Carbon in Agriculture: The Importance of Soil Quality
Carbon

Plants are one of the most important contributors to global carbon emissions. It is estimated that plants emit about 120 billion tonnes of CO2 yearly, about twice as much as human contributions. However, plants also get a lot of their carbon from the atmosphere in the form of CO2. Carbon is a crucial component of the soil for plants to grow and thrive. Plants take up CO2 from the atmosphere through photosynthesis, which converts it into sugars and other organic compounds used by plants for growth and development. Organic matter accumulates in the soil as bits of decaying plants and animals, which are then broken down by microorganisms and converted to soil organic carbon (SOC). The rate of this process is influenced by local climate and geological conditions. 

Increasing Soil Organic Carbon

Poor agricultural practices have led to the depletion of carbon from topsoils worldwide, and this has caused many historically fertile regions to struggle growing crops. When carbon is lost from the soil, it is emitted as CO2, contributing to climate change. Increasing SOC in cropland globally would not only improve soil fertility and benefit human health but is also seen as a great potential carbon sink that could drastically impact our climate. 

SOC can be increased using a variety of improved management practices. Carbon-based fertilizers, interplanting, diverse crop rotations, cover-cropping, rotational grazing, no-till/reduced-till, manure, and biochar can increase soil carbon content. When soil lacks adequate nutrition, carbon-based fertilizers can rebuild soil organic matter in topsoil while growing crops to their full potential. Carbon-based fertilizers boost primary productivity, increasing the above- and below-ground biomass, depositing organic matter in the soils, and slowly rebuilding carbon. Root and microbial exudates, formed through decomposition, improve soil stability and fertility. Long-term field experiments show soils treated with cattle manure had significantly higher SOC than fields treated with inorganic chemical fertilizers. Additionally, the soils treated with cattle manure had greater microbial activity and potential ammonia oxidation.

Source: Naylor et al., 2020

Figure 1. The microbial loop is a process in which carbon is exchanged among soil, plants, and the atmosphere. This process can be described as a cycle that starts with soil microbes converting organic materials into carbon dioxide (CO2). The CO2 can then be absorbed by plants and converted back to carbohydrates.

Soil Organic Carbon

SOC affects many soil properties, including water infiltration and retention, nutrient availability, and retention. Crops grown in fertile soils have higher nutritional content. Typical productive agricultural soils have 3-6% SOC. Without sufficient carbon, soil structure degrades, and microbial activity declines. 

The optimum carbon-to-nitrogen ratio is 24:1 – this is the ratio that microorganisms must maintain to thrive. At this ratio, decomposition rates are highest, and nutrients are cycled through the soil effectively. When the C:N ratio grows higher than 24:1, decomposition slows down as microbes must work harder to maintain the proper ratio of cellular nitrogen. Soils without a healthy C:N ratio lose the ability to raise crops economically.

Further Reading

Chan, Y. (2008). Increasing soil organic carbon of agricultural land. NSW Department of Primary Industries.

Cho, R. (2018, February 6). Can Soil Help Combat Climate Change? State of the 

Planet. https://news.climate.columbia.edu/2018/02/21/can-soil-help-combat-climate-change/

Datnoff, L. E., Elmer, W. H., & Huber, D. M. (2013). Mineral Nutrition and Plant Disease. APS Press.

Enwall, K., Nyberg, K., Bertilsson, S., Cederlund, H., Stenstrom, J., and Hallin, S. (2006). Long-term impact of fertilization on activity and composition of bacterial communities and metabolic guilds in agricultural soil. Soil Biology & Biochemistry.

Jackson, W. R. (1993). Humic, Fulvic and Microbial Balance: Organic Soil Conditioning. 

Kohnke, H. and Franzmeier, D. P. (1995). Soil Science Simplified. Waveland Press.

Marler, John B. “Biotic Fertilizers: High-efficiency Carbon Sequestration Fertilizers.” Perfect-Blend.com, https://perfect-blend.com/pdf/Brochures/HighEfficiencyCarbon.pdf.

Marschner, P. (2012). Mineral Nutrition of Higher Plants. Academic Press.

Scientific Reports, 11(1). https://doi.org/10.1038/s41598-021-98143-0

Soil Organic Carbon, chapters 1, 2 & 4. Advances in Agronomy. 2013, https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/soil-organic-carbon. Volume 118, 2013 vols.

Witzgall, K., Vidal, A., Schubert, D. I., Höschen, C., Schweizer, S. A., Buegger, F., 

Pouteau, V., Chenu, C., & Mueller, C. W. (2021). Particulate organic matter as a functional soil component for persistent soil organic carbon. Nature Communications, 12(1). https://doi.org/10.1038/s41467-021-24192-8

Yang, C., Feng, M., Song, L., Wang, C., Yang, W., Xie, Y., Jing, B., Xiao, L., Zhang, M., Song, X., & Saleem, M. (2021). Study on hyperspectral estimation model of soil organic carbon content in the wheat field under different water treatments. 

Zomer, R. J., Bossio, D. A., Sommer, R., and Verchot, L. V. (2017). Global sequestration potential of increased organic carbon in cropland soils. Nature.