Land use and management influence a variety of ecosystem processes that affect greenhouse gas fluxes (Figure 1.1), such as photosynthesis, respiration, decomposition, nitrification/denitrification, enteric fermentation, and combustion. These processes involve transformations of carbon and nitrogen that are driven by the biological (activity of microorganisms, plants, and animals) and physical processes (combustion, leaching, and run-off).
historical fire suppression and past forest harvest activities) or a combination of all three causes, the effects of which cannot be readily separated.
The key greenhouse gases of concern are CO2, N2O and CH4. CO2 fluxes between the atmosphere and ecosystems are primarily controlled by uptake through plant photosynthesis and releases via respiration, decomposition and combustion of organic matter. N2O is primarily emitted from ecosystems as a by-product of nitrification and denitrification, while CH4 is emitted through methanogenesis under anaerobic conditions in soils and manure storage, through enteric fermentation, and during incomplete combustion while burning organic matter. Other gases of interest (from combustion and from soils) are NOx, NH3, NMVOC and CO, because they are precursors for the formation of greenhouse gases in the atmosphere. Formation of greenhouse gases from precursor gases is considered an indirect emission. Indirect emissions are also associated with leaching or runoff of nitrogen compounds, particularly NO3- losses from soils, some of which can be subsequently converted to N2O through denitrification.
Figure 1.1 The main greenhouse gas emission sources/removals and processes in managed ecosystems.
Figure 1.1 The main greenhouse gas emission sources/removals and processes in managed ecosystems.
Emission and Removal Processes
Greenhouse gas fluxes in the AFOLU Sector can be estimated in two ways: 1) as net changes in C stocks over time (used for most CO2 fluxes) and 2) directly as gas flux rates to and from the atmosphere (used for estimating non-CO2 emissions and some CO2 emissions and removals). The use of C stock changes to estimate CO2 emissions and removals, is based on the fact that changes in ecosystem C stocks are predominately (but not exclusively) through CO2 exchange between the land surface and the atmosphere (i.e. other C transfer process such as leaching are assumed to be negligible). Hence, increases in total C stocks over time are equated with a net removal of CO2 from the atmosphere and decreases in total C stocks (less transfers to other pools such as harvested wood products) are equated with net emission of CO2. Non-CO2 emissions are largely a product of microbiological processes (i.e., within soils, animal digestive tracts and manure) and combustion of organic materials. Below, emission and removal processes in the AFOLU Sector are described for the major ecosystem stocks and processes, organized by ecosystem components, i.e., 1) biomass, 2) dead organic matter, 3) soils and 4) livestock.
Plant biomass, including above-ground and below-ground parts, is the main conduit for CO2 removal from the atmosphere. Large amounts of CO2 are transferred between the atmosphere and terrestrial ecosystems, primarily through photosynthesis and respiration. The uptake of CO2 through photosynthesis is referred to as gross primary production (GPP). About half of the GPP is respired by plants, and returned to the atmosphere, with the remainder constituting net primary production (NPP), which is the total production of biomass and dead organic matter in a year. NPP minus losses from heterotrophic respiration (decomposition of organic matter in litter, dead wood and soils) is equal to the net carbon stock change in an ecosystem and, in the absence of disturbance losses, is referred to as net ecosystem production (NEP).
Net Ecosystem Production (NEP) = Net Primary Production (NPP) - Heterotrophic respiration
NEP minus additional C losses from disturbance (e.g., fire), harvesting and land clearing during land-use change, is often referred to as net biome production (NBP). The carbon stock change that is reported in national greenhouse gas inventories for land-use categories is equal to NBP 2.
Net Biome Production (NBP) = NEP - Carbon Losses from Disturbance/Land-Clearing/Harvest
NPP is influenced by land use and management through a variety of anthropogenic actions such as deforestation, afforestation, fertilization, irrigation, harvest, and species choice. For example, tree harvesting reduces biomass stocks on the land. However, harvested wood requires additional consideration because some of the carbon may be stored in wood products in use and in landfills for years to centuries. Thus, some of the carbon removed from the ecosystem is rapidly emitted to the atmosphere while some carbon is transferred to other stocks in which the emissions are delayed. In non-forest ecosystems (i.e., Cropland, Grassland), biomass is predominantly nonwoody perennial and annual vegetation, which makes up a much smaller part of total ecosystem carbon stocks than in forest lands. The non-woody biomass turns over annually or within a few years and hence net biomass carbon stocks may remain roughly constant, although stocks may diminish over time if land degradation is occurring. Land managers may use fire as a management tool in grasslands and forests or wild fires may inadvertently burn through managed lands, particularly forest lands, leading to significant losses of biomass carbon. Fires not only return CO2 to the atmosphere through combustion of biomass, but also emit other greenhouse gases, directly or indirectly, including CH4, N2O, NMVOC, NOx and CO.
