Soil Carbon, Farm Woodlands, and the Woodland Carbon Code

Summary Description:

Much afforestation in recent years has taken place on agricultural land. Whilst ostensibly this is good news for climate change mitigation, the carbon balance of these new woodlands is poorly understood. Of particular importance is the need to understand how carbon stocks below ground have been affected by afforestation.

The aim of this project is to investigate the effect of tree planting on farmland on the quantity, depth and type of below ground carbon. This will be assessed through excavation of root systems and sampling of soil carbon stocks and ‘fractionation’ to determine likely residence times for carbon in the soil. The implications for the ‘Woodland Carbon Code’ will be investigated following a life cycle analysis methodology.

Using a loader to weigh branches from a felled hybrid poplar for quantifying carbon content. Silsoe silvoarable agroforestry trial (June 2011) - Matthew Upson.

Project Outcomes:

This PhD project was focused on the the storage of carbon in biomass and the soil in recently afforested agricultural environments.

In the first year (2011), the focus was on a poplar silvoarable system at the Cranfield Experimental Farm at Silsoe, and included the quantification of above and below ground biomass and the soil organic carbon (SOC) change resulting from tree planting in 1992. From May 2012 onward, the focus of the work shifted to ash woodlands planted on agricultural land in Bedfordshire. Soil carbon storage was investigated in detail at one site: Clapham Park, planted in 1998. Biomass measurements and destructive harvests were completed at an additional seven woodlands across Bedfordshire.

Measurements of soil organic carbon found that on average 60% and 40% of the soil carbon (relative to 1:5 m) was found beneath 0:2 and 0:4 m in depth respectively. Whilst tree planting in the arable system showed gains in soil organic carbon (12:4t C haô€€€1 at 0{40 cm), tree planting in the pasture at Clapham Park was associated with losses of soil organic carbon (6:1{13:4 t C haô€€€1 at 0{10 cm). Evidence from a nearby mature grazed woodland indicate that these losses may recover over a long time period (centuries). No differences in SOC associated with tree planting were found to the full 1:5 m.

Measurements of the 19 year old poplar at the Cranfield Experimental Farm at Silsoe suggest that Forestry Commission yield tables for widely spaced poplar do not provide a good fit for trees grown in an agroforestry situation. The use of these yield tables for bio-economic models is likely to have caused over-estimations of the yield of agroforestry systems as a result.

A comparison of biomass carbon and soil carbon measurements indicates that losses in soil carbon at relatively shallow depths can offset a large proportion of the carbon stored in tree biomass, but assessing changes on a site by site basis may be prohibitively expensive for schemes such as the Woodland Carbon Code. The combination of very small tree-planting-related SOC changes, with very large variation over relatively small areas, means that a large amount of expensive and labour intensive replication would be required.