This category refers to all classes of urban formations that have been in use as settlements (e.g., areas that are functionally or administratively associated with public or private land in cities, villages, or other settlement types), since the last time data were collected. Emissions and removals of CO2 in this category are estimated by the subcategories of changes in carbon stocks in biomass (both woody and perennial non-woody components), in DOM, and in soils, as summarized in Equation 2.3 in Chapter 2.
The biomass pool in settlements has woody and herbaceous components. For woody biomass, carbon stock change is calculated as the difference between biomass increment and biomass loss due to management activities. For herbaceous biomass (such as turfgrass or garden plants) in Settlements Remaining Settlements, the carbon stock change in biomass can usually be assumed to be zero.
The DOM pool in settlements contains dead wood and litter from both woody and herbaceous components. For the woody vegetation, changes in this pool can be quantified as production of coarse and fine litter from woody plants. For herbaceous vegetation, annual production of DOM is estimated as the accumulation of thatch plus production of herbaceous material such as garden waste and yard trimmings. Greenhouse gas emissions associated with Waste Sector are estimated in Volume 5 (Waste) and therefore the methods in this Chapter describe only those components of annual production that can reasonably be expected to stay on-site.
Soil C pools vary with time depending on the balance between C inputs from plant litter and other forms of organic matter and C outputs resulting from decomposition, erosion and leaching. Estimating the impact of settlement management on soil C storage will be particularly important in countries with a large portion of land in cities and towns, or high rates of settlement expansion. For mineral soils, the impact of settlement land use and management on soil C stocks can be estimated based on differences in storage among settlement cover classes relative to a reference condition, such as native lands. Although organic soils are less commonly used for settlements, C is emitted from these soils if they are drained for development due to enhanced decomposition, similar to the effect of drainage for agricultural purposes (Armentano, 1986). In addition, peat may be harvested from organic soils during settlement development, which will also generate emissions to the atmosphere.
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