Soil health and carbon capture in multi-species swards

Multi-species swards can play a central role in improving soil health and increasing carbon capture in grassland systems. 

By combining grasses, legumes and deep-rooting herbs, these swards increase above‑ and below-ground biomass, stimulate soil biology and enhance long-term soil organic matter. 

Well-managed multi-species swards help shift carbon from the atmosphere into plant material and soil while also improving water retention, nutrient cycling and system resilience – key advantages as weather patterns become more variable. 

Plants capture carbon dioxide (CO₂) from the atmosphere through photosynthesis, converting it into carbohydrates that fuel growth. 

• Carbon is stored in leaves, stems and roots 
• A proportion of this carbon is transferred into the soil during plant growth and decay 
• Higher and more consistent biomass production means more carbon capture overall 

Multi-species swards tend to maintain growth for longer across the season, increasing the total amount of carbon captured each year compared with less diverse swards. 

One of the biggest advantages of multi-species swards is the diversity and depth of their root systems. 

Many herbs and legumes develop deep roots, often extending well below the topsoil; this has various benefits: 

• Carbon deposited deeper in the soil is less prone to loss 
• Subsoil carbon is more stable than surface organic matter 
• Improves soil structure and resilience during drought 

These traits help build longer-term soil carbon stocks compared with shallow rooted grass‑only swards. 

As roots grow and die back, they leave organic material in the soil; this improves physical properties, such as structure, porosity and water retention, supports microbial activity and enhances the soil’s long-term health. 

Compared to swards with less diversity, multi-species swards produce more root exudates (carbohydrates), which drives the carbon content of soil.  

Over time, this continual input supports the build-up of soil organic matter. 

Legumes such as white clover, red clover, lucerne and sainfoin fix atmospheric nitrogen through bacteria in root nodules, which: 

• Reduces reliance on artificial nitrogen fertilisers 
• Supports companion grasses through shared and recycled nitrogen 
• Feeds soil microbial communities 

In balanced grass–clover swards containing 20–30% clover, nitrogen fixation can supply 100–200 kg N/ha per year, depending on season and conditions. 

• Soil pH ≥ 6.5 
• Adequate phosphorus and potassium 
• Well-drained, aerated soils 
• Healthy, pink nodules indicating active fixation 

Excess artificial nitrogen suppresses fixation, so legumes work best in low‑input systems. 

Multi-species swards reduce emissions in several ways. 

Methane: Some species (e.g. bird’s-foot trefoil, sainfoin, chicory) contain tannins that reduce methane production during digestion and improve protein use in in the rumen. 

Nitrous oxide: Legumes such as clover and lucerne help lower emissions by fixing atmospheric nitrogen, reducing the need for synthetic inputs. 

Carbon dioxide: By integrating nitrogen-fixing legumes, producers can significantly reduce reliance on artificial fertilisers, lowering their carbon footprint. 

Multi-species swards can attract a broad range of wildlife due to the variety of plants and the longer rest periods between grazing. 

Benefits that underpin both productivity and carbon storage include: 

• Increased soil microbial biomass 
• Improved nutrient cycling across soil types 
• Greater resilience to drought, flooding and pest pressure 

Permanent pasture makes up about 70% of the UK’s grassland area and plays a vital role in UK carbon storage. It stores approximately 2.4 billion tonnes of carbon – more than any other land-use type. 

When managed with added diversity (e.g. legumes), it also: 

• Improves microbial activity 
• Enhances soil structure 
• Supports continued carbon capture over decades 

Most commonly sown herbs and some legumes continue to grow during dry seasons due to their deeper root systems. 

However, some deep-rooted species may struggle in poorly drained or waterlogged soils, especially if grazed hard.