Keeping the grasslands growing

Grasslands are among the most extensive and heavily exploited environments in the world. The work of Professor Mark Ritchie of Syracuse University, New York, USA, explores how plants, animals and microbes interact within these complex ecosystems, and how they can be managed sustainably for local stakeholders and to mitigate global climate change.

Grasslands are thought to cover up to 40% of the earth’s terrestrial surface and contribute to the livelihoods of some 800 million people — over ten percent of the world’s population. They are also key carbon sinks (able to absorb carbon dioxide from the atmosphere), which could be harnessed to help mitigate climate change. Indiscriminate grazing of livestock on grasslands may degrade supplies of soil carbon and nitrogen, causing spiralling losses of biodiversity and productivity. Prof Ritchie’s research seeks to prevent this by exploring the network of interactions between plants, animals, humans and other organisms in grassland ecosystems, and by finding ways in which they can best work together.

Grasslands — including steppe, savanna and pampas — may look like relatively simple communities but their appearance belies the complexity beneath the soil surface. Many occur on poor soils, low in essential nutrients, but support higher-than-expected rates of productivity. Prof Ritchie’s latest project focuses on how carbon and nitrogen from the atmosphere are fixed into the soil by plants and microorganisms, contributing to grasslands’ extraordinary plant growth and more fertile soils.

Focusing on the Serengeti National Park in Tanzania, East Africa — and building on a long history of ecological research there — his three-year, National Science Foundation-funded project aims to elucidate how wild and domesticated grazing animals, rainfall, and the chemical composition of grassland soils all interact to shape the key processes of nitrogen fixation and carbon storage.

Nutrients from the air
Nitrogen is a key component of amino acids, the building blocks of life. This relatively inert molecule enters the web of life when it is ‘fixed’ by bacteria, living freely in the soil or associated with plant roots, into usable, soluble compounds such as ammonia. In many ecosystems, plant productivity is effectively limited by nitrogen availability and the activity of these nitrogen ‘fixers’.

Nitrogen-fixing bacteria have long been linked with certain groups of plants, particularly legumes (Fabaceae, the pea and bean family), with which they form a mutually-beneficial (‘symbiotic’) relationship. However, Prof Ritchie’s recent research has shown that nitrogen-fixing bacteria may also form symbiotic relationships with grass roots in the Serengeti.

By measuring the activity of bacteria that possess the particular gene that facilitates nitrogen fixation, Prof Ritchie showed that nitrogen fixation by the dominant grass species of the Serengeti, Themeda triandra, was often similar or even greater than that of the dominant legume species, Indigofera volkensii. He concluded that grasses host ‘abundant, active bacteria that can fix large amounts of nitrogen’ — potentially far more significant than previously recognised.

A complex system
Prof Ritchie believes that grassland soils harbour a complex community of plants, fungi, multiple types of nitrogen-fixing bacteria and other soil microbes, that in turn are modified by grazing, rainfall and the availability of other soil nutrients. His latest project aims, for the first time, to tease out the interactions between all these factors, in order to design management systems that maximise the sustainability and productivity of fragile grassland ecosystems. The project will achieve this, firstly, by gathering more detailed information on the rate of nitrogen fixation by bacteria associated with multiple grass species in large scale field plots covering a range of natural conditions.

Crucially, grass-associated nitrogen-fixing bacteria may be affected differently by environmental factors than legume-associated and free-living bacteria, which may have important implications for the sustainable management of grassland ecosystems. Therefore, the second aim of the project is to test experimentally how grass- and legume-associated bacteria respond to grazing, water, and soil chemical composition.

Grasslands may look like relatively simple communities but their appearance belies the complexity beneath the soil surface.

The team will also conduct experiments to test for competition between bacteria and widespread plant root-associated fungi known as ‘mycorrhiza’. Prof Ritchie believes it is possible that these otherwise beneficial fungi compete with nitrogen-fixing bacteria for nutrients supplied by their host plants and thus impact negatively on bacterial nitrogen-fixation.

Based on these empirical data, the team will then develop a computer model to integrate the effects of rainfall, grazing, competition with mycorrhizal fungi, and nutrient limitation on nitrogen-fixation, carbon storage and grass productivity. The model will show how nitrogen-fixing bacteria and fungi can affect the role of grasslands as carbon sinks, and how this could be manipulated by changes in grazing regime and vegetation management.

Managing for sustainability
Preliminary results from the study suggest that plant growth in the Serengeti grasslands is limited by both water and nutrients, including nitrogen. Prof Ritchie proposes that the bacteria associated with grass roots may be the most important source of nitrogen in many tropical and subtropical grassland ecosystems, and therefore may be crucial to productivity, ecosystem services, and the human livelihoods that depend on these ecosystems. He also suggests that the fixation associated with grass species is less susceptible to interference by grazing mammals than that associated with legume species.

However, the impact of grazing herbivores is complex, including both negative (destruction of plant material) and positive (fertilisation via dung) impacts on grassland productivity, soil carbon storage and sustainability. Results thus far extrapolated from sites in the USA and Europe suggest that maintaining a diverse community of grazing species may enhance plant diversity, depending on their natural impact. Systems of migrating domestic herbivores, mimicking the natural grazers of the Serengeti, also promote the health of grasslands in terms of plant diversity, soil carbon storage and soil fertility.

Prof Ritchie’s overarching aim is to translate his empirical research and theoretical models into real practices for protecting and enhancing grassland communities, supporting the human livelihoods that depend upon them, and mitigating climate change by restoring carbon sinks. He is already working with over 40 local human communities in Tanzania and Kenya, supported by The Nature Conservancy and other non-governmental organisations, to introduce new livestock and fire management practices. This will not only improve sustainable livestock production, but will also help retain soil carbon and potentially generate income for communities from the sale of ‘carbon credits’. Working with the natural characteristics of grassland ecosystems in this way can be of benefit to plants, animals and humans, and Prof Ritchie’s work is helping to uncover those characteristics.