The use of bioenergy has been proposed as a possible substitute for fossil fuels. However, forest bioenergy production must not compromise soil and water conservation, long-term soil and site productivity, or biodiversity and habitat quality.
It is possible to harvest biomass for energy from forests without compromising soil quality and forest productivity if sustainable management practices are adopted. Intensive forest harvesting plus residue removal may significantly affect soil quality and decrease soil productivity on some sites. Low-quality sites, when intensively harvested, are most susceptible to productivity decline. Good sites are also susceptible if management does not fully compensate for possible negative effects of harvesting and site preparation.
Loss of soil organic matter and nutrient depletion are of special concern in management systems associated with whole-tree harvesting or residue removal for bioenergy. The potential negative side-effects of forest biomass removal on soil productivity include:
- Soil organic matter depletion
- Nutrient depletion (N, P, K, Ca, Mg, B)
- Soil compaction
- Soil erosion and displacement
However, it is possible to harvest biomass for energy from forests without compromising soil quality and forest productivity if sustainable management practices are adopted. The following general principles and practices are applicable to most forest environments:
- managing soil physical properties
- preventing permanent damage (soil compaction and loss)
- ameliorating damage (soil cultivation)
- managing tree crop nutrition through
- the amount of biomass removed
- matching nutrients to tree demand
- avoiding biomass harvest on low nutrient sites
Site-specific management is essential for the maintenance of soil productivity. Due to the unique characteristics of each soil, different management approaches are needed to maintain soil fertility and physical properties in intensively harvested forests.
Machine traffic compaction causes negative effects on soil productivity. Traffic impacts can be minimised or eliminated if operations are confined to periods when soils are dry. Information on susceptibility of the soils involved should be considered and used in harvesting planning. Ripping, subsoiling, and bedding should be used to repair damaged soils.
Harvesting procedures must aim to minimise nutrient removal from forest sites. Nutrient removal in biomass can be minimised by leaving nutrient-rich leaves and needles behind through on-site drying of forest fuel. This reduces nutrient loss, increases value of the fuel, and can reduce the incidence of disease and pest attack. On-site drying is especially beneficial in pre-commercial thinning, which is carried out at a time when tree nutrient demand is high.
Wood ash from the combustion of forest biomass should be returned to forest sites to counteract long-term nutrient depletion.
If, after all possible nutrient conservation measures have been taken, soil fertility is compromised then inorganic fertiliser should be applied to maintain soil fertility. Benefit-cost should be carefully analysed before biomass is harvested for energy if fertiliser application is likely to be required.
Nitrogen (N) supply often limits growth in intensively managed forests and it is also the nutrient that is removed in greatest quantities with biomass harvesting. Practices that conserve organic matter should be encouraged to improve soil N reserve. N-fixing legumes and other non-competitive vegetation should be also managed to increase N input and cycling in forests.
- Timber harvest residues: Silvicultural implications.
- Sustainable production of woody biomass for energy.
- Biomass harvesting on forest management sites in Minnesota.
- Nutrient recycling.
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