Using digestate to grow perennial grasses on brownfield land

Project name: Cotesbach and Roxwell trials

This two year project (2012 -2014) follows on from research carried out in Germany which has investigated the benefits of growing mixes of perennial wild flowers instead of maize as an energy crop for use in AD plants. Establishing and harvesting these plants is a lot cheaper than conventional energy crops like willow and miscanthus and are therefore suitable for growing on less permanent sites.  This project looks at the effects the application of liquid fraction digestate has on the growth of perennial plants on two brownfield sites - one on a soil bund within a sand and gravel quarry at Cotesbach, Leicestershire and the other at a capped landfill at Roxwell, Essex.

The trials compare the application of digestate to compost and inorganic fertiliser and  comprises of the following six replicated treatments (including a control where no amendments were added:

1. Control (no application) 
2. Synthetic fertiliser150kg N/ha in spring, 100kg N/ha in summer
3. Compostone spring application at 250kg N/ha (75t/ha)

4. Digestate

one spring application at 250kg N/ha (74m3/ha)

5. Digestate

one spring application at 150kg N/ha (45m3/ha), plus one summer application at 100kg N/ha(29m3/ha)

6. Compost and digestateCompost at 150kg N/ha in spring and (35t/ha) digestate at 100kg N/ha in summer (29m3/ha)

For each of the treatments, the equivalent of 250kg of N/ha was applied in 2012 and again 2013( excludes the control). 

Monitoring

At both sites dry yields are determined and plant material will be analysed for nutrients and metal up take.  Soil samples will be taken before and after application of digestate and will be analysed to determine the influence of repeated applications of organic amendments on soil quality.  

Results to date

At the end of 2012 soils from both sites were analysed for physical properties, nutrient content and potentially toxic elements (PTEs).  There were no significant differences between the plots, however the soils from plots treated with digestate or synthetic fertiliser treatment on the Cotesbach site did have higher levels of available nitrogen (ammonium and nitrate).

In 2012 the plants grew sufficiently on the Cotesbach site to warrant two harvests.  Yields were relatively low (3t/ha) compared to what would be expected on agricultural (13t/ha) land however it is still too early to make any conclusions as there was a lot of variability between the replicated plots. Higher yields are expected in 2013 and subsequent years as the crop is now established.

Harvesting on the Roxwell site is scheduled for 2013 and final results will be available from March 2014.