Soils Carbon:
Soil carbon comprises 50 -70% of the total carbon assets in temperate and the cooler boreal forests. Hence, small changes in soil carbon can have significant impact on ecosystem carbon storage. Soils constitute the largest terrestrial carbon pool, containing as much as 2,344,000,000,000 tonnes (2.344 gtonnes) of basic carbon (C) to a depth of 3 meters. Forest soils, especially, contain more than double the amount of carbon than above ground biomass. Most carbon registries assume that management activities have little or no effect on soil carbon stocks if site preparation activities do not include mechanical site disturbance of more than 25% of the area. However, research has shown significant sequestration rates for temperate forest soils with rates ranging as high as 4.8 tonnes of CO2e /acre/year. (1.3 tonnes of carbon). Although soils carbon stocks accounted for nearly 48% of all forest carbon, they contributed only 2% of the total sequestration. This suggests that soil carbon stocks are relatively stable and this apparent lack of change may be the result of losses (from management activities) and gains (from increased growth). Several factors influence the interaction of soil carbon stocks and management activities:
Firstly, soil carbon dynamics vary with the dominant tree species, harvest type and silviculture system, soil type, site preparation techniques, time after disturbance, climate, and a multitude of other factors, known and unknown. There is a high amount of uncertainty compounded by studies that indicate different results. The end result being our knowledge of below ground activities is weaker than our understanding of aboveground processes.
Secondly, there are forest treatments that increase carbon stocks. Precommercial and commercial thinning treatments that leave organic material on the site have the potential to increase soil carbon 20-40%, especially on poor soils. Added carbon resources increases plant productivity as well as below ground carbon transport. These positive results are dependent upon leaving the needles, branches, and stems on the site. Removal in these biomass stocks causes changes in the microclimate and stimulates organic matter decomposition. Leaving these stocks on site has a positive effect on soil carbon pools in conifer dominated ecosystems and actually results in carbon losses in broadleaf ecosystems due to the higher content of labile (short-term) versus recalcitrant (longer term) carbon stocks in these broadleaf forests. Another important treatment recognized by Global Forest resources is prescribed fire. Carbon from charcoal following a prescribed fire (typically after a harvest) provides a significant input of charcoal carbon. Charcoal is created by the incomplete combustion of organic material and is resistant to microbial decomposition. As this charcoal is incorporated into the soil, it provides a long-term soil carbon pool as well as improved soil quality.
Thirdly, treatments that create significant soil disturbance, such as plowing, deep ripping and contour site preparation will have negative effects on soil carbon pools with potential losses of 30% or more.
Fourthly, the harvest system and type of harvest plays a significant role in soil carbon dynamics. Whole tree harvests reduce soil carbon by as much as 20%. Stem only harvests that leave organic material- needles, branches, bark, unmerchantable stems on site can result in gains of as much as 40%. Whole tree harvesting will result in significant soil carbon losses relative to the total carbon sequestered by the same forest carbon project.
Fifthly, Rotation length as well as cutting cycle is an important contributing factor to soil carbon gains and losses. Research indicates that time between entries may be a more important factor than the aforementioned harvest intensity. Research also shows that soil carbon lost due to harvest activities may be recovered in certain ecosystems and soil types within 50 years. However, the interval is longer in the cooler boreal forests.
Sixthly, precommercial and commercial thinning are both approved treatments in the Global Forest Resources Protocols. Thinning can change the soil microclimate and stimulate soil organic matter decomposition when the biomass is left on the site. Any negative effects caused by tree thinning treatments are typically offset by increased tree growth in the short to near term (10-20 years) of the residual forest.
And lastly, methods and established techniques to measure and monitor soil carbon losses and gains are not precise.
Soil carbon is not anticipated to change significantly as a result of Forest Project activities. However, soil carbon inventory must be updated if the following occur:
Site preparation activities involve the use of prescribed fire.
Wildfire burns through the Project Area.
Addition of carbon resources occurs. These added carbon resources include organic material added to the environment through precommercial thinning; branches and needles added from the commercial thinning operation and carbon added with the application of biochar-charcoal carbon.
