Uncharacterised microbial pathways are key to understanding large fluxes of biogenic reactive nitrogen gases from agronomic soils
Lead Research Organisation:
University of Warwick
Department Name: School of Life Sciences
Abstract
Atmospheric reactive nitrogen oxide gases (NOy = NO + NO2 + HONO + ...) are coupled to Earth's nitrogen cycle through an intricate network of interactions between anthropogenic activity (primarily combustion), soil nitrogen, and soil microbial activity. The biogeochemistry of soil nitrogen emissions is traditionally thought to be dominated by nitrogen gas (N2) and nitrous oxide (N2O); however, satellite, modelling, and laboratory studies show NOy emissions from soil can be of greater magnitude than these more commonly measured nitrogen gases.
NOy are important in atmospheric chemistry as precursors to ozone (O3) and play a key role in the formation of acid rain. NOy are also considered secondary air pollutants - known to worsen asthma and bronchitis, especially in adolescent and geriatric populations. In addition to anthropogenic sources, NOy gases are produced from natural non-point sources including soil (24% of total NOy emissions), wildfires (19%), and lightning (13%). However, very little is known about NOy fluxes from these natural sources even though they account for 50% of all atmospheric NOy - with this percentage increasing as vehicle and industry emissions continue to decline.
Importantly, there is a critical lack of information regarding biogenic (microbiologically derived) production mechanisms, resulting in inaccurate NOy-coupled climate model projections. This can be primarily attributed to a lack of understanding regarding the formation of major NOy species, such as nitrogen dioxide (NO2) and nitrous acid (HONO). We suspect there are yet undiscovered NOy-producing pathways, catalysed by a vast array of microbes from all three domains of life. Our hypothesised mechanisms are derived from human physiology, where NOy species are known to be important signalling molecules.
We will explore these mechanisms in agronomic soils, as soils are the largest natural source of NOy gases, and within an agronomic context as our preliminary work has shown that these soils produce significantly more NOy than other terrestrial systems such as grasslands and woodlands. We will further define agronomic-NOy with field trials of a major UK commercial crop (Triticum), including four cultivars with differing above- and belowground traits. It will be crucial to define NOy soil emissions from these different cultivars, as varying plant traits, such as specific root length, can influence the soil N-cycle microbiome - which will inevitably influence NOy emissions. Other important variables will also be explored, including fertiliser application and spatial variability of NOy flux.
Importantly, we will also attempt to determine the role of soil iron and iron speciation on N-cycle community composition and NOy fluxes. Soil iron is an important aspect of our theoretical NOy mechanism - stimulating the production of reactive oxygen species, which is a key reactant in the production of NOy. We will source soil with differing iron content from various farms throughout England to be used in wheat mesocosms studies. Soil NOy fluxes will be measured and connected to mineralogical characteristics and the N-cycle community size.
Overall, this project will determine fundamental knowledge on the biogenic production mechanisms of major NOy species, provide direct soil NOy flux measurements from a major global crop, and lead to a better understanding of coupled carbon-nutrient-mineral cycling in soil. Furthermore, this work represents a major step change in the understanding of soil nitrogen dynamics, will be one of the first to couple shotgun metagenomics and culture-dependent methods to atmospheric chemistry processes, and will represent a major advancement in atmospheric accounting of NOy.
NOy are important in atmospheric chemistry as precursors to ozone (O3) and play a key role in the formation of acid rain. NOy are also considered secondary air pollutants - known to worsen asthma and bronchitis, especially in adolescent and geriatric populations. In addition to anthropogenic sources, NOy gases are produced from natural non-point sources including soil (24% of total NOy emissions), wildfires (19%), and lightning (13%). However, very little is known about NOy fluxes from these natural sources even though they account for 50% of all atmospheric NOy - with this percentage increasing as vehicle and industry emissions continue to decline.
