Mechanisms of Pollutant Exchange at Soil-Vegetation-Atmosphere Interfaces and Atmospheric Fate

Abstract

The presence in the atmosphere of pollutants emitted from agriculture or impacting crop production, their concentration levels and their residence time depend on a series of physical, chemical and biological processes. This chapter describes the basic mechanisms and the main factors involved in air pollutant emissions and fate in agricultural systems. These include emissions to the atmosphere from fields or livestock husbandry, atmospheric transfers at various scales – from the proximity of the sources to regional and global scales –, atmospheric chemistry and deposition to ecosystem. These concepts are necessary to understand the context and various choices that can be made in terms of measuring air concentrations and emission/deposition fluxes and of modelling emissions, atmospheric fate – dispersion, transport, degradation – and deposition. The knowledge on these processes contributes to modelling the impact of agricultural sources on air quality and the impact of air pollution on ecosystems. It also helps identify levers for mitigating emissions from agriculture and impacts of air pollution on crops.

Appendix

1.1 Fundamental Aspects of Microclimate at Soil-Vegetation-Atmosphere Interfaces, Soil Conditions and Crop Physiology

For further reading, see Guyot (1998), de Parcevaux and Huber (2007), Calvet et al. (2005), Calvet (2013), Heller (2004) or Jones (2013).

1.1.1 Energy Balance

Emission, transport and deposition processes are highly dependent on energy transfers occurring at the soil-vegetation-atmosphere interfaces. Understanding energy transfer processes enables characterization of the carrier fluid (in this case, air) and state variables of the exchange interfaces (surface temperature, humidity, presence of superficial water), which can also be modified by the presence of pollutants (such as hygroscopic compounds, i.e. having an affinity with water). The energy transfers at the interfaces are expressed as energy flux through the interface (W·m⁻²). When the energy flux is positive, the interface receives energy, which generally results in an increase in temperature (de Parcevaux and Huber 2007; Guyot 1998). Conversely, when the energy flux is negative, the interface losses energy and its temperature drops. Energy exchanges are radiative (net radiation, R_n), convective (sensible heat flux, H), conductive (heat flux, G) and phase change (latent heat flux, λE), each term being expressed in W·m⁻². The energy balance at the interface is expressed as:

$$ {R}_n+H+\lambda E+G=\varDelta E $$

(10)

where ΔE is the energy stock at the interface, often considered as null for plants and soils. The terms of the energy balance (detailed below) depend on the overall hydric and thermal states of the soil profile, the regulation processes (evapotranspiration) and the physical properties of the vegetation cover (height, leaf profile, albedo, roughness). Thus, there is constant feedback between the energy transfer and the water status of this surface. The radiative energy fluxes are sorted according to their wavelength. Short-wavelength radiation (< 1 μm) consists of direct and diffuse solar radiation in its positive components (energy input), and a proportion of the reflected solar radiation (energy loss) at the interface. The reflection coefficient of short-wave radiation is called albedo (a). In addition to short-wave radiation, there is also long-wave radiation (above 1 μm) mainly in the infrared spectrum. These different types of radiations are classed by their positive component, atmospheric radiation R_a (responsible for the greenhouse effect), and their negative component, surface radiation of the interface, which is expressed as a function of its temperature. The sum of all these radiative fluxes is called net radiation, R_n. The sensitive heat flux (H) reflects the energy exchanges that modify the interface temperature controlled by the temperature gradient. The latent heat flux λE reflects the energy exchanges that result from changes in the state of water and is driven by the gradient in water vapour concentration between the surface and the atmosphere. Finally, the heat conduction energy flux (G) reflects the heat exchange by contact and is expressed according to Fourier’s law, describing the temperature gradient in the ground. These energy fluxes cause a change in the microclimate (atmospheric water content, temperature change) near the surface.

1.1.2 Water and Energy Transfers in the Soil

The soil moisture status is characterized by its water potential (h_w, m⁻¹); its water content (θ), which can be expressed as weight (kg water/kg soil) or volume (m³ water/m³ soil); or as a proportion of the volume of pores filled by water, depending on the approach and application. It evolves in time and space due to water transfer processes (described by Darcy’s law in water-saturated environments and generalized Darcy’s law or the Richards equation in water-unsaturated environments) in the soil profile as well as exchanges at the surface via the surface energy balance (described above). Similarly, the soil temperature evolves according to heat transfer processes (including conduction transfers, described by Fourier’s law). Since these statuses depend on the depth in the soil, it may be necessary to define layers, or soil horizons, by depth.