The bulk of biomass production (NPP) contained in living plant material is eventually transferred to dead organic matter (DOM) pools (i.e., dead wood and litter - see Table 1.1 for definitions). Some DOM decomposes quickly, returning carbon to the atmosphere, but a portion is retained for months to years to decades. Land use and management influence C stocks of dead organic matter by affecting the decomposition rates and input of fresh detritus. Losses due to burning dead organic matter include emissions of CO2, N2O, CH4 NOx, NMVOC, and CO.
As dead organic matter is fragmented and decomposed, it is transformed into soil organic matter (SOM). Soil organic matter includes a wide variety of materials that differ greatly in their residence time in soil. Some of this material is composed of labile compounds that are easily decomposed by microbial organisms, returning carbon to the atmosphere. Some of the soil organic carbon, however, is converted into recalcitrant compounds (e.g., organic-mineral complexes) that are very slowly decomposed and thus can be retained in the soil for decades to centuries or more. Following fires, small amounts of so-called 'black carbon' are produced, which constitute a nearly inert carbon fraction with turnover times that may span millennia.
Soil organic carbon stocks are influenced by land-use and management activities that affect litter input rates and soil organic matter loss rates. Although the dominant processes governing the balance of soil organic carbon stocks are C inputs from plant residues and C emissions from decomposition, losses as particulate or dissolved carbon can be significant in some ecosystems. Inputs are primarily controlled by decisions impacting NPP and/or the retention of dead organic matter, such as how much harvested biomass is removed as products and how much is left as residues. Outputs are mostly influenced by management decisions that affect microbial and physical decomposition of soil organic matter, such as tillage intensity. Depending on interactions with previous land use, climate and soil properties, changes in management practices may induce increases or decreases in soil C stocks. Generally, management-induced C stock changes are manifested over a period of several years to a few decades, until soil C stocks approach a new equilibrium. In addition to the influence of human activities, climate variability and other environmental factors affect soil C dynamics (as well as biomass and DOM).
In flooded conditions, such as wetland environments and paddy rice production, a significant fraction of the decomposing dead organic matter and soil organic matter is returned to the atmosphere as CH4. This can be a
2 Harvested wood or other durable products derived from biomass (e.g., clothing) products are not included in NBP; harvested wood products (HWP) are dealt with in Chapter 12.
major source of emissions in countries with a considerable amount of land dedicated to paddy rice production. Although virtually all flooded soils emit methane, net soil C stocks may either increase, decrease or remain constant over time, depending on management and environmental controls on the overall carbon balance. In well-drained soils, small amounts of CH4 are consumed (oxidized) by methanotrophic bacteria.3
Soils also contain inorganic C pools, either as primary minerals in the parent material from which the soil was formed (e.g., limestone), or as secondary minerals (i.e., pedogenic carbonates) that arise during soil formation. Inorganic soil C stocks can be affected by management, although typically not to the extent of organic C pools.
Some soil management practices impact greenhouse gas emissions beyond simply changing the C stock. For example, liming is used to reduce soil acidity and improve plant productivity, but it is also a direct source of CO2 emissions. Specifically, liming transfers C from the earth's crust to the atmosphere by removing calcium carbonate from limestone and dolomite deposits and applying it to soils where the carbonate ion evolves into CO2.
Nitrogen additions are a common practice for increasing NPP and crop yields, including application of synthetic N fertilizers and organic amendments (e.g., manure), particularly to Cropland and Grassland. This increase in soil N availability increases N2O emissions from soils as a by-product of nitrification and denitrification. Nitrogen additions (in dung and urine) by grazing animals can also stimulate N2O emissions. Similarly, land-use change enhances N2O emissions if associated with heightened decomposition of soil organic matter and subsequent N mineralization, such as initiating cultivation on wetlands, forests or grasslands.
With current state of scientific knowledge, it is possible to provide methods for estimating CO2 and N2O emissions associated with management of peatlands, and CO2 from conversion to wetlands by flooding. A methodological appendix (Appendix 3) has been included setting out a basis for development of a methodology for estimating CH4 emissions from flooded land.
Animal production systems, particularly those with ruminant animals, can be significant sources of greenhouse gas emissions. For example, enteric fermentation in the digestive systems of ruminants leads to production and emission of CH4. Management decisions about manure disposal and storage affect emissions of CH4 and N2O, which are formed in decomposing manures as a by-product of methanogenesis and nitrification/denitrification, respectively. Furthermore, volatilization losses of NH3 and NOx from manure management systems and soils leads to indirect greenhouse gas emissions.
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