Managing grassland diversity to enhance soil carbon sequestration

Lead Research Organisation: Lancaster University
Department Name: Lancaster Environment Centre


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.

Technical Summary

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.
Description The main discoveries of our research are:

1. Increasing plant diversity in agriculturally improved grasslands enhances soil carbon (C) sequestration, but these responses are related to the presence and biomass of certain plant species, notably N fixers and forbs. This was investigated in soils of different fertility the effects of plant species and functional group richness and composition on carbon (C) and nitrogen (N) stocks in vegetation, soil and soil microbes and on CO2 exchange and the loss of C and N from soil through leaching. We established plant communities from a pool of six mesotrophic grassland species belonging to one of three functional groups (C3 grasses, forbs and legumes) in two soils of contrasting fertility. We varied species richness using one, two, three or six species and one, two or three functional groups. After 2 years, vegetation C and N and soil microbial biomass were greater in the more fertile soil and increased significantly with greater numbers of plant species and functional group richness. The positive effect of plant diversity on vegetation C and N coincided with reduced loss of water and N through leaching, which was especially governed by forbs, and increased rates of net ecosystem CO2 exchange. Soil C and N pools were not affected by the number of plant species or functional group richness per se after 2 years, but were enhanced by the presence and biomass of the legumes Lotus corniculatus and Trifolium repens. Collectively, these findings, which are published in Journal of Ecology (De Deyn et al. 2009), show that changes in plant species and functional group richness influence the storage and loss of both C and N in model grassland communities, but that these responses are related to the presence and biomass of certain plant species, notably N fixers and forbs. Our results therefore suggest that the co-occurrence of species from specific functional groups is crucial for the maintenance of multifunctionality with respect to C and N storage in grasslands.
2. Positive effects of plant diversity on C sequestration are related primarily to increased rates of assimilation and allocation of C belowground to roots, mycorrhizal fungi, free-ling soil microbes, and soil organic matter pools in more diverse plant communities. This was tested in two experiments: one based on the mesocosm study reported above (De Deyn et al. 2010) and the other based on a long term field experiment at Colt Park, where different restoration treatments have been on-going since 1989 (De Deyn et al. 2011).

In the first study (De Deyn et al. 2010), we tested how plant species richness, identity and biomass influence the abundances of arbuscular mycorrhizal fungi (AMF), saprophytic bacteria and fungi, and actinomycetes, in model plant communities in soil of low and high fertility using phospholipid fatty acid analysis. Abundances of saprophytic fungi and bacteria were driven by larger plant biomass in high diversity treatments. In contrast, increased AMF abundance with larger plant species richness was not explained by plant biomass, but responded to plant species identity and was stimulated by Anthoxantum odoratum. Our results indicate that the abundance of saprophytic soil microbes is influenced more by resource quantity, as driven by plant production, while AMF respond more strongly to resource composition, driven by variation in plant species richness and identity. This suggests that AMF abundance in soil is more sensitive to changes in plant species diversity per se and plant species composition than are abundances of saprophytic microbes.

In the second study (De Deyn et al. 2011), we tested whether changes in grassland vegetation composition resulting from management for plant diversity influences short-term rates of C assimilation and transfer from plants to soil microbes. To do this, we used an in situ 13C-CO2 pulse-labelling approach to measure differential C uptake among different plant species and the transfer of the plant-derived 13C to key groups of soil microbiota across selected treatments of a long-term plant diversity grassland restoration experiment. Results showed that plant taxa differed markedly in the rate of 13C assimilation and concentration: uptake was greatest and 13C concentration declined fastest in Ranunculus repens, and assimilation was least and 13C signature remained longest in mosses. Incorporation of recent plant-derived 13C was maximal in all microbial phosopholipid fatty acid (PLFA) markers at 24 h after labelling. The greatest incorporation of 13C was in the PLFA 16:1?5, a marker for arbuscular mycorrhizal fungi (AMF), while after 1 week most 13C was retained in the PLFA18:2?6,9 which is indicative of assimilation of plant-derived 13C by saprophytic fungi. Our results of 13C assimilation and transfer within plant species and soil microbes were consistent across management treatments. Overall, our findings suggest that plant diversity restoration management may not directly affect the C assimilation or retention of C by individual plant taxa or groups of soil microbes, it can impact on the fate of recent C by changing their relative abundances in the plant-soil system. Moreover, across all treatments we found that plant-derived C is rapidly transferred specifically to AMF and decomposer fungi, indicating their consistent key role in the cycling of recent plant derived C.

3. The presence of legumes within grassland plant communities plays a key role in this enhancement of soil C sequestration. To test whether management for biodiversity restoration has additional benefits for soil C sequestration, we investigated C and nitrogen (N) accumulation rates in soil and C and N pools in vegetation in a long-term field experiment (16 years) in which fertilizer application and plant seeding were manipulated. In addition, the abundance of the legume Trifolium pratense was manipulated for the last 2 years. To unravel the mechanisms underlying changes in soil C and N pools, we also tested for effects of diversity restoration management on soil structure, ecosystem respiration and soil enzyme activities. We found that the long-term biodiversity restoration practices increased soil C and N storage especially when these treatments were combined with the recent promotion of the legume Trifolium pratense, sequestering 317 g C and 35 g N m-2 year-1 in the most successful management treatment. These high rates of C and N accumulation were associated with reduced ecosystem respiration, increased soil organic matter content and improved soil structure. Cessation of fertilizer use, however, reduced the amount of C and N contained in vegetation. These findings, which are published in Journal of Applied Ecology (De Deyn et al. 2010), show that long-term diversity restoration practices can yield significant benefits for soil C storage when they are combined with increased abundance of a single, sub-ordinate legume species. Moreover, we show that these management practices deliver additional ecosystem benefits such as N storage in soil and improved soil structure.
Exploitation Route Our results formed the basis of a follow grant from BBSRC (BB1009000/1) and a Defra funded project exploring the potential to use red clover to enhance carbon stocks in soil.
Sectors Agriculture, Food and Drink,Environment

Description Trials have been set up to test how effective the addition of red clover is for enhancing C stocks in soil across a range of grasslands, funded by Defra. This work will feed into management recommendations if successful.
First Year Of Impact 2012
Sector Agriculture, Food and Drink,Environment
Impact Types Policy & public services