Depending on the weather forcing and water saturation of the soil, different hydraulic regimes exist: infiltration, hydrostatic equilibrium or capillary rise. This variability in water transfers is the largest in the thin surface layer of the soil, where the water content changes rapidly. It also implies that the ground surface can be subject to extreme local conditions, without intense meteorological forcing. In addition to local soil and climate conditions, the soil characteristics in terms of texture (sand/silt/clay distribution), density ρ_b (kg·m⁻³) or organic matter (OM) content must be known in order to calculate the soil transfer functions required to calculate the h_w relationships (θ) and the hydraulic conductivity K_w (m·s⁻¹). During a rainy season, depending on the soil, slope, and the presence of crops that can intercept varying degrees of rain, surface runoff can occur and generate horizontal transfers. While water transfers determine the ability of the soil to transport the different solutes in the soil, the water status and temperature of the soil also determine the distribution of compounds between the different phases (gases dissolved in soil water or adsorbed), which in turn determine the availability of the compound for the different transfers (in aqueous or gaseous form) and their degradation.

Finally, it is important to note the high spatial heterogeneity in soil conditions, which may lead to heterogeneity in the soil structure, which can increase preferential transport and lead to rapid transfers of solutes into the deeper soil (e.g. root passage, earthworms, fissures).

1.1.3 Soil Biophysical and Chemical Conditions

Soil condition is characterized by its water status, chemical status and biological status. The pH of the soil solution is one of its important characteristics for physicochemical balances and microbial functioning, as well as its cation exchange capacity (CEC). Soil organic matter is an essential component, as it conditions the chemical fertility of the soil, its structural stability and its microbial functioning. It can also influence water flows by modifying water adsorption processes. This organic matter consists of both inert and living organic matter (root, microbial biomass, mesofauna), which impacts the physical and chemical state of the soil and the biogeochemical transformations that take place there. Many processes and interactions take place in soil, and its state varies considerably, both spatially and temporally (with highly variable scales of space and time). For further reading, see Calvet (2013).

1.1.4 Biological Functioning of Plants and Vegetation Cover

The presence of vegetation cover modifies ambient conditions at the ground-atmosphere interface by creating a microclimate specific to the vegetation cover. A quick review of the general functioning of plants, whether cultivated or part of a natural ecosystem, will be helpful here. Plants are autotrophic organisms. They produce their own organic matter from mineral salts extracted from the soil and carbon dioxide (CO₂) in the atmosphere, which is assimilated by the leaves by using solar energy through the photosynthesis mechanism. Plants need different elements to survive and grow. The first is light, which is necessary for the photosynthesis process, which then provides energy, water and soil from which nutrients are derived; the second is air, from which CO₂ is extracted. Although plants have different morphologies, they all have various organs: a stem bearing leaves and buds, and roots. These different organs are made up of tissues, organized in sets of specialized cells. The roots and the aerial part are two interdependent systems due to the conductive tissues that run through them. The xylem ensures the transport of raw sap (water and mineral salts), the phloem carries the elaborated sap (i.e. the organic and soluble compounds) from the leaves, where photosynthesis takes place, to the storage organs (tubers, roots, etc.) and fruits or grains.

The plant cells that make up these different organs are delimited by the plasma membrane and contain within them the vacuole (used to store water, nutrients and waste), the nucleus, and all intracellular organelles (namely the chloroplasts involved in the photosynthesis process). All of these elements are surrounded by a medium called cytoplasm. Plant cells are connected to each other through plasmodesms, channels that pass through the cell walls of plants to form the pathways for water, solutes, phythormones and phytopathogenic viruses that spread through the plant. The cytoplasm of these connected cells forms one single compartment in the plant, and the intracellular continuum thus obtained constitutes the symplasm. Outside the plant cells, the extracellular continuum formed by all the interstices of cell walls and dead xylem cells constitutes the apoplast. Water and solutes can use this route to infiltrate organs without entering a cell.

Roots allow the plant to be firmly and durably secured in the ground, but their main role is to draw up water and mineral salts that are essential for plant growth. This absorption is achieved through a multitude of absorbent hairs at the root tips, ensuring a very large contact surface at the root-soil interface. Water enters the roots through a hydrostatic mechanism.

