SAEON Grasslands Carbon Project: A platform for facilitating local and international collaboration

By Lindokuhle Dlamini, SAEON Grasslands Node

Lindokuhle Dlamini collects soil samples for bulk density and moisture calculation

The French collaborators have spent two weeks on exchange visits in South Africa

Grasslands are the second largest biome in South Africa, with high plant diversity

The Grasslands Carbon Project is part of research conducted at SAEON’s integrated long-term ecological research sentinel site being developed at Cathedral Peak.

The primary aim of the sentinel site is to contribute to our understanding of how global change is impacting the essential services that grassland ecosystems provide to society.

Grasslands are the second largest biome in South Africa, with high plant diversity. They provide many benefits (ecosystem services) to humans, which include food provision and valuable rangelands for commercial and communal animals.

Classified as a Strategic Water Source Area*, the grasslands of the uKhahlamba Drakensberg are well-known for their contribution to South Africa’s water supply, particularly for Gauteng and KwaZulu-Natal. Grasses use less water than trees and an anomalously high proportion (about 40–60%) of rainfall, compared with other systems, is converted to streamflow, primarily through the soil, before being transported through rivers to important dams. These mountainous grasslands are also rich in wetlands.

A lesser-known service that grasslands provide is the removal of atmospheric CO₂ through plant production.

The bigger picture: Oceans and ecosystems absorb CO₂

Healthy ecosystems absorb CO₂ naturally and are responsible for reducing the rate of increase of human-generated CO₂ in the atmosphere. This service has been vital in mitigating the exponential increases in human-driven CO₂ emissions we are already seeing, and the consequential change in climate and earth systems resulting from these heat-trapping greenhouse gases.

Globally, oceans and terrestrial systems absorb about 50% of human-related CO₂ emissions (Ballantyne et al., 2015, Peylin et al., 2016). Carbon, ecosystems and climate interact through carbon cycling, influencing each other directly as well as indirectly through feedbacks.

Ecosystems can be neutral, sources or sinks of carbon over time. Changes in these interactions resulting from shifts in climate, elevated CO₂ (fertilisation), plant productivity and land-use change, have the potential to enhance (as sources of CO₂) or suppress (as sinks of CO₂) climate change by altering atmospheric CO₂ concentration.

Plants absorb carbon dioxide from the atmosphere and use this to build plant material. A proportion of carbon sequestered by plants lands up in the soil as soil organic carbon.

Reasons for studying carbon in fire climax grasslands

Internationally there is a strong focus on tree planting for carbon sequestration. But is this the right strategy for South Africa?

According to a study by Valentini et al. (2014), Africa is considered a small sink of carbon (i.e. absorbs more carbon than it releases) in an average year (-0.61 PgC.yr-1). In South Africa, over 60% of the terrestrial carbon stocks are stored in grassland and savanna ecosystems. These are systems that are designed to burn.

Notably, more than 90% of these carbon stocks are stored in the soil, mostly in the form of soil organic carbon. This is a very safe place for carbon to be. Our preliminary findings show that there are relatively high soil organic carbon stocks in the Drakensberg Mountains at the Cathedral Peak LTER master site. Here it should be noted that about two-thirds of grass productivity are below ground.

With international pressure mounting for more tree planting, given we are a water-scarce country and given our grasslands seem to be doing a good job at putting carbon into the soil, we need to understand more about regional carbon dynamics in our mesic grasslands to direct appropriate adaptation and mitigation policies.

Another important factor to consider for South Africa is that unlike many areas in the northern hemisphere where the land is already degraded, we have large tracts of land that are still intact.

While plants take in CO₂, soils respire CO₂

Soils store three times more carbon than the atmosphere. Some carbon is released through soil respiration.

Soils have small invertebrates (such as ants) and microbes (microorganisms such as bacteria, viruses, nematodes, fungi, mites and more) living, feeding on dead plants and animals, and reproducing. Like humans, microbes do not like working or leaving their rooms (colonies) when it is cold, so they reduce their feeding behaviour and day-to-day activities, which means they are less active, feeding less on dead matter (plants and animals) and hence release less carbon dioxide.

