1
Potential and Challenges of Carbon Sequestration in Soils

Erfan Sadatshojaei1*, David A. Wood2 and Mohammad Reza Rahimpour3

1Department of Chemical Engineering, Shiraz University, Shiraz, Iran

2DWA Energy Limited, Bassingham, Lincoln, United Kingdom

3Department of Chemical Engineering, Shiraz University, Shiraz, Iran

Abstract

Terrestrial soils, by volume, represent the most significant land-based carbon store on our planet. Over time, soils absorb carbon from a wide range of organisms as they respire during life and decompose after their demise. Carbon currently residing in the upper soil layers constitutes more than the combined quantity of carbon in land-surface vegetation and the atmosphere. Retaining and ideally boosting that carbon store in soils and preventing that carbon entering the atmosphere is of paramount importance in the fight against climate change. Almost 50% of global soils within about 1 m of the surface have been disturbed by agriculture releasing at least some of the carbon they store to the atmosphere. Carbon ideally needs to become mineralized in soils if it is to be stabilized and sequestered in the subsurface over the long term. Unfortunately, a significant portion of carbon in soils has a relative rapid turnover time, or low residence time, and is returned to the atmosphere as carbon dioxide via soil respiration processes. Whereas, it takes much longer for some of the soil carbon to be converted to stable mineralized forms. Soil erosion, as well as tillage, plays a significant role in releasing some soil carbon to the atmosphere. Converting significant areas of croplands and grazing lands to forests, grassland, and wetlands is the best option currently available for increasing the soils uptake of carbon from the atmosphere. Additionally, plant large quantities of perennial deep-rooted, fast growing bioenergy crops, such as switchgrass and miscanthus, can increment the carbon storage potential of grassland soils. The aggressive implementation of such actions has the potential to increase global soil carbon storage by between 0.5 and 2.0 Pg C a-1 for several decades. This could only be achieved in association with large-scale reforestation and robust steps to mitigate anthropogenic soil disturbance and natural erosion.

Keywords: Soil as a carbon store, carbon turnover time, climate change mitigation, soil disturbance, organic matter decomposition, humification and peat formation, carbon mineralization, soil C sequestration potential

List of Abbreviations and Units

1 petagram (Pg) = 1015 grams = 1 gigatonne

1 kilogram (kg) of carbon is equivalent to 44/12 of carbon dioxide

1 hectare (ha) = 107,639 square feet = 10,000 square meters

1 acre = 43,560 square feet = 4047 square meter

10,000 parts per million (ppm) = 1%

CO2 = carbon dioxide

NPP = net primary production

1.1 Introduction

Today, more than ever, the impact of the advancements in a wide range of technologies and artificial intelligence are influencing progresses and development of human life across the world. In this regard, we can point out ultrasonic application [1], carbon dioxide issue [2], medical research [3-8], and new chemical methods [9] are all advancing rapidly with their impacts being felt more widely. "Sequestration" of combines both the capture of carbon dioxide (CO2) and its long-term isolation from the atmosphere and ocean, storing it safely and securely for thousands of years.

Carbon sequestration in soils, to absorb some of the unwanted CO2 in the atmosphere, mainly involves adopting improvements in land management. This means adopting practices, on a large scale, that convert more atmospheric CO2 into carbon stored in soils than current practices achieve. The main potential to improve carbon management techniques applies to cropland and grazing lands [10]. These improved land use and carbon management techniques strive to increase the rate of biomass entering the soils and/or by reducing the rates of turnover of organic carbon already residing in the soils and by increasing the quantity of soil carbon that becomes mineralized. Through carbon sequestration in soils, CO2 is to a degree stabilized in soils on a semi-permanent basis. However, to achieve this, the CO2 needs to be converted into other materials. These chemical changes are initiated primarily through organic processes. CO2 is involved in a complex cycle which includes a circulation through the soil influenced by a range of micro-biological activity. Through this circulation, CO2 becomes available in soils to be dissolved by percolating rainwater. This leads to the formation of carbonic acid in near surface fluids.