The leaves are the main organs at the plant-air interface, with exchanges taking place according to two pathways: the cuticle and the stomata. The leaf generally consists of a petiole and a limb with veins (the conductive vessels) of very different shapes. A protective layer, called the epidermis, covers the plant’s aerial organs. The outermost part of the epidermis that is in contact with the air is the cuticle. The cuticle’s main physiological roles are related to its generally hydrophobic nature: it maintains a water-poor area on the plant surface, which protects the plant from pathogens (including germination and fungal spore development), and limits the loss of water, ions and polar solutions (sugars, organic acids, etc.) from the plant. The plant cuticle is covered by epicuticular waxes (Barthlott et al. 1998). These structures, composed of small, highly variable crystals associated with the presence of trichomes, confer roughness to the cuticle and play an important role in the wettability of leaf surfaces (Koch et al. 2008), while increasing surfaces that are potentially reactive to pollutants. Most of the leaves are green, i.e. with chlorophyll pigments. Chlorophyll is essential for photosynthesis. This biochemical mechanism allows the production of the organic compounds that make up the plant and are essential for its metabolism. It requires not only light, water and mineral salts coming through the xylem from the roots to the leaves, but also a carbon source, CO₂, which the leaf draws from the atmospheric air. CO₂, water vapour, and gaseous pollutants penetrate into the leaves through the microscopic pores of the leaves called stomata, then integrate the cells of the leaf tissues. Stomata open and close according to microclimatic conditions and plant regulation.

Plants play an important role in the surface energy balance. They are mainly made up of water, which constitutes from 80% to 95% of their total weight. They draw this water from the soil through their roots. Foliar transpiration causes sap to rise from the roots along the stems and to the leaves via the xylem using the energy provided by solar radiation. This suction phenomenon is very powerful: coupled with cohesion forces that hold the water column, it brings the water to the top of even the tallest trees. Transpiration is also a key element in regulating plant temperature. The transformation of liquid water to water vapour consumes huge amounts of energy (latent heat of vaporization) at constant temperature, which prevents the plant’s temperature from rising excessively under high solar radiation. The transpired water passes through the stomata, and in the case of water stress, the stomata close, limiting water loss for the plant. Transpiration is a specific and essential manifestation of plant behaviour towards water. Its intensity is such that the quantities of water stored in a plant and those used by its metabolism are very small compared to those that pass through the plant. Evapotranspiration almost exclusively regulates a plant’s water needs.

Plants growing in stands create what is called a plant canopy. Leaves are the main surfaces of exchange with the atmosphere. Their organization in space partly determines the effects of climatic factors: the penetration of wind and radiation into the different strata of the vegetation cover depends on the cumulated leaf surfaces from the top of the cover to the stratum in question. The total cumulative leaf area corresponds to the leaf area index (LAI, in m² of leaves per m² of soil). Similarly, fine roots and their absorbent hairs are the main surfaces of exchange with the soil and a total root area index (RAI, in m² of root surfaces per m² of soil) is defined.

Crop establishment for cultivated species usually begins with seeding. The different physiological stages of canopy growth and development to harvest include foliar development, flowering, fruiting and maturation. However, not all plant organs remain functional throughout the crop cycle: over time, ageing causes slow and progressive degradation of their functions and triggers a physiological process called senescence.

1.1.5 Effect of Agricultural Practices

In the context of agricultural fields, the implemented practices modify the state of the soil in ways that may be short or long term. For example, irrigation changes water levels and soil temperature in the short term, but it can also change albedo or surface roughness. Tillage (ploughing, stubble incorporation, seedbed preparation) causes a mixture of the different layers of soil impacted, thus drastically and instantly modifying its porosity, humidity and temperature. By incorporating any crop residues on the surface into the soil, it changes the local organic matter content and microbial activity in the soil (Boiffin et al. 2020). The energy and water balances of the surfaces may also be altered. Indeed, the energy absorbed by the surface is closely linked to the surface radiative properties (albedo, emissivity): thus, an increase in albedo (e.g. through crop establishment or crop residues) reduces the energy available at the surface and therefore the amount of heat entering the ground. The impact is generally negative on temperature, but this may depend on changes in water characteristics associated with this modification. In addition, these changes in surface properties result in a change in latent (λE) and sensitive (H) heat fluxes and consequently in the temperature of the surface and the surrounding air. In the absence of tillage, crop residues left on the soil lead to two opposite mechanisms: albedo increases while soil evaporation decreases (Ceschia et al. 2017), and surface horizons are generally acidified in relation to the accumulation of organic matter (Benoit et al. 2014). Inputs can also change the state of the environment: manure input, for example, provides a significant volume of water, alters the soil water content, temperature and surface conditions (e.g. albedo) due to the dry matter remaining on the surface. Other examples include the choice of crops, their density and the planting date, which profoundly modify the surface characteristics and temporal dynamics, as well as the energy and water transfer.