Soil in Carbon Footprint and Agriculture

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1		Ekrem Ozlu	--	2740	2022-10-24 01:56:43	\|
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This entry is adapted from the peer-reviewed paper 10.3390/biology11101453

Global attention to climate change issues, especially air temperature changes, has drastically increased over the last half-century. Along with population growth, greater surface temperature, and higher greenhouse gas (GHG) emissions, there are growing concerns for ecosystem sustainability and other human existence on earth. The contribution of agriculture to GHG emissions indicates a level of 18% of total GHGs, mainly from carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). Thus, minimizing the effects of climate change by reducing GHG emissions is crucial and can be accomplished by truly understanding the carbon footprint (CF) phenomenon.

carbon footprint agriculture soil management carbon sequestration

1. Introduction

The debate about the anthropogenic impact of climate change on a global scale has been increasing over the last 50 years as the detrimental impact of increased temperatures is now widely recognized by the scientific and the non-scientific communities at the same time. Earth surface temperatures are expected to rise between 1.6 to 5.8 °C by end of the century, parallel to the population growth rate and greenhouse gas (GHG) emissions ^[1]. With 95% confidence, the anthropogenic impact in natural agroecosystems has been found responsible for the mainstream climate change ^[2] observed since the late 1800s. Therefore, decreasing these GHG emissions to the atmosphere is an important task that might be achieved through a keen understanding of the carbon footprint (CF) of human activities. Besides quantifying total GHG impacts, knowledge of the CF can provide a strategy with an inclusive GHG record to distinguish expected reductions from its major sources. Thus, CF calculations might be a successful tool to guide the reduction in emissions and understand the risk of global warming. The term ‘carbon footprint’, which has become extremely common and is now a worldwide concept ^[3] was defined as “a measurement of the total GHG emissions caused directly or indirectly by an individual, an organization, even a product, and is expressed as a carbon dioxide equivalent (CO₂e)” by Gao, et al. ^[4]. As such, CF is a weight in units of kg or Mg of carbon per person or activity. Several methods have been allocated to determine estimates of CF, ranging from basic online tools to complex models, life-cycle analysis, or input-output-based methods and tools ^[3].

The global food and agricultural industry is challenged by various issues, among the most important of which is the necessity for the industry to produce a higher quality and a sufficient food supply that cater for the increasing global population. At the same time, the main environmental conditions within agricultural production and animal husbandry are heavily scrutinized by the public and governmental and non-governmental organizations. Previous attempts have indicated that 70% to 90% of the environmental impacts of agricultural production occur during the primary production process ^[5], which further indicates the importance of CF as part of the ecological footprint.

Supply and energy-intensive agricultural practices were reported to have high CF which has increased the global energy budget by about 10 times since the beginning of the 20th century ^[6]. Atmospheric CO₂ concentration has been increasing dramatically since the beginning of the industrial revolution, when the estimated CO₂ concentration was about 310 ppm, compared to the current atmospheric CO₂ concentration of 418 ppm measured on 1 July 2021, according to the Keeling Curve (https://keelingcurve.ucsd.edu/ (accessed on 1 July 2021)). The Kyoto Protocol established an obligation to mitigate the increases in GHG emissions to 108% of its 1990 records by 2012 ^[7]. Besides the stationary energy, industrial, and transport sectors, the agricultural sector must be strongly committed to applying its tools to diminish GHGs and climate change ^[7]. China [10151 million metric tons (MMT) CO₂] and the USA (5312 MMT CO₂) have been reported as the leading countries in terms of atmospheric CO₂ emissions, followed by India (2431 MMT CO₂), Russia (1635 MMT CO₂), and Japan (1209 MMT CO₂). However, in terms of methane (CH₄) emissions, tropical South America and Southeast Asia are the global leading regions, followed by China, Central Eurasia, Japan, and Southern Africa. Most of the CH₄ emissions on a global scale are related to agricultural and waste activities (191 MMT CH₄ yr⁻¹) or wetlands (167 MMT CH₄ yr⁻¹). In particular, India and China are the countries to have the most CH₄ emissions because of agriculture and waste. However, most CH₄ emissions come from wetlands in Tropical South America. The Intergovernmental Panel on Climate Change (IPCC) reported that GHG emissions as a result of waste management were around 3% of the total emissions in 2010 ^[8].

