16.4: Emissions Reduction via Agricultural Management

Management of agricultural lands has historically been a major contributor to climate change, amounting to approximately 25% of global greenhouse gas emissions. When plants and animals are harvested from working lands, the associated carbon and nutrients are harvested as well. Fertilizer can replace some of the nutrients harvested, and plant growth can bring new carbon into ecosystems, but rarely do we replace all the carbon and nutrients that are lost. Fertilization, irrigation, and biomass burning, as well as practices that disturb soils such as plowing and tillage, can increase emissions of all three of the major greenhouse gases. Human land use over the last 12,000 years has resulted in the loss of an estimated 116 petagrams of SOC in the top 2 meters of soil, globally. Deforestation, primarily in the tropics, results in the loss of approximately 1.7 petagrams organic carbon per year from ecosystems. In order to slow climate change, and bend the curve, greenhouse gas emissions from working lands must be reduced. There are several possible approaches for reducing emissions that can yield significant greenhouse gas savings, including improved fertilizer, tillage, water, and residue management, as well as matching crops to appropriate soils and climates, and incorporating fallow periods. Improved grazing land and livestock management, together with better manure management, are additional practices that are known to reduce emissions. Taken together at a global scale, these practices have been estimated to have the potential to save over 3 petagrams in CO₂ equivalents (CO₂e) per year. Below, we detail two examples of approaches that can reduce emissions and offer valuable co-benefits.

Nitrogen fertilizer

Nitrogen fertilizer comes in organic and inorganic forms and is widely used in agriculture to enhance plant growth. Inorganic nitrogen fertilizer can be a large source of greenhouse gas emissions, from production to field application. The manufacturing of inorganic nitrogen fertilizer is a carbon-intensive activity. A lot of energy is required to convert dinitrogen gas to ammonia during fertilizer production. In 2004, the fertilizer industry used approximately 1% of the world’s energy, with 90% of that used to produce ammonia. Producing the 119 million metric tons (MMT) of nitrogen fertilizer applied to soils globally in 2018 resulted in at least 492 MMT of CO₂ emissions (values calculated using Statista 2014). This is assuming that natural gas was used in the manufacturing process; if coal was used, the energy cost was higher. To compound the problem, nitrogen is often applied to fields in excess of plant requirements. This extra nitrogen fertilizer stimulates microorganisms in the soils that make nitrous oxide gas, and nitrous oxide emissions increase exponentially with the amount of nitrogen fertilizer added.

At the field scale, there are several approaches that can lower greenhouse gas emissions from fertilizer use. Careful monitoring of plant requirements could significantly lower the amount of nitrogen fertilizer needed for agriculture. There are important co-benefits from this relatively simple action. Less fertilizer applied means that less fertilizer will need to be produced, lowering the carbon footprint of fertilizer manufacturing. Lower fertilizer application rates will also lower nitrous oxide emissions. More efficient fertilization application could save the farmer money, helping to support a more financially sustainable agricultural industry. And finally, less fertilizer use can help reduce nitrogen runoff and pollution of waterways. Some additional ways to lower greenhouse gas emissions associated with fertilizer use include the following:

• Use low-carbon or no-carbon fuels in fertilizer manufacturing.
• Capture biosolids and wastewaters and convert them to nitrogen amendments; this also helps remove nitrogen pollution from waterways.
• Use organic nitrogen and slow-release fertilizer. If the fertilizer is released slowly, it can result in lower emissions and have a lower overall carbon footprint.
• Use buried or drip irrigation. Supplying only the amount of water the plant needs can minimize overwatering that can stimulate nitrous oxide emissions.
• Use nitrogen-fixing cover crops. Nitrogen-fixing plants are species that can pull nitrogen from the atmosphere and supply it to soils. Nitrogen-fixing plants in the legume (pea) family are often used as cover crops during fallow periods (see below). Nitrogen-fixing cover crops can also stimulate nitrous oxide emissions but do not result in the energy costs associated with inorganic nitrogen fertilizer production.

The total greenhouse gas savings from these improved practices have not yet been estimated at a global scale, but models suggest the results will be very promising.

Livestock waste

Animal waste is another large source of greenhouse gas emissions on working lands. Animal agriculture accounts for approximately 20% of the non-CO₂ greenhouse gas emissions, globally. Almost half of these emissions come from manure management, primarily on dairies and feedlots. Manure is generally stored in piles or slurry ponds, which create favorable conditions for the production of nitrous oxide and methane. Storage of manure presents additional problems in the form of biological and chemical hazards to human and ecosystem health. Manure applied to fields can stimulate plant growth but has also been shown to be a large source of soil nitrous oxide emissions and to contribute to nitrate pollution of waterways.

Livestock waste is carbon-rich material. One alternative management for livestock waste is anaerobic digestion. Anaerobic digestion is the process of controlled microbial decomposition under anoxic (no-oxygen) conditions. The microbes that perform the anaerobic decomposition produce methane gas. While methane is a greenhouse gas, it can also be used as a biofuel and can be captured directly from the digestor. Thus, processing livestock wastes through anaerobic digestion lowers emissions from traditional waste storage and produces a valuable fuel source. It should be noted that when the methane fuel is utilized for energy, CO₂ is the by-product. The CO₂ by-product is considered to be carbon-neutral and not a contributor to climate change. This is because the livestock waste was derived from CO₂ recently captured from the atmosphere (via the plants that the animals consumed), and in carbon accounting schemes, this relatively fast cycling of CO₂ is considered to result in no net change in atmospheric CO₂ concentrations.

Anaerobic digestion does not completely decompose the livestock waste, leaving a partially decomposed material called digestate. The digestate is nitrogen rich and can stimulate nitrous oxide emissions if applied directly to soils. Composting the residual digestate can reduce nitrous oxide emissions. Anaerobic digestion may not remove harmful chemicals (for example, hormones, antibiotics) or microorganisms (for example, pathogenic bacteria), thus original feedstocks, such as the livestock manure, and the ultimate digestate must be monitored closely to avoid contamination of soils and waterways. As yet, anaerobic digestion is not widely adopted in the US or globally, but policy and financial incentives can increase the use of this technology. Concerns over costs, reliability, and leakage remain, but this technology holds considerable promise for emissions reduction.