Soils
The key to maintaining soil productivity includes the following practices. Bioenergy harvesting has a number of potential environmental impacts on soils. There are also mitigating practices for these impacts (Table:Potential Environmental Impacts of Bioenergy Harvesting on Soils).
Conserving Organic Matter
Soil organic matter can be conserved by reducing the impact of those operations with high probability of removing the forest floor and generally disturbing the soil surface, such as machine traffic and disturbances associated with harvesting and site preparation operations (at right). Conserving organic surface horizons in managed forests will help maintain the single most important source of plant-available managing nutrients in forest soils (the forest floor), and reduce the need for fertilizer amendment to correct future nutrient deficiencies. Managers can train machine operators to reduce soil disturbances from machine traffic and schedule operations during seasons when soil moisture conditions contribute to resisting machine ruttings and scarification. Allowing machine traffic and performing certain operations during adverse soil conditions can lead to a loss of organic matter.
Conserving or Improving Soil Physical Properties
Machine traffic across harvesting blocks can be kept to a minimum in order to reduce the potential for adverse changes to soil physical properties from compaction or displacement (below left). Managers also need to consider scheduling machine work for times when moisture conditions are not high and therefore the potential for machine damage is relatively low on some sites. The use of specialized equipment, such as high flotation tires or advanced techniques such as boom-forwarding, can be adapted to prevent machine damage. Should soils be damaged, they should be repaired using appropriate techniques for the site. For example, on some sites compaction can be partially reversed by ripping and cultivation. Scarified soils can be amended by spreading harvest residues. Soils with high water tables can be mounded and bedded (below right).
Managing Nutrients
Nutrient management is essential for maintaining or improving the availability of essential nutrients to trees and therefore to maintaining soil productivity. The foliage of harvested trees should be left on site in order to retain the relatively high proportion of nutrients contained in this part of the tree. This practice may not significantly reduce the availability of high-quality biomass for bioenergy feedstocks, since foliage is of less value than woody tree components. If foliage retention is not possible, forest managers should consider the application of fertilizer (at right) or wood ash to sites that have been diagnosed as requiring nutrient amendments for specific deficiencies. Elements of special concern include nitrogen, phosphorous, and basic cations (Ca, Mg, K). Wood ash amendment is also recommended on soils which have become acidified by acid rain and associated air pollution.
Preventing erosion
The potential for overland flow of rainfall and soil erosion (at left) can be reduced by minimizing compaction and the amount of bare soil exposed during harvesting and forwarding operations. Forest managers and logging supervisors must take special care to minimize soil scarification on sites with slope greater than 10%. Extra caution should be employed when considering tillage and road construction operations on erosion-prone sites.
While these guidelines can be generally applied in all areas, each site should be evaluated and managed with site-specific goals for maintaining or improving soil quality in mind. To find out more about the soils at a specific location, go to Web Soil Survey, a service of the Natural Resources Conservation Service.
Encyclopedia ID: p1279
