Paper sharing seminar, 11.10.2017,Tharangika Ranatunga

Soil nitrate reducing processes- drivers, mechanisms for spatial variation, andsignificance for nitrous oxide production, Giles et al (2012). Frontiers inMicrobiology. 3, 407
Introduction
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Agricultural soils - 60% of global N2O emissions primarily through microbial driven soil processes.
eg. Denitrification, DNRA and even nitrification
Denitrification reduce NO3- through various intermediate steps to N2 .
DNRA reduce NO3- to NH4+
In both processes, N2O is a by-product.
Why it is critical to understand the effects of complex soil environment on both nitrate reducing
communities and soil’s abiotic conditions? Due to strong gradients in abiotic factors over shortdistances
Factors vary both spatially and temporally in soil.
Great understanding of dynamics of denitrification and DNRA will allow the development of
more effective mitigation startegies focused on areas of important N2O sources.
Denitrification is controlled by several factors: O2 and water content, nitrate, carbon, pH,temperature and is catalyzed by enzymes: narG, nirS/nirK, norB,nosZ respectively.
Studies focus on individual factors that can alter nitrate reduction in the rhizosphere are neededin order to fully understand the effects of roots on nitrate reducing communities and rates ofthis process.

Spatial variability in soil
 Abiotic conditions will play an important role in determining the spatial arrangement ofdenitrification.
 Especially true with rhizosphere root growth and inputs can change large changes in soil
conditions over a small distance.
 Unless consideration is made for spatial variability when a sample strategy is designed, there is
an implicit assumption : Any system is homogeneous in regard to microorganism and resource
distribution/ any variation is averaged due to the relatively large sample size taken.
 These may not be useful assumptions. Why? Soluble compounds move through soil by mass
flow, hydrodynamic dispersion, diffusion.
 In such a system with no flow, spread of a soluble resource, for eg. organic carbon from a point
source, , would be equal in all directions, with the concentration of resource inversely
propotional to the distance from the source.
 Concentartion gradients would spread by diffusion until evenly distributed throughout thesystem.
 In such equilibriums, denitrifying microorganisms expected to be randomly distributed with noregard to species and with activity dependant on a local, but evenly distributed resource.
 Drivers of soil abiotic heterogeneity - water content and soil structure as they influencetransport and location of chemical comounds.
 Differences in soil particle shape, size – no two flow paths are identical In unsaturated bulk soil, water is held as afilm around soil particles by capillary action leaving air
spaces in many pores.
 Pore retain water will also deped on soil particle size, geometry and connectivity.

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Dynamic nature of soil nitrate reduction – at any point, any time, locally dependant on therelevant factor that is limiting, either directly or through long-term control of nitrate reducer
abundance in that microhabitat.
Roots introduce an extra level of spatial complexity.
Within rhizosphere, resources originate from the plant roots, but the flow of waters is also
driven toward the root by plant uptake.nitrate reduction is likely to be high in the rhizosphere,
where C source is high and O2 is lower because of root respiration (related studies are with andwithout plants, shown that denitrification is higher with planted systems). Studies have foundroot exudates to stimulate denitrification.
Studies of individual factors that can alter nitrate reduction in the rhizosphere needed to fully
undertand the effects of roots on nitrate reducing communities and rates of this process.

Functional gene studies

Due to limitations of sampling size, averaging the contribution of community members on alarge range of habitats.
To link community structure and function, samplings need at relevant spatial scale.
Studies need to focus spatial and temporal scale relevant to organisms concerned.