Short rotation coppicing
Short rotation coppicing differs from simple tree husbandry, being more akin to an alternative crop grown under intensive arable production. Typically using spe-cially bred, fast-growing varieties or hybrids, often of various Salix or Populus species, SRC involves establishing plantations which are then harvested on a sus-tainable basis, to provide a long-term source of biomass material for combustion. There is often a substantial land requirement associated with SRC and routinely a 2 – 4 year lead-in period. Once established, however, a yield of between 8 – 20 dry tonnes per hectare per year can reasonably be expected, with a calorific value of around 15 000 MJ/tonne. Harvesting the crop forms a rotational cycle, as different sections of the plantation reach harvestable size, year on year. In this form of energy cropping, the trees themselves are effectively pruned, rather than felled, regrowth ensuring a continuing supply. Utilisation is by burning, usually in the form of chips or short lengths, most commonly for heating purposes in one form or another. In addition, the potential for producing electricity is becoming increasingly important.
The practicalities and limitations of generation from such a fuel source largely lie beyond the scope of the present work to examine. In general, though, ensur-ing continuity of supply and adequate production can be problematic. In addition, while much interest has been shown in the idea of using the biomass produced by a number of individual growers in a single generator, the logistics and transport costs are major obstacles to overcome. It is possible to characterise any given fuel in terms of its calorific value per unit mass, which is referred to as its energydensity (ED). Clearly, high ED confers obvious advantages in terms of storageand delivery. Wood, however, is a relatively low energy density fuel and hauling it to a centralised facility, thus, becomes costly, both in economic and environ-mental terms, especially over long distances. There is a clear advantage, then, in maximising the final yield of energy cropped trees and integrated biotechnology can assist in this regard.
The climate of the growing location, the irrigation needs of the particular trees being grown, the available nutrients in the soil and the management regime all play major deciding roles in the ultimate delivered biomass energy to land area ratio. While the climate must be simply accepted, the last three production variables can be optimised by judicious interventions.
The irrigation requirements of SRC have been the subject of much debate and consternation over the years. In this respect, some confusion has crept in between the needs of poplars and willows. While the former has a very deep tap root and in close planting can lower the water table by up to 10 times its grassland level, the latter has a much shallower root system, making no greater a demand than a normal crop like winter wheat or sugar beet (MacPherson 1995). Even so, at the equivalent of conventional arable requirements there still remains a large irrigation need and it is obvious that for locations with soils of poor water-holding capacity, this could form a major constraint, however well suited they might otherwise be for biomass production.
The potential for nutrient and humus recycling from biowaste back into the soil, via composted, digested or otherwise biologically treated material was mentioned. Without digressing into detailed examination of the general options open for the utilisation of such soil amendments, they do have water-holding applications and form another example of the natural potential for environmental biotechnologies to self-integrate.
Much of the evidence for this has come from the field, with research conducted throughout the UK highlighting the major water-holding benefits to be gained by large-scale use of biowaste compost. It has been shown that at an application rate of around 250 tonnes of composted material per hectare, the land is able to hold between 1000 and 2500 tonnes of rainwater (Butterworth 1999). Perhaps the most significant evidence in this respect comes from the trials of large-scale compost treatment in the loose, sandy soils of East Anglia, which seem to suggest that this would allow SRC crops to be grown without any further watering in all but the most exceptional of years (Butterworth 1999). According to the same study, even under such circumstances, the additional irrigation required would be very greatly reduced. The same work established that relatively immature composts are particularly effective in this respect, as they can absorb and retain between two and 10 times their own weight of water. The situation appears similar for dewatered AD digestate, when applied to soil and permitted to mature in situ. Digestate sludges are often aerobically stabilised in a process sometimes rather inaccurately termed ‘secondary composting’; this approach simply extends the same idea. The end result of this process is a high humus material, with good microbiological activity and excellent water-retaining properties, which appears to match the performance of ‘true’ composts at similar application levels. More-over, it would also seem that biologically derived soil amendment materials likethese, applied appropriately to soils either as a surface mulch or ploughed-in, can not only lower supplementary watering demands enormously but also largely offset any tendency to drought stress in the growing biomass. In addition, the leaching of nitrate from the soil is also lessened significantly.
