1.2.7 Uncertain Impacts of Soil Erosion and Redistribution on Its Carbon Store
Soils are not static systems. Climatic and geological processes act continuously to impact soils. Several of these processes disturb and redistribute soils due to erosion and transportation actions associated with the forces exerted by blowing winds and/or flowing water. Soil erosion ultimately moves the organic content of soil from its place of original formation to another, or ultimately distribute portions of soil from one location to several subsequent locations. Similar to the anthropogenic consequences of tillage, natural erosion processes lead to considerable soil disruption that is likely to increase the degradation rate of its organic matter [38]. However, if soils are merely displaced by a single erosion event, for example, by a landslip, relatively small amounts of carbon loss are likely to result. Unfortunately, the continuous and repeated nature of erosive forces and events leads soil components to gradually make their way, via fluvial actions, into ocean sediments. Although, the carbon that does ultimately arrive in ocean sediments may then be buried for many millennia and isolated from the ocean and atmosphere [38], much of the carbon temporarily fixed by the original soil formation processes is lost or partially degraded along the way. This makes it almost impossible to quantify on a global scale what quantities of carbon stored in soils is ultimately returned to the atmosphere by the geological forces of erosion. In the short term also, the quantities of crop residues entering a soil system is reduced by the actions of erosion at the surface, typically resulting from extreme or seasonal weather-related disturbances.
1.2.8 Fire Impacts on Soil Characteristics
Across the world, fire is relatively frequent but intermittent natural phenomenon impacting large tracts of land. In addition to natural causes of land fires (lightning strikes, volcanic activity, etc.) there are anthropogenic causes such as crop residue burning, forest clearance, accidents, and arson. Isolated, high-latitude wetlands are much less likely to experience natural land fires caused by lightning strikes [39] than hotter drier regions close to human population centers, where substantial quantities of dry biomass at the land surface is more easily ignited.
Seasonal burning to contain vegetation is a land management technique used in some areas. Although this removes biomass and temporarily reduces carbon input to the underlying soils [40], this practice can ultimately stimulate new, more rapid vegetation growth that will, for a time, enhance carbon input to the soil. The addition of some “charcoal” or charred carbon-rich material to the soil following episodes of burning also contributes to the inert carbon matter in the soil. This component, and artificially produced biochar additions, acts to stabilize some of the carbon within fire-affected or biochar-treated soils. Whether induced naturally or for land management purposes, there is a risk that land fires can ultimately ignite the underlying soils and peats. If this occurs, it releases huge quantities of CO2 to the atmosphere, and it reduces or removes the carbon store in the soils that have taken millennia to accumulate. Such outcomes have been observed associated with forest clearances in Brazil and Indonesia and are likely intermittent natural events over geological time scales.
1.3 Carbon-Sequestration Potential of Specific Vegetation Zones and Ecosystems
Fundamentally, each type of land-surface vegetation type has characteristic carbon quantities and fabrics in its underlying soils. For instance, in temperate climates carbon contents tend to be higher than the global average and increase as latitude increases within the temperate zones. Reductions in temperature and increases in waterlogging of soils at higher temperate latitudes are the main causes of this trend. These changes tend to enable a constant stream of biomass to enter the soils but reduce organic matter degradation. The global average estimated carbon density per unit area of soil in various vegetated zones and ecosystems are listed in Table 1.1.
1.3.1 Croplands
Soils underlying arable croplands typically possess low carbon soil contents (less than about 150 t C ha−1), because they are either under cultivation or have been cultivated in the past [14]. Typically, such soils are maintained under the condition that causes them to lose their carbon to the atmosphere and by leaching at a rapid rate. they present a major challenge for sustainable future sequestration of carbon in soils.
Table 1.1 Estimated soil carbon complements in the top 1 m of common vegetated systems. The ranges provided indicate uncertainty in the global average carbon density for each major vegetated system considered (after Hester et al., 2010) [14].
| Ecosystem | Carbon density (t C ha-1) |
|---|---|
| Wetland | 643 |
| Cropland (arable) | 80–122 |
| Tundra | 127–206 |
| Deserts and semi-deserts | 42–57 |
| Temperate grassland and shrubland | 99–236 |
| Tropical savanna and grassland | 90–117 |
| Boreal forest | 247–344 |
| Temperate forest | 96–147 |
| Tropical forest | 122–123 |
1.3.2 Grasslands
Grasslands typically display higher soil carbon contents rather than cropland soils. Table 1.1 displays the carbon density of undisturbed naturally occurring grassland soils. Some crop rotation schemes combine periods of grassland cultivation with periods of arable crop growth. Soils associated with such systems tend to have carbon densities higher than croplands but not as high as natural undisturbed grassland soils. The soil carbon density tends to increase for areas that are laid to grass for higher fractions of the crop rotation cycle. Extensive cultivation of fast-growing bio-energy grasses with deep root systems, such as switchgrass and miscanthus, has the potential to substantially increase the carbon density of grassland and cropland converted to grassland.
1.3.3 Woodlands
Woodlands consisting of conifer, deciduous or mixed tree types, particularly in high-latitude forests tend to have soils with carbon densities similar to, or somewhat higher than, undisturbed natural grasslands. Higher latitude coniferous forests tend to retain much higher soil carbon densities than deciduous temperate woodlands [41].
Sun et al. (2019) [22] evaluated the long-standing impacts of temperature, litter inputs, soil characteristics, and vegetation type on the carbon content in the top 20 cm of soils