Human activity interferes with the Earth’s carbon cycle through the release of carbon dioxide and through changes in land use. Croplands cover a large proportion of Earth’s surface, but we are still understanding how cropland soils store and release carbon. Dr Jürgen Augustin and Dr Steffen Kolb from the Leibniz Centre for Agricultural Landscape Research in Germany are using an integrated methodological approach to monitoring carbon fluxes in eroded cropland soils and resolve mechanisms regulating them.
The Earth runs on a system of biogeochemical cycles, where water, carbon, energy, and various other nutrients are constantly recycled, moving between sources (releasing materials and processes) and sinks (storing materials and processes). We can monitor the dynamics of these sinks and sources around the Earth and understand how human activity impacts them. The carbon cycle is of particular interest to climate scientists, as the increasing concentration of carbon dioxide gas in the atmosphere changes the balance of carbon, causing complex feedback effects throughout the rest of the system. Along with the increase in anthropogenic carbon dioxide from fossil fuels, changes in land use also affect the natural cycling of carbon and thus foster climate warming.
Soil plays a key role in the global carbon cycle, which is why changes in land use can impact the amount of carbon stored or released back into the global cycle. Soil acts as a reservoir for organic carbon from soil organic matter, which is the residues from plants, soil microbes, and animals that can be decomposed by soil life. Soil organic matter is thus continuously formed and decomposed by soil organisms, building a seemingly stable (over decades and centuries) and large carbon storage in terrestrial ecosystems. Scientists think that soil stores as much as 2 to 3 times more than the atmosphere and biosphere combined. This means that any significant changes to the soil environment will change its ability to store carbon, potentially releasing it into the atmosphere and reducing the level of carbon dioxide that can be captured.
Dr Jürgen Augustin and Dr Steffen Kolb from the Leibniz Centre for Agricultural Landscape Research (ZALF) in Germany are interested in the impact of erosion on soil carbon dynamics. Erosion particularly affects croplands (land designated for the cultivation of crops), and since the security of our food production hangs in the balance as our climate continues to warm, understanding the relationships between the effects of erosion and carbon in soil is vital for modelling changes to the carbon cycle and maintaining adequate plant productivity.
What is soil erosion?
From a geological perspective, the process of erosion removes rock and soil material, either by the action of wind or water. For cropland soils, the removal of material via water dominates, along with tillage erosion. This is movement of the soil via the action of tillage, such as ploughing and turning of the soil to prepare it for crops. Erosion of soil mixes up the distinct layering, moving subsoil (soil underneath the uppermost layer) into the topsoil. The topsoil is nutrient and carbon rich, and on slopes it will slowly move into depressions or can be completely washed away by the action of water.
The mixing of subsoil into topsoil changes a range of environmental conditions within the soil, including the pH, oxygen, and plant nutrient levels. It also affects the type and amount of microbes living in the soil, as different bacteria thrive in different conditions. These microbes utilise carbon and other nutrients from different areas within their environment, either drawing it from the organic matter already within the soil or taking it from plant deposits. This interaction of crops, soil and microbes is a complex system and therefore challenging to study.
Studying carbon flux in soils
A key focus for Dr Augustin and Dr Kolb is to resolve the ongoing discussions around what impact erosion in croplands has on the carbon cycle, which in turn affects the anthropogenic carbon dioxide contribution towards climate change. As erosion alters soil carbon storage, it is important to understand the relationship clearly and be able to quantify it for accurate use in carbon cycle modelling. Some research has indicated that erosion could enhance the role of soil as a source of carbon dioxide, whereas other studies have noted erosion can act to enhance the ability of soil to sequester carbon dioxide.
This is the question that Dr Augustin and Dr Kolb hope to answer with their work by developing integrated methods of measuring the carbon fluxes into and out of soils. They have already identified evidence that suggests more highly eroded soils extract more carbon dioxide via crop plants from the atmosphere to build new soil organic matter, as the subsoil that has been moved to the surface contains less carbon for use by microbes on balance. If this is truly the effect of erosion on cropland soils, it may help us to identify practical methods for increasing their role as a carbon dioxide sink, aiding in our approach to tackling climate change. With their project, called CropRhizoSOM, the team hopes to confirm whether this is indeed the case.
An integrated methodological approach
Dr Augustin, Dr Kolb and their colleagues have developed an integrated approach to study the movement of carbon input and output of cropland soils. Their process includes studying the crop rhizosphere (the area of soil around the roots of a plant), the microbiome (the microorganisms living in the rhizosphere), and the soil organic matter to understand how carbon moves between them. In simple terms, crops add organic carbon into the soils through their decomposition, which is driven by the microbiome within the soil. This means that the soil microbiome is acting as a regulator for carbon within the soil, releasing it from the plant material, but also removing it from the soil and releasing it into the atmosphere, depending on the type and amount of microbes.