As an aside, it is interesting to note that this ability to retain large amounts of water, together with its naturally high organic content has led to the use of compost in the construction of artificial wetlands. The USA has been particularly active in this area, in part due to the fact that federal environmental regulations encourage the creation of this type of habitat as a means of water treatment. This approach, which has been discussed more fully in an earlier, has as its main goal the manufacture of a wetland which behaves like a natural system in terms of both its hydrology and biology. To achieve this, a humus rich, biologically active medium, which closely replicates the normal physical and chemical properties of local soils is required. Biowaste-derived composts have been found to contribute well as constituents of manufactured wetland soils, often allowing vegetation to become established on such sites more quickly than usual (Alexander 1999).
To return to the issue of minerals, one of the chief potential bulk end uses of biowaste-derived compost is as a horticultural amendment and fertiliser replace-ment. There is no clear consensus between those working in the field as to how much nutrient is removed from the system when SRC wood is harvested, esti-mates for nitrogen loss ranging between 30 kg and 150 kg per hectare. A study by the UK’s Forestry Commission produced figures of 135 kg per hectare for nitrogen and 16 kg of phosphate, which is around one-fifth the demands made by a cereal crop. On this basis, it seems unlikely that nutrient removal would be a limit on fertile sites and certainly not for the first few harvest cycles. In the case of soils with naturally low fertility, or those which have been used for coppice cropping for some years, supplementary mineral input may well be required. Clearly, if biowaste-derived material is used for its water-holding prop-erties, the concomitant humus and mineral donation would represent what might be described as a gratuitous benefit. Process integration in this fashion brings evident economic advantages to any commercial coppicing operation.
There is another way in which composts can help SRC. Direct competition from other plants is one of the largest factors in poor coppice crop growth and may even lead to outright failure in some cases. Uncontrolled grass or weed growth around the trees in their first season can reduce the dry matter yield by a fifth and halve their overall growth. Even after they have become properly established, weed control remains an important part of optimising the energy crop’s performance, particularly where a soil’s intrinsic water-holding and/or nutrient levels are less than ideal. Heavy mulching has been used very successfully in many operations and, as is obvious from the previous discussions, biowaste soil amendments areideal candidates for use in this role. It is clear that the benefits of weed suppression as a means of maximising the harvested energy yield will also apply to many other biomass crops.
In general terms, it is possible to summarise the agricultural benefits of compost as the addition of humus material and nutrients, which improve soil structure and fertility, respectively. Compost brings with it a readymade microbial community which can significantly augment the complement already present in naturally impoverished soils. With better physical structure, aeration is improved and root growth facilitated. The ability of biowaste-derived material to contribute to a soil-nutrient replacement programme, and thereby lead to a reduction in propri-etary chemical fertiliser use, has been a consistent finding in numerous studies. This also represents a further prospective contribution on two relevant sides of the intervention triangle. Firstly, in reducing nitrogenous inputs, it may play a useful part in reducing the farm’s pollution potential. Secondly, it becomes an example of cleaner production, since by biocycling nutrients back into the chain of biomass utility, it forms a closed loop system in respect of both minerals and energy. There may still be further ‘clean’ benefits to come, since research at the University of Kassel on a range of plants, including cabbage, carrots, potatoes and tomatoes has found that the use of compost was associated with an improved nitrate to vitamin C ratio in the final product. Moreover, in structurally deficient soils especially, compost appears to produce better results than it is possible for artificial fertilisers alone to achieve. Even so, most investigations have concluded that while high application rates generally tend to give relatively big increases in crop yield, at lower levels the effect is less significant, being very largely attributable to the compost’s humus enhancing effect.