1.2.1 Weather Conditions and Fluctuations
Prevailing weather conditions substantially influence the quantity of carbon present in soils. This is not simply due to differences in specific climatic zones and geographical environments that clearly plays a substantial role. It is well established that the carbon content of soils reduces from the equator to the poles, mainly as a consequence of climatic variations [23]. Specifically, it is biophysical variables such as humidity and temperature conditions in a soil that determine the rates of microbial and bacterial metabolism. Up to a point higher temperatures and humidity tend to increase metabolic rates; whereas, water-saturated soils, starved of oxygen result in a decrease in metabolic rates. It is the latter conditions that are responsible for massive deposits of peat, formed in cooler and wetter regions and tropical regions, resulting in massive carbon sinks developed over millennia. In many environments, soil water content is the dominant factor in determining the quantity of carbon preserved in peats. In cool or warm arid environments, the lack of soil moisture tends to inhibit the degradation of organic matter but carbon input tends to be low in such soils, leading to their low NPP.
Of course, fluctuating weather conditions also influence soil erosion rates in specific areas. Even if soil temperature and humidity are well balanced to fix large quantities of carbon, that is not much use as carbon sequestration sink if large quantities of the soils produced are frequently eroded by deluges or floods related to periodic or seasonal extreme weather events.
As soils tend to hold less carbon at higher temperatures, Sun et al. (2019) [22] considered the potential release of carbon to the atmosphere from forest soils based on a range of global warming scenarios. They estimated that by the end of this century, the top 20 cm of forest soils globally would release some 6.58 Pg C to the atmosphere if there was just a 1°C increase in global mean air temperature. On the other hand, they suggested that those soils could release as much as 26.3 Pg C to the atmosphere if there was a 4°C increase in global mean air temperature. In either case, the addition of carbon to the atmosphere would likely exceed carbon uptake from reforestation and forest growth.
Rate of decomposition and availability of carbon in soil to microorganisms is often used to distinguish two type of organic carbon in soil: labile and recalcitrant [24]. Labile soil carbon tends to be associated with microbial biomass, dissolved organic matter that is easily oxidized and broken down. Recalcitrant organic carbon in the soil are compound that are resistant to microbial decomposition. Recalcitrant organic carbon tends to be associated with soil mineral particles. Zhang and Zhou (2018) [25] estimate that more than 80% of mineralized soil carbon is derived from the recalcitrant pool of soil organic matter in the temperate forests of northern China. In those forests, the quantity of mineralized soil organic carbon slightly increased with soil moisture content. Mineralization of soil carbon clearly plays a key role in the carbon cycle in terms of determining soil’s ability to store quantities of carbon in a stable manner over long periods of time. That ability is strongly influenced by temperature and moisture content.
1.2.2 Plant and Natural Biomass Inputs
The quantity of carbon in a soil is positively correlated with its input rate of biologically derived material. Increasing that input rate is therefore likely to increase a soil carbon stock. In agricultural regions, this can be achieved, to an extent, by minimizing the time the land remains fallow without vegetation, although in arid regions fallow periods are often essential to restrict overuse of limited water resources. Introducing higher-yield, fast-growing crops and grass leys into crop rotation sequences tends to increase the amount of organic material entering the soil due to the higher biomass production above ground [26]. For instance, a crop rotation including deep-rooted grasses enhanced the organic content in soils from savannah environments by as much as 70 t C ha−1 [27]. Ensuring that maximum quantities of the residual wastes from crop harvests are allowed back into the soil in agricultural regions enhances soil organic content [28]. Increasing biomass production with the aid of nitrogen-rich fertilizers tends to work well in temperate climates but not so well in tropical climates [29]. However, excess nitrogen in rivers and in the oceans surrounding major river deltas is known to have a detrimental effect on biodiversity.
1.2.3 Organic Enrichment Treatments
Introducing organic supplements to agricultural land that is over and above the carbon input resulting from the rotation cycle of crops grown on the land does act to increase the resulting carbon stored in a soil. The simplest way to achieve this is to extensively mulch land with composts and manures. In some developing countries and communities, manure/dung is dried and combusted as low-grade domestic and communal fuel, rather returned to the land. Composted municipal waste and sewage sludges are also applied as organic supplements to farmland at agricultural scales. These can introduce harmful chemicals, such as heavy metals, into soils, so their compositions have to be carefully monitored. Such treatments can also have negative impacts on the biodiversity of both flora and fauna supported by the land if applied inappropriately. Arable farmland generally responds well to such supplements, but grasslands growing on poor soils can be damaged by them [30, 31].
1.2.4 Tilled and Ploughed Agricultural Land
Whereas tilling prepares land for cultivation by mixing particles as a till is dragged through a soil ploughing tends to fragment clumps of soil and bury weeds and crop residues by penetrating deeper into the soil layers. Both activities are generally referred to as tillage. These activities typically result in higher carbon losses from cultivated soils than undisturbed grassland or forest soils, which typically possess the lower levels of carbon in their undisturbed condition. Deforestation of land to create new land for cultivation often leads to their soils being rapidly degraded in carbon and certain mineral components by groundwater leaching and enhanced microbial metabolism that increases the rate of soil respiration. Increased soil oxygen levels and changes in their temperature and humidity by tillage disruption are typically the causes of enhanced carbon losses from soils, both in the form of CO2 to the atmosphere and as leached organic compounds deeper into the subsurface [32]. Replacing ploughing with less disruptive tilling using shallow-tine equipment can reduce carbon losses from a soil substantially. Whereas ploughing can result in greater than 25% carbon loss, replacing it with shallow tilling can result in just 5% or 6% carbon loss [33]. Soil carbon content can be reduced rapidly the introduction of arable cultivation techniques involving some level of tillage. Buhre et al. (2005) [34] recorded carbon losses of up to 11% from soils subjected to just one cultivation cycle.
1.2.5 Pasture Managed for Livestock Grazing
Grasslands are typically able to sustain higher soil carbon contents than soils of arable land not treated with organic supplements. Moreover, best practices in grazing land management can improve the organic density of grass sward, that is, the surface soil layer constituted by a mass of grasses and their roots. This, in turn, may increase soil carbon levels by greater than 1.0 t C ha−1 a−1 [13, 28]. However, poor grassland management, which tends to be the norm in deforested areas, associated with over-grazing with livestock exacerbates carbon loss, soil erosion, and reductions in biodiversity at the macro and micro levels.
1.2.6 Irrigated Arable Lands and Their Associated Drainage
Water movements through soils and water retention by soils have complex interactions with soil carbon levels. Whereas, decomposition rate of organic