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Planting Biomass Crops: Assessment of Options to Reduce Soil Carbon Loss

The aim of this Defra project (NF0441) is to assess different establishment methods for Short Rotation Coppice (SRC) in old-growth pasture and to develop strategies to maximise soil carbon sequestration.

Research question

The establishment of bio-energy crop plantations of short rotation coppice (Willow) trees is intended as one component of a government strategy to mitigate net CO2 emissions in accordance with U.K. commitments to the U.N. Kyoto Protocol on Climate Change. In addition to the provision of "carbon neutral" fossil fuel replacement, bio-energy plantations may also contribute to CO2 emissions mitigation by enhancing sequestration of carbon in soil organic matter in the underlying soils. A particular concern is the effect of establishing SRC on grassland. Cultivation may stimulate initial losses of carbon offsetting the later benefits of sequestration of C into soil organic matter.

Reducing soil tillage is likely to lower the initial losses of carbon from the disturbance of the organic matter. Research funded by Defra allowed an experiment to be set up at Wellesbourne to test the effects of different cultivation treatments on the establishment and subsequent yields of short rotation coppice on a heavy soil. Conventional autumn ploughing will be compared against banded tillage methods, where only soil immediately in contact with the planting material is disturbed. The experiment will point to methods which promise to reduce initial soil disturbance and subsequent CO2 emissions

The experiment will point to methods which promise to reduce initial soil disturbance, but it will be difficult to generalise the effects on CO2 emissions Therefore, a review of algorithms will be made to asses the possibility of predicting the effects of these reduced tillage methods on overall C sequestration will also be undertaken. To support this soil samples will be collected from established grassland of different ages, including Wellesbourne, to determine the relative effects of age of grassland on losses of CO2. This will enable the break-even point – when sequestration exceeds emissions – to be estimated for a range of sites. This information will be incorporated into a decision support tool to maximise the benefits of short rotation coppice crops established after grassland.
Strategies of managing the changeover from grassland to short rotation coppice, while ensuring maximum C sequestration, will be a key output of the Defra funded project.

Experiment

An experiment was set up at Wellesbourne; on a clayey river Alluvium, Thames Series soil after 13 years setaside grass. The Willow variety Tora, one of the more vigorous and high-yielding of the currently available energy crop varieties, was planted in May 2009 after a range of different planting and cultivation methods.

Five cultivation treatments were compared ranging from autumn conventional ploughing followed by the Step planter to plots that were hand planted into undisturbed soil. Other reduced tillage treatments included direct planting with Egedal and Step planters. Two treatments compared the Egedal and Step planters after strip tillage. The observations made so far have demonstrated the importance of pest and weed control. The main cultivations and planting operations are sumarised in the pictures below

ploughed-plotp1150989.jpg step-ploughp1190484.jpg
Field in the autumn after ploughing Step planter being used on ploughed land
strip_tillp1190462.jpg strip-detail.jpg
Rotary cultivation being used to strip till planting rows for willow in May Close up of Rotary cultivator

The percentage take of cuttings in the direct-planted plots was poor and very poor in the hand-planted plots. This was due to repeated grazing of the young shoots by slugs. The growth rate of those cuttings that did establish in these plots was generally poor with many plants failing to grow away strongly. The percentage take in the ploughed and strip-tilled plots was significantly better and growth rates were also good. The strip tillage followed by Step planting was comparable to conventional ploughing. Across all treatments, the Step planter provided better results than the Egedal, possibly due to fewer gaps within the rows where no cuttings were planted (assessments were made on an area basis). The shoot counts in Table 1 show the number of cuttings with one or more visible shoots.

In all plots weed populations were a problem, see below, and without considerable inputs of time this may have led to failure which emphasises the importance of controlling weeds when establishing SRC. For commercial planting it is essential that the soil is well-structured, provides good contact with the cuttings and that the cuttings can grow with little or no competition.

bindweed.jpg buttercup.jpg common_vetch.jpg creeping_thistle.jpg 
Bindweed Butter cup Common Vetch Creeping Thistle
 dandelion.jpg  fat-hen.jpg fools_parsley.jpg  mayweed.jpg 
Dandelion Fat Hen Fools Parsley Mayweed
 groundsel.jpg  hogweed.jpg  mayweed.jpg  sowthistle.jpg
Groundsel Hogweed Mayweed Sow Thistle

It is considered feasible to direct-plant SRC at the commercial scale into grassland but ideally grassland that has been tightly grazed so there are fewer invasive, perennial weeds present. Leatherjackets and slugs must be controlled. Establishment costs will be reduced considerably if direct-planting does prove effective and as more grassland is likely to be offered for SRC production in the future this can only provide a positive message to prospective growers.

At the first coppicing in March 2010 the tallest stems were recorded from the Step planted plots following ploughing or strip-tilling. The Egedal planted plots after strip tillage were next tallest. Because of the poor establishment referred to above, the direct-planted and hand-planted treatments performed poorly. Dry matter yields reflected the variations in shoot height.

TABLE - Results of main growth assesments in 2009/10

Treatment

3 June 09

Shoot count

per m single row

6 August 09 Shoot count

per m single row

6 August 09

Mean shoot height cm

CoppicingShoot height cm

Coppicing DW yield kg/ha

Plough + Step

1.31

1.48

92.4

138

347

 

Strip-tillage + Egedal

0.58

1.23

68.3

93

105

 

Strip-tillage + Step

1.35

1.56

100.7

148

368

 

No tillage + Egedal

0.35

0.79

43.8

54

20

 

No tillage + Step

0.79

1.08

46.0

58

31

 

Hand plant

0.54

0.52

47.8

56

10

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Shoot counts also measured on 17th, 24th June, 2nd, 16th and 24th of July.