Managing grassland diversity to enhance soil carbon sequestration

Humans have long recognised the importance of soil organic matter, or humus, for farming because it is crucial for the maintenance of soil fertility and crop production. In recent years, however, scientists and politicians have also started to get interested in this material, and especially the carbon contained within it. This is because much of the Earth's carbon is stored within soil and the loss of this carbon to the atmosphere, as carbon dioxide, would greatly exacerbate climate change. Scientists are also recognising that by changing the way that agricultural soils are managed, it might be possible to increase the amount of carbon that is stored in soil, which could mitigate human-induced increases in atmospheric carbon dioxide. This is called soil carbon sequestration and it has been estimated that through judicious management, the world's agricultural and degraded soils have the potential to sequester 5-to-15 per cent of the global fossil-fuel emissions. The potential for agricultural soils to sequester carbon depends on how changes in farming management influence the balance between carbon loss from soil through decomposition by soil microbes and carbon gain from plant growth. One potential way to manipulate this balance is by increasing the diversity of the plant community. This could potentially increase the amount of carbon that enters soil from plant material, and may also affect the activity of the soil microbes that breakdown this material. Using plant diversity to do this is very appealing, since there are many conservation benefits of increasing biological diversity. Also, the enhancement of plant diversity is a major objective of current environmental policy, especially in grassland. This is what this project is concerned with: testing, in agricultural grassland, how changes in plant diversity of agricultural grassland affects soil carbon sequestration through altering the amount of carbon that enters soil from plants and the rate that it is broken down by soil microbes. We also aim to test how this balance is affected by fertilizer application, which could potentially alter the balance between soil carbon storage and its breakdown by soil microbes. Overall, our studies will help inform land managers and farmers of the optimum management for securing, and increasing, carbon stores in agricultural soils. Joint with BB/D523419/1.

Understanding the factors that regulate soil carbon (C) storage, or sequestration, has long been a major research theme in agriculture. This interest has been driven largely by a historical recognition of the importance of soil organic matter accumulation for the maintenance of soil fertility and crop production, which is at the core of sustainable agricultural management. In recent years, however, soil C storage has risen high on the political and scientific agenda due to growing interest in the extent to which agricultural soils can sequester C, which could help to mitigate human-induced increases in atmospheric CO2. Indeed, it has been estimated that through judicious management, the world's agricultural and degraded soils have the potential to sequester 5-to-15 per cent of the global fossil-fuel emissions. Managing the world's agricultural soils to increase the soil C pool is clearly a win-win strategy: not only does it enhance soil quality, thereby sustaining food production, but it also helps to mitigate against human-induced increases in atmospheric CO2. The capacity of agricultural soils to act as a C sink depends on how changes in management influence the balance between C loss through decomposition and C gain from primary productivity. One potential route to manipulate this balance in agricultural systems is through increasing the diversity of the plant community, which is widely recognised as a major driver of both plant productivity and the make-up and activity microbes that decompose plant material in soil. Managing for botanical diversity is also a major objective of agri-environmental policy, especially in grassland. This is what this project is concerned with: testing, in permanent agricultural grassland, how variations in plant diversity and composition influence the soil microbial community, and ultimately the incorporation and fate of plant-derived C in soil, thereby affecting soil C sequestration. We also aim to test how these plant-soil relationships are governed by soil nutrient availability, since it is widely viewed that soil C storage places an additional demand on the availability of other nutrients in soil. This will be tested using a combination of long-term field experiments and model grassland communities, coupled with the use of novel stable isotope approaches that enable quantification of the flux of plant-derived C into soil and its transfer to microbial and other soil pools of varying stability. Joint with BB/D523419/1.