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gradient in China content of the 0- to 20-cm soil layer and its fractions across three wet forested sites of contrasting climatic zones in China. They found that as mean annual temperature increased organic carbon in the soil samples reduced, whereas annual litter-fall and soil microbial respiration increased. Carbon compounds displaying greater stability were better preserved in those forest soil samples from warmer climates. This was considered to be the most likely caused by the more rapid breakdown of less stable carbon molecules in warmer climate forest soils.
1.3.4 Temperate Wetlands and Peat Bogs
Temperate wetlands often associated with bogs, swamps, and extensive peat accumulations display the highest carbon densities of terrestrial soils. Peats can achieve carbon densities greater than 1000 t ha−1 [42], or 5 to 10 times the levels of the other vegetated zones described.
1.3.5 Induced Changes in Vegetation and Land Conditions
Most attempts to change the vegetation type on a land area, particularly from forests or natural undisturbed grasslands to croplands [43], lead to a reduction in soil carbon density. Land-use alteration from one type to another invariably involves soil disturbance that leads to more CO2 lost from the soil to the atmosphere. Eventually, the modifications induced to vegetation type at the land surface will establish a new carbon equilibrium within the underlying soil. Of course, converting cropland to permanent grassland or woodland does ultimately lead to an increase in the carbon content of the underlying soils. In Europe, it is estimated that poorly managed and damaged cropland and gazing land with improved land management practices applied could offer additional carbon sequestration potential of between 0.1 and 0.6 t C ha−1 [14]. Based on the areal extent of such poorly managed land, the incremental soil carbon sequestration potential in Europe is estimated at between about 60 and 175 Tg C a−1.
1.3.6 Warm Temperate and Tropical Vegetated Zones
The average carbon densities in forest soils of high-latitude Europe are known to be about three to five times higher than average carbon densities in soils of low-latitude Europe, with a 10-fold difference in their potential as carbon sinks [44]. On the other hand, with the exception of tropical peatlands, the carbon density of tropical soils is typically less than temperate soils and many times less than the vegetation that grows upon it. Soil carbon losses are high due to higher soil temperatures and water leaching rates. Potential carbon sequestration rates in tropical soils are about half those of temperate soils. Deforestation, a common practice in many tropical regions for conversion to cropland and grazing land, further exacerbates the relatively low carbon stock in tropical stores by releasing much of it rapidly to the atmosphere, especially where over grazing leads to drastic soil erosion.
1.4 Estimates of Global Potential for Carbon Sequestration in Soils
There are several published estimates of the global potential for carbon sequestration in soils, summarized by Paustian et al. (2019) [45], that suggest a range for the technical potential for additional carbon uptake by soils of between 2 and 5 Pg (gigatonnes) of CO2 per year (~0.5 to 1.5 Pg C a−1). To achieve the top end of that range, the best of currently available land and carbon management practices would need to be adopted for most croplands and grass lands around the world. These practices would also need to involve a drastic reduction in soil disturbance, measures to combat soil erosion, and be accompanied by a substantial increase in reforestation.
If and when such practices are adopted, they should provide an initial boost to the soils carbon uptake from the atmosphere. However, such a boost would likely only last for some decades until the better managed cropland and grassland soils and reforested regions achieved their new and higher carbon storage equilibrium. Technology advances in producing beneficial soil supplements cheaply, for example, biochar residues from biofuel production [46], and genetically engineered perennial and annual grain crops with deeper and larger roots [47], if widely utilized in the medium term, have the potential to increase the carbon uptake by soils by a further 0.5 to 1.0 Pg C a−1. This could achieve up to about 2 Pg C a−1 of additional global soil carbon storage capacity [45].
1.5 Conclusions
The greatest potential to improve carbon sequestration rates in soils is by converting croplands into (or back into) woodlands, grasslands, and wetlands. Whereas intensive addition of farmyard manure, slurries, and general biomass waste into cropland soils can significantly increase their carbon densities, most intensive soil carbon intervention projects on croplands are less effective in the long term than converting such lands to natural forests, grasslands, and wetlands. Grassland and converted cropland carbon storage potential could be improved by more extensive growth of perennial deep-rooted, fast growing bioenergy crops such as switchgrass and miscanthus. Whereas, the top 2 m of soils currently hold about 2,400 Pg of carbon globally, there is a general consensus that improved land management practices could add to this by between 0.5 to 2.0 Pg C a−1. A key limitation on the soils ability to rapidly uptake additional carbon from the atmosphere in coming decades is the long residence time of carbon in existing soils. This suggests that it would take significant time for the additional carbon added to soil from biomass to be converted and stabilized by carbon mineralization. Once carbon is mineralized it is more likely to be retained and sequestered long term in the ground rather than be cycled back into the atmosphere. Even in mineralized form, some soil carbon is frequently disturbed and displaced by the relentless forces of erosion, particularly over geological time scales. These factors mean that attempts to sequester more carbon in soils, while beneficial in the long term, can only constitute a relatively small component of a short-term global effort to sequester more carbon from the atmosphere to rapidly combat rising CO2 levels in the atmosphere that are fueling climate change. However, efforts to prevent soil erosion and reduce soil disruption by agriculture are essential to preserve the carbon store currently held globally in soils to prevent as much as possible of that carbon being displaced to the atmosphere and contributing incrementally to CO2 levels in the atmosphere.
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