The team use a variety of different methods to build the overall picture of the exchange of carbon in cropland soils. Until now, methods for measuring carbon flux have been limited by either the spatial variations within field locations (soils varying over small areas), or by temporal variations (not sensitive enough to track seasonal or sub-seasonal changes).
One key aspect of their approach is to track the movement of carbon through the plant-soil-atmosphere system by isotopic labelling. This is done by tracking a particular isotope of carbon. An isotope is a form of an element that has a slightly different number of neutrons in its nucleus. The element carbon, in its most common form, has six protons and six neutrons in its nucleus, giving it an atomic mass of 12. However, there is a radioactive isotope of carbon that contains six protons and eight neutrons, giving it a slightly higher atomic mass of 14. Scientists refer to this as 14C or Carbon-14, and we can use instruments to detect its quantity within gas samples.
By using 14C labelling, the team were able to track the passage of this isotope from the plant into the subsurface and quantify how much was moved during different periods of plant growth. They also set up a series of experiments that studied the effect of soil erosion on the distribution of carbon. The team manipulated the surface soil within a field to simulate the effects of strong erosion, and then used pot experiments to cultivate plants from both the eroded and non-eroded sites. They discovered that erosion creates an immediate carbon sink effect in soils, confirming that cropland erosion could provide longer-term soil carbon sequestration.
Tackling spatial and temporal variation
Part of Dr Augustin and Dr Kolb’s integrated approach is to use an automated method at field sites to take measurements of carbon dioxide fluxes in different ecosystems. They hope to overcome the spatial and temporal variations that limit other methods to quantify the changes in soil organic carbon over small-scale areas and short time periods. Dr Augustin, together with further scientists, has developed a robotic chamber system that measures the carbon dioxide fluxes over a plot area, which they used to measure carbon stock changes in soils with varying levels of erosion.
They discovered that their robotic system could measure changes in carbon dioxide flux between night and day. It was also able to see other pattens influenced by external factors such as the weather and different land management practices. The team successfully identified changes in plant productivity caused by the different levels of erosion in the soils, correctly hypothesising that increased erosion leads to lower plant productivity.
Overall, they were able to prove that the apparatus and methodology they have developed could quantify the changes in soil carbon stocks over short periods of time, along with the effects of erosion and land management. This information is crucial for building accurate carbon flux models of the soil-plant-atmosphere system, which feeds into the global carbon cycle and helps scientists to better understand how human activity can affect the cycle and what we can do to mitigate changes to our climate.
- Vaidya, S., Schmidt, M., Rakowski, P., et al. (2021). A novel robotic chamber system allowing to accurately and precisely determining spatio-temporal CO2 flux dynamics of heterogenous croplans. Agricultural and Forest Meterology, 296, 108206. Available at: https://doi.org/10.1016/j.agrformet.2020.108206
- Remus, R., Kaiser, M., Kleber, M., et al. (2018). Demonstration of the rapid incorporation of carbon into protective, mineral-associated organic carbon fractions in an eroded soil from the CarboZALF experimental site. Plant Soil, 430, 329-348. Available at: https://doi.org/10.1007/s11104-018-3724-4
- Remus, R. & Augustin, J. (2016). Dynamic linking of 14C partitioning with shoot growth allows a precise determination of plant-derived C input to soil. Plant Soil, 408, 493-513. Available at: https://doi.org/10.1007/s11104-016-3006-y
Drs Kolb and Augustin study the effect of topsoil manipulation on carbon gas fluxes.
German Research Foundation (DFG)
Prof Dr Michael Sommer, Leibniz Centre for Agricultural Landscape Research (soil erosion research, soil manipulation experiments)
Prof Dr Jürgen Augustin gained a Diploma in Agricultural Sciences (University of Halle, 1979) and a doctorate (Dr. agr.) in Physiology and Nutrition of Plants (University of Halle, 1985). Since 1992 he has been working for the Leibniz Centre for Centre for Agricultural Landscape Research (ZALF) in Müncheberg, Germany. Dr Augustin also holds an Honorary Professorship in Resource Management at the University of Halle. He was project coordinator of “CarboZALF – Carbon Dynamics of Arable Landscape Under Climate Change”, 2008-2019, and is currently Head of the Working Group “Isotope Biogeochemistry & Gas Fluxes”.
Prof Dr Steffen Kolb (PhD) gained a Diploma in Biology (University of Göttingen and Max Planck Institute in Bremen, 2000), a doctorate in environmental Microbiology (University of Marburg and Max Planck Institute in Marburg, 2004) and Habilitation in Microbiology from the University of Bayreuth. Currently, he is Professor at the Faculty of Life Sciences, Humboldt University Berlin, as well as Head of the Working Group “Microbial Biogeochemistry”, Research Area “Landscape Functioning” at ZALF.
ZALF – Leibniz Centre for Agricultural Landscape Research e.V.
Eberswalder Str. 84, 15374 Muencheberg, Germany
Dr Jürgen Augustin
T: +49 (0)33432 82 376
Dr Steffen Kolb
T: +49 (0)33432 82 326