On a global scale, soils store about 1,500 Pg (petagrams, equivalent to 1,500 gigatonnes) of organic carbon to a depth of one meter, increasing to 2,400 Pg to a depth of 2 m [11]. This means that carbon residing in upper soil layers amounts to more than the combined quantity of carbon in land-surface vegetation and the atmosphere. A little less than 50% of soils globally have been or are in use for agriculture, both cropland and grazing land. The soils involved in cropland activity have almost all been disturbed by some form of tillage. Organic matter within soils can vary between about 1% and 10%.

Subsequently, that carbonic acid reacts with basic cations leading to the creation of secondary carbonates in the short-term, on the scale of years, forming mineralization in near-surface rocks, leading to sustained processes that persists over geological timescales. The creation of secondary carbonates comes mainly from sub-surface weathering and diagenesis reactions with silicate minerals containing calcium and magnesium. Such reactions generate free positively charged ions (cations). Many of these free cations go on to combine with CO2 to form carbonate minerals, particularly calcite and dolomite [12]. However, these pervasive carbonate forming diagenetic processes tend to progress too slowly in their natural cycles to be practically exploited for carbon sequestration purposes. Nevertheless, they do involve substantial quantities of CO2, particularly in alkaline and saline soils present in dry and semi-dry zones [13]. Consequently, the inorganic sub-surface carbon cycle cannot be considered as significant or viable for rapid carbon sequestration in the soils typically found in the soils of wet and temperate zones.

On the other hand, organic carbon can cycle through soils, some returning to the atmosphere very rapidly. In the organic dimensions of the carbon cycle, atmospheric carbon dioxide is stabilized through the photosynthesis conducted by plants, algae, and cyanobacteria to form a range of organic compounds. Although the living organisms initially form glucose during photosynthesis, they transform it into diverse organic compounds, such as cellulose, hemicellulose, and lignin mostly; materials that are useful for biological growth and tissue formation. However, other complex organic materials such as protein, lipids, including more intricate compounds used to provide various benefit to plants and bacteria, are also formed. Land plants direct a significant portion of photosynthetic products to their roots, some of which are released to the soil as soluble carbon compounds; products termed as rhizoexudates [14].

Figure 1.1 Schematic diagram illustrating the biological contributions to the carbon cycle via terrestrial soils.

When plants and bacteria die, their organic constituents are dispersed in soils through decomposition by soil micro-organisms. That decomposition releases much of the CO2 they captured during photosynthesis making its way out of the soil to return to the atmosphere (Figure 1.1). This organic matter-soil decomposition cycle contributes CO2 output to overall soil respiration that includes respiration of plant root and flora and fauna that live in the soil. In addition to the contributions of plants, algae, and cyanobacteria to the carbon cycle through soils, there is a substantial sub-cycle that is related to contributions from animals. The animals consume CO2 in the form of food, with animal excrements and corpses returning to the soil and being decomposed along with plant, algae, and cyanobacteria remnants.

1.1.1 Soil Decomposition Processes

Animals that live in the soil vary from clearly visible spineless animals such as woodlice, centipedes, and earthworms, to smaller, microscopic-scale animals, the mesofauna, including mites (arthropods), springtails (Collembola a hexapod), and enchytraeidae worm-like (microdrile oligochaete) creatures. Some of the smallest animals in the soil, such as nematodes and protozoa, are among the most effective in the soil decomposition processes [15]. Soil organisms of all sizes and types collectively consume plant, animal, and bacterial debris in the soil. They do this by communition, or the reduction of material from one average particle size to a smaller average particle size, using various physical and biochemical techniques. Fungi and bacteria play a key role in breaking down the structural fabrics of plant materials. These groups are able to convert cellulose and lignin into soluble materials applying complex enzymes to do so. Subsequently, the soluble materials produced are absorbed by the organisms and further metabolized.

Initially, dead plant material located above the ground are, for the most part, decomposed above ground on the soil surface. The soil organisms, weather, and industrial-scale anthropogenic...