Although microbes help decompose and store carbon in the soil, they also release some carbon from the soil. When the soil is warm, microbes come out of hiding and start working, respiring CO₂.

Worldwide, there is an inverse correlation between temperature and soil organic carbon stocks, which generally increase with a decrease in mean annual temperature (Stockmann et al., 2013).

Soil carbon and climate change

Scientists and the general public are concerned about carbon released by burning fossil fuels. However, as a natural process, soil microbes release between 44 and 77 billion tons of carbon annually (Hawkes et al., 2017), which is more than all fossil fuels combined.

Climate change has people worried about the future, with targets being set to reduce human-related carbon dioxide emissions into the atmosphere. The expected increase in global temperature as a result of global warming (i.e. climate change) introduces uncertainties around carbon dynamics in ecosystems, including our carbon-rich grasslands.

Mesic grasslands in the Drakensberg are temperate, with cold winters. This, combined with high grass production in summer months, means that while large amounts of carbon are put into the soil through plant production, the low temperatures result in low microbial activity in winter, which may be the reason why we are finding such high carbon stocks in these.

What will the result of global warming be on this dynamic? Increased temperatures and longer “summer” season conditions may result in microbes staying active for longer, releasing more CO₂ from the soil pool.

By contrast, multiple lines of evidence have linked an increase in temperature to an increase in the amount of grass produced. If there is more grass production, this means more photosynthesis, which translates to more absorption of CO₂ from the atmosphere, and hence more carbon stored, which is a good thing.

Through several mechanisms, including longer growing seasons, photosynthesis across the world land ecosystems has been increasing for the past 30 years (Vicca, 2018), which adds important new information on the mechanisms influencing ecosystems carbon sink and on the key role vegetation plays in climate change mitigation.

Therefore, an increase in temperature in these mountainous grasslands could lead to two possible outcomes:

Increase in carbon sequestration: An increase in temperature could lead to nutrient enrichment (from increased microbial activity) and a prolonged growing season, leading to increases in the amount of grass produced, and thus a longer photosynthetic period (green period).
Increase in soil respiration: When we talk about microbes and respiration, we mean the amount of carbon dioxide (CO₂) released back into the air when the microorganisms in the soil break down and decompose organic or carbon-containing matter. An increase in temperature, mostly soil temperature, will lead to increased microbial activity and organic matter decomposition, and increase CO₂ releases, which will ultimately contribute to more warming.

This means that if these grasslands were a carbon sink, they might shift to be a source of carbon (releases more carbon than they absorb) and vice versa. However, at this stage we are not even sure whether these grasslands are a sink or source over time and in relation to climatic factors.

Overall, gaps in our knowledge about carbon in these grasslands make it difficult to predict their future response. Coupled with that, is the role of rainfall and fire in the sequestration of carbon in these systems.

Another problem facing our grasslands is land degradation caused by land-use change or mismanagement. Mismanagement of grasslands sometimes leads to low grass production and less soil cover. This leads to soil erosion, which further reduces carbon stored in these soils.

Contrary to this, compared with developed countries, Africa and South Africa in particular have a significant number of undisturbed, healthy ecosystems. With the United Nations declaring the next 10 years as years of ecosystem restoration, the question remains, should we not invest as much effort in protecting pristine healthy ecosystems?

Investing in appropriate measurements to answer critical questions

The SAEON Cathedral Peak LTER master site in the Drakensberg offers an ideal study platform to investigate these questions.

Historically, different catchments within the site were subjected to different treatments such as fire exclusion, “normal” burning regimes maintaining “pristine” grasslands, and catchments that were previously planted with pine that are now degraded as a result. This allows for a comparative approach providing a unique opportunity to study the effect of land-use change, altitudinal gradient and climate on carbon dynamics.

Soil respiration is one of the major sources of uncertainly in climate models, where small changes can have big impacts on model outcomes. For this reason, the SAEON Grasslands Node has invested in systems for measuring soil respiration. We now have a growing data set on soil respiration that has already captured over 18 months of near continuous soil respiration in the “pristine” grassland catchment.

This, with additional site measurements taking place, can now be used to investigate the amount of carbon released into the air, soil respiration dynamics over time and in relation to climatic variables as well as the processes that drive these dynamics.