Importantly, there is a critical lack of information regarding biogenic (microbiologically derived) production mechanisms, resulting in inaccurate NOy-coupled climate model projections. This can be primarily attributed to a lack of understanding regarding the formation of major NOy species, such as nitrogen dioxide (NO2) and nitrous acid (HONO). We suspect there are yet undiscovered NOy-producing pathways, catalysed by a vast array of microbes from all three domains of life. Our hypothesised mechanisms are derived from human physiology, where NOy species are known to be important signalling molecules.
We will explore these mechanisms in agronomic soils, as soils are the largest natural source of NOy gases, and within an agronomic context as our preliminary work has shown that these soils produce significantly more NOy than other terrestrial systems such as grasslands and woodlands. We will further define agronomic-NOy with field trials of a major UK commercial crop (Triticum), including four cultivars with differing above- and belowground traits. It will be crucial to define NOy soil emissions from these different cultivars, as varying plant traits, such as specific root length, can influence the soil N-cycle microbiome - which will inevitably influence NOy emissions. Other important variables will also be explored, including fertiliser application and spatial variability of NOy flux.
Importantly, we will also attempt to determine the role of soil iron and iron speciation on N-cycle community composition and NOy fluxes. Soil iron is an important aspect of our theoretical NOy mechanism - stimulating the production of reactive oxygen species, which is a key reactant in the production of NOy. We will source soil with differing iron content from various farms throughout England to be used in wheat mesocosms studies. Soil NOy fluxes will be measured and connected to mineralogical characteristics and the N-cycle community size.
Overall, this project will determine fundamental knowledge on the biogenic production mechanisms of major NOy species, provide direct soil NOy flux measurements from a major global crop, and lead to a better understanding of coupled carbon-nutrient-mineral cycling in soil. Furthermore, this work represents a major step change in the understanding of soil nitrogen dynamics, will be one of the first to couple shotgun metagenomics and culture-dependent methods to atmospheric chemistry processes, and will represent a major advancement in atmospheric accounting of NOy.
Technical Summary
Soil microbes most responsible for producing NO include chemoautotrophic prokaryotes and heterotrophic bacteria and fungi; however, definitive microbial mechanistic evidence does not exist for soil nitrogen dioxide (NO2) and is very sparse for nitrous acid (HONO), even though studies have shown that NO2 and HONO are both directly emitted from soil with a large proportion being attributed biological processes. A possible mechanism exists that relies on heterotrophic activity, leading to the production of extracellular reactive oxygen species, specifically superoxide. Kinetics studies show superoxide can react with NO, forming peroxynitrite, which then undergoes rapid decomposition in the presence of CO2 to form volatile NO2. NO2 can either be emitted from soil to air or be converted to nitrite via Fe(II). Under acidic conditions, nitrite is protonated to HONO. Thus, we aim to prove that the production of NO2 and HONO is dependent on both nitrogen cycling and superoxide producing microbes.
This project will be undertaken within an agronomic context because of the importance of agricultural soils in terrestrial NOy production. We will perform field experiments to differentiate NOy flux from different cultivars of wheat - with fluxes being measured from the rhizosphere and bulk soil of each crop. Previous work has shown that root density can alter the abundance of N-cycle microbes and nitrogen cycle rates, thus we have selected four cultivars that have differing specific root length. We will also include an in situ experimental component where differing levels of fertiliser will be added to the three crops, creating a fertiliser gradient. The question here is whether the alleviation of nitrogen will enhance NOy production in the rhizosphere though increases in dissimilatory N-cycle taxa - possibly creating an NOy hotspot.
This project will be undertaken within an agronomic context because of the importance of agricultural soils in terrestrial NOy production. We will perform field experiments to differentiate NOy flux from different cultivars of wheat - with fluxes being measured from the rhizosphere and bulk soil of each crop. Previous work has shown that root density can alter the abundance of N-cycle microbes and nitrogen cycle rates, thus we have selected four cultivars that have differing specific root length. We will also include an in situ experimental component where differing levels of fertiliser will be added to the three crops, creating a fertiliser gradient. The question here is whether the alleviation of nitrogen will enhance NOy production in the rhizosphere though increases in dissimilatory N-cycle taxa - possibly creating an NOy hotspot.
