Agriculture and Pollution

If you've ever driven by a livestock yard and gotten a whiff of that particular smell, then you've experienced agricultural air pollution first hand. However, that scent is actually not the air pollutant we're concerned about. The methane released from cows during digestion is, in fact, an odorless gas, but it's responsible for contributing to greenhouse gas pollution of the atmosphere. Yes, I'm talking about cow farts here, or enteric fermentation if you prefer an academic term.

Of all the human-caused greenhouse gas emissions, agriculture is responsible for a significant portion of total emissions—about 30%¹. Enteric fermentation constitutes a considerable fraction of those emissions. Read on to learn more about the many effects of agriculture on pollution and how we are learning to reduce these negative impacts.

The Effects of Agriculture on Pollution

Agriculture is a type of land use in which an ecosystem is altered to support plant or animal life for food production. This land alteration inherently disrupts the ecosystem's natural processes and cycles, but the level of disruption depends on the management practices in use.

What is Agricultural Pollution?

Agricultural pollution can refer to the many different forms of pollution that result from land uses aimed at food production.

From monocropped fields of corn to pastures of organically raised cattle, nearly every type of agriculture produces some form of pollution to varying degrees.

Types of Agricultural Pollution

Pollution is a broad term that stretches out to include negative effects on three main components of the environment—air, water, and soil. The atmosphere interacts with agricultural byproducts much differently than, say, water, so it's important to understand the differences between the three types of agricultural pollution.

To better understand the many effects of agriculture on pollution, let's dive into the different types of agricultural pollution and the major agricultural pollutants associated with them.

Agriculture as a Cause of Air Pollution

Greenhouse gases are the heat-trapping gases that are essential to maintaining life here on Earth. Without them, we'd freeze. However, these gases have been accumulating in excess in our atmosphere, largely due to human activities like the burning of fossil fuels.

Natural processes of plant respiration and decomposition contribute to seasonal fluxes of greenhouse gases like carbon dioxide (CO₂). That's because plants and soils hold a lot of carbon locked up in the form of organic matter. However, some agricultural practices can release excess greenhouse gases, which contribute to air pollution as this locked up carbon is decomposed and respired out as a heat-trapping gas.

The major agricultural pollutants associated with air pollution are CO₂, methane (CH₄), and nitrous oxide (N₂O). Each of these pollutants are created through different processes in agriculture, so let's take a closer look at how each one is produced.

Fig. 1 - Greenhouse gas emissions from agriculture have been rising as industrialized agricultural practices increase around the world.

Carbon Dioxide

CO₂ is primarily released from two agricultural practices: land-use change and tilling. Deforestation often occurs when land is cleared for agricultural use, and it's the leading agricultural cause of CO₂ pollution². Clearing the land of the carbon-holding biomass results in a release of this carbon in the form of gaseous CO₂. This release also occurs when fields are burned before planting. Similarly, tilling, or breaking up the soil before planting, exposes buried organic matter to the air, where it is more quickly decomposed and respired out as CO₂.

Methane

One molecule of CH₄, or methane, traps about 25 times more heat than a CO₂ molecule when in the atmosphere. There are two principal ways in which methane is released in agriculture: rice production and livestock digestion and waste from ruminant animals.

Microbes involved in enteric fermentation, a type of digestion in ruminant animals like cows, release CH₄ as they break down grasses. Their excrement continues to release CH₄ into the atmosphere as it decomposes.

Furthermore, rice is often grown in flooded rice paddies, which creates anaerobic conditions for soil microbes. With reduced access to oxygen in rice paddies, microbes switch their metabolic pathways and release CH₄ as a byproduct of respiration.

Nitrous Oxide

If you thought CO₂ and CH₄ were strong greenhouse gases, make way for nitrous oxide, or N₂O. It's got over 250 times the warming potential than a molecule of CO₂. This potent gas is a major agricultural pollutant that comes from the application of nitrogen containing agrochemical fertilizers and the decomposition of animal waste.

N₂O is a natural byproduct of the nitrogen cycle, in which atmospheric N₂ is converted into plant available forms of nitrogen. When agrochemical fertilizers are applied to a field, often in excess, microbes begin to break the nitrogen down. This results in the release of N₂O in quantities much greater than what is normally possible within natural ecosystems.