Enhancing the site’s research potential through local and international collaboration

For several years we have been developing collaborations centred on soil carbon with the University of Burgundy in Dijon, France. The laboratory in Dijon is superbly set up for the processing of field samples for various analytical procedures for soils gas and water samples. This has provided additional expert input into the theoretical background for soil carbon studies as well as a sharing of ideas and alternative techniques.

“With such a wealth of data being collected for some time on site now, with added international collaboration, the time is ripe for students to harness this opportunity to further their studies and advance our status of knowledge regarding carbon dynamics in this system,” says Grasslands Node coordinator Susan J van Rensburg. She adds, “The international collaboration with our French colleagues also provides significant benefits to South African students with respect to exposure.”

EFTEON (Expanded Freshwater and Terrestrial Environmental Observation Network) intern Lindokuhle Dlamini’s intention for his PhD is the quantification of soil respiration (SR) and temperature sensitivity of SR in these grasslands, coupled with fine-scale soil organic carbon characterisation. In collaboration with the French researchers, Lindo wishes to understand carbon dynamics of fire climax grasslands. Lindo has been working on a carbon project as well as utilising the multidisciplinary nature of SAEON’s Grasslands-Forest-Wetlands Node since April 2018.

In addition, Rowena Harrison is also registered with the Node and working on the Grasslands Carbon Project. While Lindo’s main interest is soils, Rowena is focusing on wetlands and water flows, and carbon stocks and fluxes associated with these. Read more about Rowena’s project in the upcoming newsletter!

Methodology

Lindo is using the long-term LI-8100A Automated Soil Gas Flux System owned by SAEON (See Figure 1), which measures the amount of CO₂ released into the air from the soil and the amount stored (CO₂ fluxes). It is an eight-chamber system that measures these fluxes continuously in space and time (different areas at different times). It is the only one of its kind we know of permanently deployed in natural systems in South Africa at present.

Figure 1. Long-term LI-8100A Automated Soil Gas Flux System

An added (cheaper but non-continuous) approach is the use of soil-chamber-based manual techniques (see Figure 2), which measures relatively the same thing, but done manually. This method is now being used in three contrasting land-use catchments (degraded, fire exclusion and grasslands) to compare land-use impacts, complementing the focus on continued measurements in the grasslands site. Lindo does monthly gas sampling using this technique, after which the gas samples are sent to France for analysis.

Figure 2. Soil chambers for manual soil respiration measurements, together with the soil microstation that measures relative humidity, air temperature, soil temperature and soil moisture

Other fine-scale soil carbon characterisation will require Lindo to take soil samples, prepare these and send them to France for detailed analyses down to molecular level.

French collaborators

Fondly referred to as “the Frenchies” by the South African team, our collaborators include Olivier Mathieu, Jean Leveque, Mathieu Thevenot and Philippe Amiotte Suchet from the University of Burgundy. They have spent two weeks on exchange visits in South Africa for the last three years and have established a solid relationship with the SAEON Grasslands-Forests-Wetlands node.

The Frenchies provide a fully equipped lab for analysis whereas SAEON provides an exciting study site, which Rowena and Lindo will use for their PhD research. In turn, Lindo and Rowena will go on an exchange visit to France once a year learn how to conduct the various analyses required using state-of-the-art equipment in the Dijon lab.

The French team with two South African students. From left: Lindokuhle Dlamini (SA student), Jean Leveque, Olivier Mathieu, Mathieu Thevenot, Philippe Amiotte Suchet and Rowena Harrison (SA student)

The stage is set for great science. The fact remains, the climate is changing. Whether that will change soil microbes’ functions, soil respiration, and the overall soil organic carbon dynamics, is now in Lindo’s hands to answer.

“I am very excited and motivated to find the answers to these questions and understand in more detail the role of soil carbon in the carbon dynamics of these grasslands under a changing climate and across different land uses,” says Lindo.

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SAEON Grasslands Carbon Project: A platform for facilitating local and international collaboration

The bigger picture: Oceans and ecosystems absorb CO₂

Reasons for studying carbon in fire climax grasslands