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Storing carbon in the soil is a nitrogen problem

Since its beginning, agriculture alone has released 116 billion tons of carbon—about half the carbon stored in the soil—as carbon dioxide (CO2) into the atmosphere [1]. To put into perspective, 116 billion tonnes is as heavy as 632 million Boeing 747 airplanes. With the rising urgency of climate change, the race to pull human-caused CO2 out of the atmosphere is ever more intense. Agriculture, specifically regenerative agriculture, is now in the spotlight to restore carbon into the soil. However, soil carbon restoration is limited by another element often overlooked in the context of the climate crisis: Nitrogen.

Glossary

  • Regenerative Agriculture: A farming approach that aims at increasing the stable soil organic carbon through specific management practices (no-till, cover crops and integrating livestock) and reduced synthetic inputs while also gaining additional environmental, economic and social benefits.
  • Biological Nitrogen Fixation: Conversion of nitrogen (N2) in the atmosphere into plant-available form, mainly ammonium (NH4+), by a specialized group of soil microorganisms.
  • Carbon sequestration: Capturing the carbon dioxide in the atmosphere for long-term storage in plants, soils or the ocean to mitigate climate change.
  • Organic Matter: Materials derived from plants (crop residues) and animals (manure) applied in either undecomposed or decomposed forms (compost). When applied to soil as a source of plant nutrients, organic matter can be referred to as organic fertilizers. Organic fertilizers differ from synthetic fertilizers in that the latter is produced industrially from petroleum-derived materials.
  • Volatilization: Loss of nitrogen applied to the soil into the atmosphere through the conversion of ammonium (NH4+; a plant-available form of nitrogen) to ammonia gas (NH3).
  • Precision Agriculture: A farming method that uses advanced technology—including GPS, remote sensing, drones, sensors and software—to support farm management decisions in growing crops more efficiently and sustainably.

Regenerative Agriculture

Regenerative agriculture, as the name suggests, regenerates the soil carbon through agricultural practices mainly no-till—where the soil is left undisturbed by avoiding tillage—growing cover crops, plants grown to cover the soil between the main crops, and incorporating livestock. Applying organic matter and avoiding synthetic inputs (herbicides, pesticides and chemical fertilizers) are also considered regenerative practices. These practices leverage plant and soil-microbe partnerships to pull down atmospheric CO2 through plant photosynthesis, add organic matter to the soil and anchor it as stable soil carbon through microbial action.

The carbon cycle is closely linked with the nitrogen cycle. Nitrogen plays a vital role in the plant and microbe processes that convert CO2 into stable soil organic carbon. In fact, one ton of nitrogen is required to add around 12 tons of carbon into the soil [2]. Without enough nitrogen, plants and microbes cannot effectively sequester carbon in the soil. This need for nitrogen for effective carbon sequestration presents several challenges.

The Nitrogen Dilemma

First, naturally occurring nitrogen sources are limited. Nitrogen is usually the most limiting nutrient in agro-ecosystems. In regenerative agriculture, this limitation is compensated by adding external organic nitrogen sources such as animal manure, crop residue, compost and biological nitrogen fixers (microbes that convert nitrogen (N2) in the atmosphere into plant- and microbe-available forms (NH4+) in the soil). Unlike chemical fertilizers, organic sources are a less concentrated source of nitrogen and there is only so much manure and crop residues available.

Expanding the area under regenerative agriculture for climate change mitigation means a larger requirement of manure to supply the required nitrogen. Increasing livestock to meet the greater demand for manure invariably increases greenhouse gas (GHG) emissions through methane burped by the cattle, applied manure, and emissions from producing feed and fodder for the livestock. Also, manure available on-farm could be insufficient. Sourcing manure from far-flung livestock production areas is an additional GHG source.

The other challenge is that nitrogen is extremely volatile and dynamic in agricultural systems. A large portion of nitrogen added to the soil is lost through runoff, leaching and volatilization. Although the nitrogen runoff with manures is lower than with chemical fertilizers, the problem persists [3,4]. Most of the added nitrogen is washed away to the nearby water bodies, where it pollutes water and stimulates algal blooms, threatening aquatic life. The runoff nitrogen from the fields also leads to the emission of nitrous oxide (N2O). Although referred to as ‘laughing gas’, nothing about N2O is a matter of laughter here; far from it. N2O stays in the atmosphere for over 100 years, depletes the ozone layer, and is about 300 times more potent than CO2 in heating the atmosphere. Therefore, depending on the emission rate, unchecked N2O emissions from the applied manure can at least partially cancel out any benefits gained by carbon sequestration.

Solutions are…complicated

One of the proposed solutions to prevent nitrogen runoff is growing cover crops, a key component of regenerative agriculture. Cover crops cover the soil throughout the year to reduce nitrogen runoff and off-site N2O emissions.

