7] and could have long-term consequences for these important microbial communities and ecosystem processes in the future [8, 9]. Short-term additions of N fertilizers can result in an increase in the abundance of fast-growing diazotrophs [10] These microbial communities may use resources from fertilizers to support their own vegetative growth, instead of fixing nitrogen [11], which is known to be an energy-expensive process [12]. Fertilization could benefit copiotrophic and facultative N fixers that are
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