As part of agriculture and waste activities, rice is responsible for a significant amount of CH₄ emissions, especially in India and China. Similarly, CH₄ and nitrous oxide (N₂O) emissions from agricultural lands represent 59% and 84% of the total CH₄ and N₂O gas emissions in Australia, respectively ^[9]. In Australia, livestock is the largest source of emissions as a part of the agricultural sector, producing about 62.8 MMT CO₂, which represents almost 70% of the total agricultural GHG emissions ^[7]. Measuring the impact of the dairy industry is complex and varies with climate and management practices which influences not only the GHG emission per kg of energy-corrected milk or meat invention but also the runoff of phosphorus (P) from agricultural fields.

The agricultural sector has recently advocated switching chemical sources with organic active elements to biological or biodynamic growing methods. Using chemical fertilizers in agriculture has been shown to increase GHG emissions, especially N₂O, thus contributing to enhancing climate change issues ^[10]. Therefore, agricultural practices need to be reviewed and readapted to be environmentally friendly. Following, the CF concept is discussed under both mechanistic and practical approaches to assessing the contribution of agricultural activities to global sustainability.

2. Agriculture and Carbon Footprint

One of the largest sources of GHG emissions is agriculture, which emitted about 10–12% of the total global GHG in 2005 whereas this value has increased to 13.5% (CO₂: 25%; CH₄: 50%; and N₂O: 70%) by 2009 (Second largest source) and to 18% by 2011 ^[11]. Both scientific and public importance of the CF of agricultural inventions bounds up with the quantity of GHG emissions due to agricultural management practices such as tillage, inorganic fertilization, and harvesting ^[12], pesticides, manuring, waste management, composting, biochar addition, and crop photosynthesis capacity. Therefore, sustainable agricultural practices need to be investigated to challenge these issues.

Controlling agricultural management by assessing the agricultural CF might be an option for mitigating GHG emissions and hence climate change. Recently, some experiments have addressed the agricultural CF under different managements such as tillage, cropping systems ^[13], and nitrogen fertilizer rates ^[10]. However, reports do not contain sufficient information concerning responses of CF of crop production to integrated technologies with different agricultural strategies.

Agricultural practices need a significant amount of energy due to machinery processes. Therefore, enhancing energy use efficiency and lowering CF related to crop production is an essential requirement. Since GHG emissions are from soils and originated from biological activities which are sensitive to soil properties ^[14], it draws progressively more attention to increasing production efficiency and decreasing CFs together. Many studies have documented the importance of soils to decrease conventional energy use, provide clean energy, and hence understand low-C agriculture. Reducing GHG emissions should be in place by the time the sustainability of soil health/quality is secured or improved. Due to critical direct and indirect effective components in agricultural GHG emissions, understanding the mechanisms in the complex and dynamic soil system, and their intercorrelation with climate change issues, is crucial. A better understanding of climate change impacts on SOC needs a determination of the expected influences that climate change has on the comparative extent of soil C inputs and losses.

3. Role of Soil in Carbon Footprint and Agriculture

The soil is an important source and sinks of atmospheric C due to agricultural applications and management strategies on a global scale. A large amount of organic C and nitrogen are stored by soils which vary through the soil profile and cause GHG emissions associated with decomposition and microbial activities. If all other factors in the C cycle stayed steady, a difference of 1% soil C content would result in about 8 ppm CO₂ alteration in the atmosphere, and this 8 ppm CO₂ response might be lowered by considering the potential mediating responses due to photosynthesis and oceanic exchange ^[14]. It is a consequent result that global soils and soil management have the potential to either enhance or reduce atmospheric GHGs and climate change. Therefore, the relations between photosynthesis and GHG emissions as impacted by agricultural management practices especially focusing on soil and the related systems should be considered.