Similar to agrochemical fertilizers, animal waste is high in nitrogen. This is why manure is often applied as a fertilizer on organic farms. While the manure is indeed from organic sources, it can introduce excess nitrogen, and microbes release N₂O into the atmosphere as they decompose the animal waste.

Agriculture as a Cause of Water Pollution

Runoff from agriculture contributes to water pollution in surface waters (e.g. streams, rivers, and lakes) as well as in groundwater. Water that enters agricultural land either through rainfall or irrigation can suspend pollutants and transport them in water runoff. The major agricultural pollutants in water include agrochemicals, animal waste, and sediments.

Agrochemical Runoff

Agrochemical pesticides and herbicides are applied to fields to deter pests like insects and weeds. Since these agrochemicals are formulated to kill certain forms of life, they are a major pollutant when they are washed away with water runoff. As they are deposited into lakes and rivers, they can accumulate and become toxic to aquatic life.

Agrochemical fertilizers are used to increase plant growth, but they are often applied in excess. Nitrogen and phosphorus fertilizers can be washed out of the soil with runoff and deposited downstream. Eutrophication can occur when nitrogen or phosphorus from agrochemicals enrich a body of water beyond natural levels.

Phytoplankton in the water quickly utilize the nutrients and proliferate in algal blooms. Their uncontrolled growth depletes the water of oxygen, leaving fish and other aquatic organisms to die. In addition, these algal blooms can release byproducts that are toxic to humans.

Fig. 2 - Signs of eutrophication in the Caspian Sea. Notice the green and turquoise coloration from algal blooms occurring in the North, where a river brings contaminated agricultural runoff into the sea.

Animal Waste

Similar to the nutrient enrichment that occurs from agrochemical fertilizers, animal waste can contain high concentrations of nitrogen. As runoff water carries manure away and into surface waters or groundwater system, nitrogen is released from the manure and left to accumulate. Once again, eutrophication will take place. Animal waste can also contain other contaminants and pathogens.

Agriculture as a Cause of Soil Pollution

Soil is an indispensable natural resource for agriculture. This is true for both crop production and livestock raising, as animal feed will be impacted by the health of the soil in which it is grown. Agriculture can pollute soil through two primary processes: the accumulation of contaminants in soil and the loss of soil through erosion.

Contamination

Soil contaminants like salts and heavy metals can accumulate from the use of agrochemicals and irrigation. Soil salinization can occur when waters used in irrigation are high in dissolved salts. These salts are left behind to accumulate when the water evaporates. Heavy metals can enter soil through the application of agrochemicals. As metals and salts accumulate to toxic levels, soil microbes and plant growth can be negatively affected.

Erosion

Erosion or loss of soil is initiated by agricultural practices that physically disturb soil structure, such as tilling. Breaking apart soil aggregates makes soil particles more susceptible to being lost to wind or water erosion. Some forms of contamination from agrochemicals can also contribute to weakening soil aggregates.

Soil transported by water runoff can accumulate around water sources and contribute to water pollution. Excess sediment becomes suspended in water and can block out sunlight, constricting aquatic plant growth.

Fig. 3 - Sediment deposits from erosion and runoff of nearby agricultural fields are constricting a stream in Tennessee.

Major Agricultural Pollutants

Here's a recap of the types of agricultural pollution and their associated major agricultural pollutants.

Ways to Reduce Agricultural Pollution of Water

While agricultural pollution has been increasing, there are many efforts underway to reduce these impacts. Agricultural water pollution is of particular concern because contaminated water sources can severely impact both human, animal, and aquatic populations' health.

Fewer Agrochemicals

One solution is to scale down the use of agrochemicals, which should only be applied in appropriate amounts at the proper time of year. This can diminish agrochemical runoff and prevent eutrophication downstream. Similarly, animal waste and be composted before field application to more closely control the input of nutrients into the soil. Pesticide use can also be reduced with integrated pest management, where plants that naturally deter pests are planted alongside cash crops³.

Riparian Buffers

Riparian buffers are areas of vegetation planted around water sources to provide a natural buffer from agricultural pollution. They reduce the runoff flow of agrochemicals entering a water source from agricultural fields, and they provide habitats for native plants and animals. The roots of trees and shrubs soak up some of the excess nutrients before they can pollute the water.

Fig. 4 - A riparian buffer around a stream in Iowa slows the flow of runoff from the surrounding agricultural fields.