How stable isotope analysis of soil processes informs strategies for climate change: an interview with Prof. Liz Baggs

Understanding and quantifying the processes taking place in our soils can provide insights to guide new strategies for addressing and adapting to climate change. With stable isotope analysis, it is possible to measure tiny differences in the microbiological processes in the soil around plants (known as their rhizosphere). By comparing the differences between crop genotypes, it could be possible to select them for their ability to enhance soil carbon accrual, and regulate nitrogen availability for more sustainable production. 

Research led by Professor Liz Baggs, Professor of Food and Environmental Security at the University of Edinburgh, is providing potential responses to global challenges, particularly in tackling nitrogen emissions and enhancing soil carbon sequestration. As she told Elementar, her work “looks at how carbon from plants drives microbial processes within the soil, and how that influences whether carbon inputs into soil are stored or lost.” 

Soil carbon sequestration refers to carbon that is stored and stabilized within soil rather than being emitted. It relies on the accumulation of carbon within the soil in stable forms which are less subject to attack by microbes, reducing turnover and loss. Professor Baggs’ work aims to understand these processes to help inform ways to further lower carbon and nitrogen losses. 

The analysis of stable isotopes is critical to this work, both in terms of tracking the movement of carbon or nitrogen within the rhizosphere, and in distinguishing the processes that occur within the soil. In her work with Dr Eric Paterson and Dr Maddy Giles at the James Hutton Institute, Professor Baggs uses stable isotopes to track the movement of carbon to measure soil carbon sequestration potential, and how this interacts with nitrogen availability and losses. 

Professor Baggs explains: “We use an approach developed by Dr Paterson of growing plants in an environment that is either enriched in or depleted in ¹³C-CO₂ The plants photosynthesize that CO₂ and the altered δ¹³C signature comes through the plant into the rhizosphere. We can then trace how much of the carbon is respired, and how much is mineral associated, which allows us to calculate the potential for net accumulation. We use the respired carbon’s isotopic signature to determine whether it has come from the plant or from soil organic matter. We couple that with imaging of the location of the plant-derived ¹³C in soil, and the relationships with nitrogen processes, such as those resulting in emission of the greenhouse gas nitrous oxide”.

Understanding the influence of plant-derived carbon on soil processes in this way may help us develop more targeted sustainable crop production for climate change adaptation and mitigation. As Professor Baggs explains: “We are looking at the plant as a management tool to manipulate those processes.” 

Professor Baggs’ work uses stable isotopes to understand the microbial processes that are occurring in the soil around particular crop cultivars, which could have significant implications for climate change mitigation strategies in the global agriculture sector. As Professor Baggs explains: “We can apply ¹⁵N-enriched fertilizer to the soil, for example as ammonium nitrate. We can then calculate which microbial processes are occurring within the soil using the δ¹⁵N signature of the N₂O that is emitted.”

There could be important implications of this work for climate change mitigation. Much existing research into lowering emissions from agricultural soils focuses on optimising nitrogen application rates, because applying excess inorganic nitrogen leads to greater emissions. However, the insights of Professor Baggs’ group could offer an alternative strategy.

The production of nitrous oxide in the soil can result from the activities of several different microbial groups, each of which has different drivers. One of these processes, denitrification, not only produces N₂O, but can also reduce it to N₂. Understanding the dominant processes in a plant’s rhizosphere can therefore help to inform mitigation strategies for N₂O emissions, by management to either lower N₂O emissions, or to enhance the reduction of N₂O to N₂. Professor Baggs’ team has pioneered an approach that is based not on preventing the microbiological processes, but harnessing them.

She explains: “Denitrification has dinitrogen, rather than nitrous oxide, as its end product. Rather than trying to stop this process, we want to drive the conversion to dinitrogen, which is environmentally harmless. Carbon is often limiting for that final reduction step in denitrification, and so we are exploring the role of plant-derived carbon in driving that reduction to N₂. Other processes have nitrous oxide as the end product, and where those processes dominate, they may result in more net N₂O being released. Our ability to monitor these processes allows us to develop different approaches for mitigation.”

In collaboration with Dr Paterson and researchers at CIMMYT (the International Maize and Wheat Improvement Center), work on the influence of maize genotypes on soil carbon and nitrogen is informing genotype selections in trials in Sub-Saharan Africa. The influence of crops on release of nitrogen from soil organic matter may lower dependency on inorganic fertilizers, provided nutrients are replenished, for example through addition of crop residues, in what Professor Baggs calls a “circular nutrient economy.” Reducing dependency on inorganic fertilizers is anticipated to result in more sustainable production, especially important under a changing climate. 

The development of any new process or technique involves a degree of uncertainty, and that element of innovation is what drew Professor Baggs to her field of study and led her to pioneer the stable isotope approach for distinguishing between microbial processes related to nitrous oxide and for understanding interactions between the carbon and nitrogen cycles.

The other important motivating factor for Professor Baggs’ research is the scale of the applications it enables. “If we can get information about different genotypes and their ability to source nutrients from soil organic matter, and introduce that information into breeding selection programs, we may have a viable way to improve situations for smallholder farmers in sub-Saharan Africa. The potential impact that understanding these microscale mechanisms can have in the real world is what really interests me now.”