Abstract
Grassland soils serve as a biological sink and source of the potent greenhouse gases (GHG) methane (CH4) and nitrous oxide (N2O). The underlying mechanisms responsible for those GHG emissions, specifically, the relationships between methane- and ammonia-oxidizing microorganisms in grazed grassland soils are still poorly understood. Here, we characterized the effects of grazing on in situ GHG emissions and elucidated the putative relations between the active microbes involving in methane oxidation and nitrification activity in grassland soils. Grazing significantly decreases CH4 uptake while it increases N2O emissions basing on 14-month in situ measurement. DNA-based stable isotope probing (SIP) incubation experiment shows that grazing decreases both methane oxidation and nitrification processes and decreases the diversity of active methanotrophs and nitrifiers, and subsequently weakens the putative competition between active methanotrophs and nitrifiers in grassland soils. These results constitute a major advance in our understanding of putative relationships between methane- and ammonia-oxidizing microorganisms and subsequent effects on nitrification and methane oxidation, which contribute to a better prediction and modeling of future balance of GHG emissions and active microbial communities in grazed grassland ecosystems.
Highlights
Methane (CH4), as the second most potent greenhouse gas (GHG) after carbon dioxide (CO2), is considered to be responsible for ~20% of the anthropogenic global warming effect [1]
It is noticeable that pMMO of methane‐oxidizing bacteria (MOB) and ammonia monooxygenase (AMO) of ammonia-oxidizing bacteria (AOB) are homologous members [6], which are grouped into the copper-containing membrane-bound monooxygenase (CuMMO) family [7]
Greenhouse-gas emissions, methane, and ammonia oxidation activity The in situ GHGs measurements revealed that total N2O emission was 0.134 kg ha−1 in the ungrazed soils over the two years, and was significantly lower than that in the grazed soils (0.212 kg ha−1)
Summary
Methane (CH4), as the second most potent greenhouse gas (GHG) after carbon dioxide (CO2), is considered to be responsible for ~20% of the anthropogenic global warming effect [1]. Aerobic oxidation of CH4 in soils by methane‐oxidizing bacteria (MOB), known as methanotrophs, represents the largest biological sink for atmospheric CH4 [3]. Methanotrophs have the unique ability to grow on CH4 as their sole source of carbon and energy. They are ubiquitous in the environment and play a major role in the removal of the greenhouse gas methane from the biosphere before it is released into the atmosphere [4]. It is noticeable that pMMO of MOB and ammonia monooxygenase (AMO) of ammonia-oxidizing bacteria (AOB) are homologous members [6], which are grouped into the copper-containing membrane-bound monooxygenase (CuMMO) family [7]. The evolutionary links between MOB and AOB, together with the similar molecular structure of their substrates (NH3 and CH4, respectively), lead to functional similarities enabling them to oxidize both NH3 and CH4, neither AOB nor MOB are capable of growing on the alternative substrate [9,10,11]
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