3.1. Soil Types

Soil type is a significant factor that impacts GHG emissions directly or indirectly by influencing soil structure and soil wetness ^[15]. For instance, N₂O emission is reported to be higher from clay loam soils in comparison to those from loam soils ^[7]. Soil bulk density and clay content are significant factors for the comparison in terms of N₂O in spring. Further, ^[16] reported higher N₂O and CH₄ emissions from Histosols in comparison to Gleysols and Plaggic Anthrosols whereas differences in CO₂ emissions were not significant between Histosols and Gleysols. Similarly, Butnan, et al. ^[17] reported that total CO₂ and CH₄ emissions had a positive correlation with the addition of higher volatile matter that contained biochar in the coarse-textured low-buffer Ultisol but it was not correlated in the fine-textured high-buffer Oxisols. N₂O emission had positive influences on the Mn-rich Oxisols potentially due to differences in mechanisms indicating microbial activities, soil aluminum and manganese toxicities, and soil pH impact on these soils ^[17]. Therefore, soil type is an important component in CF predictions besides soil health indicators.

3.2. Soil Health (Feedback Mechanism)

The understanding of mechanisms behind complex and dynamic soil systems is important to better understand the impacts of agricultural management practices on soil and environmental health. For instance, the application of manure as a soil amendment can be an option for enhancing soil quality and mitigating climate change ^[10]. However, it is more important to know how manure impacts a particular soil property and what differentiations are caused by these changes.

Even though GHG production is mainly a biological process, soil physical properties also impact biology by changing the physical environment of soil microbes. Both static and dynamic properties of soils are impacted by C which in turn affects C sequestration potential indirectly. The higher SOC and the lower soil bulk density indicate a higher degree of compaction in the soil. SOC is also significant and positively correlated with soil aggregate stability, soil structure, and erosion refers that SOC clarifies a significant amount of the variability of stable aggregates which is vital due to its positive influences on plant growth and the environment. Improving soil health indicators may result in more GHG emissions but they will also increase the C sequestration capacity of the soil through photosynthesis and hence mitigate climate change issues. Previous studies reported the significance of aggregate size distribution, inter-aggregate porosity, and gas diffusivity as leading to the degree of anaerobiosis and denitrification in soil.

Soil structure is one of the most important components in CF phenomena due to its influence on GHG emissions through microbial activity, WFPS, soil metric potential, volumetric water content, aeration, relative gas diffusivities, and air permeabilities, and restricted aeration ^[15]. For instance, poor structure lowers the relative gas diffusivities, and air permeabilities, and restricts aeration which are relevant indicators for N₂O and CH₄ flux and aeration status ^[15]. Moreover, Ozlu and Kumar ^[10] reported the relationship of volumetric moisture content of the soil with air temperature, WFPS, and hence GHG emissions.

Furthermore, an explanation for soil temperature influences on higher N₂O emission might be possible by anaerobic microsites as higher respiration and oxygen requirements ^[18]. The capacity of oxygen (C), CO₂, N_2, and N₂O (D) to exchange on soils due to pore sizes (A) and total porosity is a controlling factor for GHG emissions, Figure 1. Such mechanisms are significantly determined between aggregates partially by gas diffusion rates ^[15]. Arah, et al. ^[19] conducted research in southeast Scotland to estimate soil N₂O emissions by using Fick’s Law and evaluated gas diffusion rates and N₂O fluxes. In addition, crop roots are significantly important especially due to their roles in soil aggregation. Crop roots keep soil particles together and help to develop soil aggregates (B). Soil structural quality may not always be described by considering all these properties and mechanisms but it is determined from certain results of soil physical properties such as porosity, water retention, air permeability, hydraulic conductivity, gas diffusivity, aggregate stability, and penetration resistance ^[15]. Considering these processes and properties in soil structure phenomena may help to better understand soil structural development and its role in CF. For example, soil water retention indicates the interaction of soil moisture content and soil water potential which also influence soil redox conditions whereas soil water retention under field capacity is significantly and positively linked with SOC. Similarly, SOC is associated with total porosity, saturated hydraulic conductivity, and soil bulk density ^[20]. Further, increases in soil bulk density and soil strength are the results of soil compaction which also reduces soil macro-porosity and water infiltration ^[21].

