Fertilizer use and emissions in agriculture

Agriculture is a significant emitter of greenhouse gases, but agricultural soils are also an important carbon sink. Emissions of greenhouse gases are mostly related to the following activities:

CO₂ emissions: CO₂ from the applied urea and ABC, land conversion to cropping, use of agricultural machinery, livestock production;
N₂O emissions: Use of nitrogen fertilizers, manures and nitrogen-fixing legumes, as well as microbial conversion of other nitrogen sources in agricultural soils;
Methane (CH₄) emissions: Livestock and irrigated rice production.

With regard to climate change, inappropriate nitrogen management may increase the atmospheric concentration of N₂O, a greenhouse gas considered 296 times more potent than CO₂. The Intergovernmental Panel on Climate Change (IPCC) estimates all N₂O emissions from soils at some 2.1 Pg CO₂-eq per year (of which about 605 Tg CO₂-eq per year related to mineral fertilizer use). (Smith et al. 2007).

It is important to manage all sources of agricultural nitrogen in an integrated manner because the nitrogen from manure and nitrogen-fixing legumes also contributes to N₂O emissions. Reduction of N₂O losses should not result in increased losses of other N compounds (e.g. NH₃ , NO_3^-) to the environment.

The conversion of forest or natural land to cropping results in an estimated 5.9 Pg CO₂-eq emissions annually. This is nearly three times the annual N₂O emissions from soil, underscoring the importance of preventing land conversion from forests to crops.

When managed properly, fertilizer use fosters carbon capture

To obtain a more complete idea of the greenhouse gas emissions performance of the fertilizer industry throughout its products’ life cycles, it is important to weigh emissions against the energy and carbon capture that fertilizer use promotes. When fertilizers are used properly, they help plants to produce more energy than is consumed during the production, transport and application of fertilizers. They also encourage the conversion of carbon dioxide to biomass through photosynthesis, although the length of time during which the carbon is bound will depend on whether the biomass is used immediately, ploughed into the soil, part of a perennial plant or used for bioenergy/biomaterials.

Using Fertilizer Best Management Practices (FBMPs) to improve agricultural GHG balances

The IPCC has identified three ways to make agriculture’s overall carbon balance more positive. Appropriate fertilizer use can contribute to all three.

• Reducing emissions of greenhouse gases

Agricultural releases of CO₂ , N₂O and methane (CH₄) can be reduced through better management of carbon and nitrogen flows in agricultural ecosystems. Improving nutrient use efficiency is key to achieve these reductions. The fertilizer industry promotes Fertilizer Best Management Practices (FBMPs) in many countries to help farmers reduce emissions from fertilizer use. In Canada, the industry is working with government officials to develop a protocol through which improved fertilizer management can be translated into carbon credits for farmers.

• Enhancing CO₂ removals from the atmosphere

The net flux of CO₂ between agricultural land and the atmosphere is currently estimated to be more or less in balance, with net CO₂ emissions at around 40 Tg CO₂ per year (IPCC 2007). Concerning future mitigation, the IPCC estimates that some 89% of agriculture’s future mitigation potential is based on soil carbon sequestration, 9% on reducing CH4 emissions, and only 2% on reducing N₂O emissions from soils (UNFCCC 2008). Judicious fertilizer use helps increase cultivated soil carbon reserves by increasing the photosynthetic conversion of CO₂ to biomass that is subsequently converted, in part, to soil organic carbon. The potential gain could be very large in the case of degraded soils, as in much of Sub-Saharan Africa, with positive knock-on effects for other sustainable development priorities, including land rehabilitation, food security and, ultimately, poverty alleviation.

• Avoiding (or displacing) emissions

There are two techniques for avoiding emissions. The first is to increase productivity and thus prevent conversion of additional land to cropping and the significant emissions that result. The second consists of using agricultural products as bioenergy sources or as raw materials for industrial processes that would otherwise use fossil fuels as feedstock.

“An effective way to reduce emissions associated with the conversion of land to agriculture is by intensifying agriculture, that is, producing more crops on land already in production” (UNFCCC 2008). In this scenario, the overall use of agricultural inputs such as fertilizers would increase, but improved management practices would make it possible for overall GHG emissions per unit of agricultural output to be lower than if the additional crops were produced on land newly brought under cultivation.

A corollary of increasing food production per area of land is the use of agricultural crops as feedstock for bioplastics, pharmaceuticals and other industrial processes. Biofuels are one of the best-known examples. The overall implications for greenhouse gas emissions will depend on knock-on effects for food production and land-use change, whether transport distances increase and several other factors.

Fertilizer Best Management Practices (FBMPs) are essential in order to minimize unwanted impacts of intensified agriculture. They also increase the resilience of agricultural systems to the expected impacts of climate change. For example, higher levels of soil organic matter (carbon) enhance the ability of soils to retain moisture.

IFA has developed a global framework for the elaboration and adoption of site- and crop-specific FBMPs that can result in lower N₂O emissions during crop production, and in greater carbon sequestration in agricultural soils, among other positive outcomes.