Building capacity for enhancing Bioenergy Sustainability through the use of the Global Bioenergy Partnership Indicators
Global Bioenergy Partnership (GBEP)
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Abstract: This section elaborates on estimation procedures and good practices for estimating change in forest soil C stocks. It does not include forest litter, which is a dead organic matter pool. Separate guidance is provided for two types of forest soils: 1) mineral forest soils, and 2) organic forest soils. The organic C content of mineral forest soils (to 1 m depth) typically varies between 20 to over 300 tonnes C ha-1 depending on the forest type and climatic conditions (Jobbagy and Jackson 2000). Globally, mineral forest soils contain approximately 700 Pg C (Dixon et al. 1994), but soil organic C pools are not static due to differences between C inputs and outputs over time. Inputs are largely determined by the forest productivity, the decomposition of litter and its incorporation into the mineral soil and subsequent loss through mineralization/respiration (Pregitzer 2003). Other losses of soil organic C occur through erosion or the dissolution of organic C that is leached to groundwater or loss through overland flow. A large proportion of input is from above-ground litter in forest soils, so soil organic matter tends to concentrate in the upper soil horizons, with roughly half of the soil organic C in the upper 30 cm layer. In some forest ecosystems, rooting zones of trees extend considerable deeper than 30 cm, which can increase the share of soil organic carbon in deeper layers (Nepstad et al. 1994). Changes in soil carbon stocks in response to management actions such as thinning and clearcutting have been detected below 20?30 cm, but not in all studies or all depths (Achat et al. 2015a; James and Harrison 2016; Gross et al. 2018). Moreover, the scarcity of measurements increases uncertainty related to soil carbon stock changes deeper in soil. The C held in the upper profile is often the most chemically decomposable, and the most directly exposed to natural and anthropogenic disturbances. This section only deals with soil C and does not address decomposing litter (i.e., dead organic matter, see Section 4.2.2).