Figure 1. Soil aggregates and gas exchange within the soil pore system by SEM images.

Soil texture is also a key identifier of soil property not only due to its effects on soil structure but also on soil functions under different land use and soil management practices such as tillage and compaction. NO emissions were reported to be the highest in coarser soil textures whereas soils with finer pores cause the higher formation of CO₂, N₂O, and CH₄ (under anaerobic conditions) ^[22]. Soil texture further provides structural hot spots for microbial activities but there is no certain proof for the correlation between aggregate sizes and N₂O fluxes ^[15]. In contrast, soil aggregate stability with the soil matrix, compaction, and distribution of organic C fractions are important for C stabilization ^[23] and hence GHG emissions. Microbial activities, root respiration, chemical deterioration treats, and fungi activity cause higher soil GHGs ^[24] depending on soil pH and C/N ratio ^[25]. Soil pH increases have been reported to increase CO₂ and N₂O which are significantly impacted by management and fertilization such as liming and manure. Similarly, N₂O emissions are negatively associated with the C/N-ratio, where CO₂ and CH₄ emissions are positively related to the C/N-ratio ^[26].

Therefore, it can be stated that the addition of N sources may increase soil water retention and compatibility owing to increases in biomass production and C input ^[27] but may also increase GHG emissions ^[13] due to higher microbial activities. Now the question is how the increase in soil health will help to mitigate the climate change issues if improving soil organic matter and soil structure itself will produce higher GHG emissions. For instance, the addition of organic amendments such as manure can improve soil health and mitigate climate change issues ^[28]^[29] by providing higher C sequestration capacity than causing increases in GHG emissions. Photosynthesis is one of the most important key processes in which C stabilization is a key property.

3.3. Carbon Stabilization and Storage

The SOC might be stabilized by three mechanisms in soil: physical protection, chemical composition, and biological stabilization ^[23]. Therefore, it is important to understand the C cycle especially before C turns into kinetic form and causes much bigger issues for our planet. Soil C storage is the largest sink of C on the planet with 2500 Pg (petagram, 1 Pg = 1015 g) C in top 1 m soil depth ^[30]. The C loss and soil C sequestration are two components of building this C in soils depending on management practices such as reduced tillage, good quality of pasture, green manures, manures, composts, and other sources of organic matter. The quality and quantity of soil organic matter, therefore, have a critical role in C balance worldwide.

Baldock, Wheeler, McKenzie and McBrateny ^[14] stated two types of the biologically stabilized SOC which are responsible for the biological capability of a particular form of SOC and the indicate biological capacity (decomposition) of SOC depending on biochemical recalcitrance, functional capacity, genetic potential, environmental properties, biochemical reactions, and physical protection of soil. In addition, the most labile organic matter fractions are water-extractable organic materials which are critical sources and influence CO₂ emissions. Stable organic matters in soils, which are resistant to decomposition and stay in soils for a long time, may be referred to as humus. Owing to their roles in soil physical protection, aggregate formation and cation exchange capacity are vital to stabilizing soil organic matter ^[20]^[23]. Due to their role in decomposition, higher microbial communities generated humus over time increasing soil health which provides improvements in healthy crops, yields, profits, stable and good soil structure, and thus C sequestration ^[20]. Therefore, healthy soils do not necessarily decrease GHG emissions but increase C sequestration more.

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Ekrem Ozlu

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