Legume cover crops (for example, clovers, vetches and peas) are also an additional source of nitrogen. Symbiotic nitrogen-fixing microbes in the legume crops add nitrogen to the soil through biological nitrogen fixation (BNF). This symbiosis speeds up carbon sequestration while reducing nitrogen loss. Legumes are more desirable as the nitrogen derived through BNF is relatively more stable (less volatilization, N2O emissions and leaching) compared to manure and compost. However, despite the nitrogen stabilizing effects of legume cover crops, they are no panacea. Legume cover crops can still result in N2O emissions from the soil surface.

Legumes, because of their higher nitrogen content and production of surplus nitrogen, stimulate N2O emissions from the soil when nitrogen is not added externally. Non-legume cover crops result in lower N2O emission, however, unlike legumes, cannot fix atmospheric nitrogen. Mixed cover cropping, with both legumes and non-legumes, is preferable as it significantly reduces N2O emissions [5].

Owing to the critical role of nitrogen in carbon sequestration and the limited availability of organic nitrogen sources for regenerative agriculture, we need more than just additional nitrogen sources for effective sequestration. We need advanced technologies to solve the challenges of nitrogen demands and associated runoff, leaching and N2O emissions.

Slow-release organic nitrogen fertilizers and nitrification inhibitors—chemicals that prevent the conversion of nitrogen into volatile forms—can keep applied nitrogen in the soil for longer and can reduce loss.

The nitrogen requirement for carbon sequestration through regenerative agriculture is also site-specific. Regions with high naturally occurring nitrogen in the soil are better suited to sequestering carbon without high external nitrogen input. Adopting locally appropriate nitrogen management techniques, therefore, is crucial to minimize nitrogen loss and N2O emissions. Precision agriculture technologies can reduce nitrogen surplus in the fields by tailoring the nitrogen application in a site-specific manner. Through precision agriculture, nitrogen can be applied at the right amount, at the right time and in the right place, thereby reducing nitrogen losses.

Cultivation of genetically modified crops is mostly discouraged in regenerative agriculture. This could be a tremendous loss of opportunity for carbon sequestration. Genetically improved crops that need less nitrogen could be beneficial to curb the adverse effects of nitrogen losses. Proponents of regenerative agriculture should consider incorporating genetically improved crops for their potential to aid carbon sequestration and reduce nitrogen requirements.

Cows on a hilly pasture, fenced in with electric fencing
Photo by lou lou on Unsplash

Policy Implications

There are claims that regenerative agriculture alone can sequester enough carbon in the soil to reverse climate change [6]. While this claim is debatable, regenerative agriculture certainly has the potential to sequester a substantial amount of carbon in the soil globally. Regenerative practices also generate additional benefits, such as reduced water usage, increased biodiversity and improved economic wellbeing. Given the potential as a natural climate solution to reduce emissions, regenerative agriculture has caught the attention of policymakers on an international scale. However, large-scale implementation of regenerative agriculture should be considered with some caution.

The critical role of nitrogen is often overlooked when discussing carbon sequestration through regenerative agriculture. The trade-off between increased soil carbon and N2O emissions should be accounted for. Policies aimed towards mitigating climate change through regenerative agriculture should also consider nitrogen as an essential part of the ‘carbon sequestration equation’. Nitrogen management to reduce N2O emissions should be given serious consideration for effective carbon sequestration in soil. Policies should also accelerate comprehensive research and development for integrated and site-specific nitrogen management while incentivizing large-scale deployment of regenerative agriculture.

Researched and written by Ajwal Dsouza.

References

  1. Sanderman, J.; Hengl, T.; Fiske, G.J. Soil Carbon Debt of 12,000 Years of Human Land Use. Proc. Natl. Acad. Sci. 2017, 114, 9575–9580.
  2. Ranganathan, J.; Waite, R.; Searchinger, T.; Zionts, J. Regenerative Agriculture: Good for Soil Health, but Limited Potential to Mitigate Climate Change 2020.
  3. King, K.W.; Torbert, H.A. Nitrate and Ammonium Losses from Surface-Applied Organic and Inorganic Fertilizers. J. Agric. Sci. 2007, 145, 385–393, doi:10.1017/S0021859607006946.
  4. Kirchmann, H.; Bergström, L. DO ORGANIC FARMING PRACTICES REDUCE NITRATE LEACHING? Commun. Soil Sci. Plant Anal. 2001, 32, 997–1028, doi:10.1081/CSS-100104101.
  5. Basche, A.D.; Miguez, F.E.; Kaspar, T.C.; Castellano, M.J. Do Cover Crops Increase or Decrease Nitrous Oxide Emissions? A Meta-Analysis. J. Soil Water Conserv. 2014, 69, 471–482, doi:10.2489/jswc.69.6.471.
  6. Reversing Climate Change through Regenerative Agriculture. Regen. Int. 2018.