Understanding the role of microbial necromass in soil organic matter
The Microbiology Society is undertaking a project entitled A Sustainable Future as part of our 75th Anniversary, which aims to highlight the Sustainable Development Goals (SDGs) to our members and empower them to use their research to evidence and impact the goals. Earlier this year, we put a call out to our members to submit case studies in the following three areas: antimicrobial resistance, soil health and the circular economy.
This case study was written by Dr Kate Buckeridge, a Postdoctoral Research Associate at the University of Edinburgh and by Dr Jeanette Whitaker, a Principle Scientist at the UK Centre for Ecology & Hydrology. It focuses on Soil Health; maintaining the health of our soils has gained increasing prominence in recent years. Soils are essential for the global food system and regulate water, carbon and nitrogen cycles but are put under pressure from population growth and climate change.
What are the challenges/needs that this research/initiative addresses?
Protecting and increasing terrestrial carbon storage and controlling nitrogen pollution, while meeting food and climate change targets, are global challenges. Soil stores more carbon than the atmosphere and vegetation combined1 and contains a small but reactive component of the global nitrogen pool2. As a result, achieving small increases in soil carbon storage and more efficient management of soil nitrogen could have consequences for global biogeochemical cycles underpinning food security, pollution and climate change SDGs.
How can we increase soil carbon and more efficiently manage soil nitrogen?
Our understanding of how carbon is stored in soil has traditionally focused on the quantity and quality of plant carbon inputs to soil. However, growing evidence over the last decade has revealed that most of the carbon stored in soil is in the form of microbial products and residues, or ‘necromass’3. This means that soil microbial physiology – specifically, biomass growth and necromass production – is key to controlling soil carbon storage. Soil nitrogen management is struggling to move away from fertiliser towards organic amendments and microbial necromass is nitrogen-rich. Therefore, filling the gaps in our understanding of how microbial necromass is produced, recycled, and protected in soil is an immediate challenge which could help us predict how much soil carbon we can store and the consequences for nitrogen management.
What findings and solutions were provided by this research/initiative?
In the U-GRASS project, part of the Natural Environment Research Council (NERC) funded Soil Security Programme, we used laboratory and field experiments to understand the controls on microbial necromass recycling, stabilisation and persistence in soil. Our experiments used a model necromass – lab-grown Escherichia coli – that we labelled with stable isotopes to trace its fate into live microbial biomass (i.e. necromass recycling), to the atmosphere as CO2 (i.e. microbial respiration), or to a more stable position on soil mineral surfaces (i.e. stabilisation). We then measured the persistence of this mineral-stabilised necromass over time, in both the lab and the field, to quantify necromass carbon sequestration potential. We carried out these soil microbial experiments on farms with different management intensities, different soil properties, and across a broad UK precipitation gradient to understand the impacts of land management and climate change.
We showed that historical precipitation was the main control on how necromass was recycled in soil4, suggesting that climate can be used to predict necromass recycling in soil carbon sequestration models. We also describe how the characteristics of specific microbes influence necromass recycling, which will be useful for further explorations on necromass production and recycling. We demonstrated that a common composite version of land management intensity (i.e. tillage + grazing + inputs) was not a useful predictor of necromass recycling or soil carbon storage, and that studies moving forward should focus on understanding the impacts of individual management components on soil carbon sequestration4, which will require maintaining research partnerships with the farming community.
We also demonstrate that microbial necromass stabilisation is strongly influenced by abiotic adsorption to soil minerals, and that the identity of the microorganisms that formed the necromass influences the stabilisation success5. This suggests that necromass physical properties, such as cell morphology or chemical structure, may influence necromass stabilisation and possibly soil carbon sequestration.
How can this research/initiative support the transition to a more sustainable future?
Soil carbon is the primary indicator of soil health, determining a soils climate resilience, nutrient and water cycling capacity and plant productivity. However, we are currently losing carbon from our managed soils globally due to intensive agriculture. To move to a more sustainable agricultural system we need to maintain existing soil carbon stocks and reverse historic losses. This is challenging as there is still significant uncertainty over the mechanisms of soil carbon persistence which limit our ability to predict how and where gains can be achieved. These novel experiments exploring the microbial mechanisms of necromass stabilisation are helping us to overcome those limits by understanding how climate and land management interact to controls the dynamic pools of carbon in soils, to inform future land management strategies.
What is the future for research and innovation in this area?
As we gain increased understanding of the way necromass is produced, recycled and stabilised in different soil types, under different management practices, and climate scenarios we can start to explore ways to promote necromass production and stabilisation while minimising soil CO2 and N2O emissions and nitrogen leaching from agricultural soils. This may include soil additives or promoting best management practices that enhance microbial necromass stability. This is an exciting research area receiving attention from researchers globally, which will in time inform sustainable management practices that deliver nitrogen-efficient, climate-friendly agriculture.
1. Ciais, P. et al. Carbon and other biogeochemical cycles. Clim. Chang. 2013 Phys. Sci. Basis. Contrib. Work. Gr. I to Fifth Assess. Rep. Intergov. Panel Clim. Chang. 465–570 (2013).
2. Gruber, N. & Galloway, J. N. An Earth-system perspective of the global nitrogen cycle. Nature 451, 293–296 (2008).
3. Liang, C., Amelung, W., Lehmann, J. & Kästner, M. Quantitative assessment of microbial necromass contribution to soil organic matter. Glob. Chang. Biol. 25, 3578–3590 (2019).
4. Buckeridge, K. M. et al. Environmental and microbial controls on microbial necromass recycling, an important precursor for soil carbon stabilization. Commun. Earth Environ. 1, 1–9 (2020).
5. Buckeridge, K. M. et al. Sticky dead microbes: Rapid abiotic retention of microbial necromass in soil. Soil Biol. Biochem. 149, 107929 (2020).
About the authors
Dr Kate Buckeridge is a Postdoctoral Research Associate at the University of Edinburgh and Dr Jeanette Whitaker is a Principle Scientist at the UK Centre for Ecology & Hydrology.