Linking soil pH and phosphorus management to potential N2O emissions and nitrogen cycling microbial communities
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2022-08-15Author
Grau Butinyac, Meritxell
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Abstract
Greenhouse gases (GHGs) in the atmosphere create the greenhouse gas effect, which
allows maintenance of global temperatures. However, anthropogenic production of
GHGs has caused dramatic increases of GHGs in the atmosphere, inducing global
warming and resulting in substantial climatic changes. Mitigating GHGs emissions is thus
a key priority to reduce the impact of climate change. Nitrous oxide (N2O) is a potent
greenhouse gas that is not only involved in global warming but that also causes damage
to the ozone layer. Soils are one of the major sources of N2O emissions. Excess
application of nitrogen (N) on agricultural soils via synthetic fertiliser and manure can
result in N losses due to leaching and N2O emissions. The N transformations that facilitate
these losses are driven by soil microbial communities that respond to changes in their
environment. These changes such as availability of inorganic N, soil pH and phosphorus
(P) levels can be a consequence of agricultural management practices.
Increases in soil pH have been linked to a reduction of N2O emissions through
impacts on the microbial community at the functional and structural level, making pH
management a possible approach for mitigating emissions. However, there is a need to
assess whether the impact of pH on microbial communities involved in N transformations
is conserved across a wide range of agronomic scenarios. Soil pH also affects P
availability in the soil, creating an interaction effect of these soil properties that could also
dictate both N2O emissions and microbial communities involved in the processes, but the
role of P management, and of this interaction, on N2O production rates, and N cycling
microbial communities, is poorly understood. The overall aim of this thesis was to
investigate the impact of soil pH on microbial community structure and functional
communities involved in N cycling processes, and to assess if this relationship was
maintained across a geoclimatic gradient and a wide range of soil types. Also, this thesis
aimed to investigate the long-term interaction between soil pH and P on these same N
cycling microbial communities, and associated processes, to better understand their
possible role in reducing N2O emissions from arable and grassland soils. This was
achieved through qPCR quantification of functional, prokaryotic, and fungal
communities, sequencing of prokaryotic and fungal communities, and laboratory
incubations for the measurement of potential denitrification and nitrification. The abundance of denitrifier and nitrifier functional communities, and of
prokaryotic and fungal communities were strongly impacted by both geoclimatic region
(including soil type) and pH treatment. This effect of pH treatment was primarily positive
on the abundance of the microbial communities present across pH treatments; however,
the relationships present sometimes varied between sites and between sampling times
within a site. Potential N2O emissions were influenced by pH treatment while potential
nitrification rate was influenced by pH and P treatment interaction. This interaction effect
was also observed on crenarchaea and denitrifier (nirK, nirS and nosZII) gene
abundances. P treatment influenced fungal and nitrous oxide reductase (nosZI) gene
abundances while pH treatment shaped the prokaryotic or fungal community structure.
Overall, these results demonstrate microbial communities are shaped by
agricultural management, with soil pH being a strong factor determining the functional
and structural community, but also indicating P has a role in influencing these same
processes and microbes. Understanding the response of microbial communities to
management practices will be key for future mitigation of greenhouse gas N2O from
agricultural soils and reducing the impact of agriculture on the environment.