The SWAT MAPS advantage: Precision Nitrogen for sustainable carbon sequestration

Nitrogen (N) plays a critical but complex role in carbon (C) sequestration in soils, influencing both the accumulation and decomposition of soil organic matter (SOM). Depending on how it is managed it can either enhance or reduce the storage of carbon in soil. Proper management of nitrogen fertilization is essential in maximizing soil carbon sequestration while minimizing the loss of carbon as carbon dioxide (CO2) through microbial respiration.

One of the primary ways nitrogen influences carbon sequestration is by enhancing plant productivity. Nitrogen fertilization increases plant biomass production, which in turn leads to higher inputs of organic material into the soil through root exudates, fine root turnover, and crop residues. These carbon inputs provide the raw material for SOM formation, an essential process for long-term carbon storage in soil. When plants have adequate nutrition, their increased photosynthesis results in greater carbon fixation from the atmosphere, subsequently enriching the soil organic carbon pool. (Tiefenbacher et al., 2021)

Sequestration of carbon in SOM is inherently tied to nitrogen because organic matter contains both elements in varying proportions. As SOM builds up, it sequesters not only carbon but also nitrogen, effectively tying up nitrogen in forms that are less available to plants and microbes. While this is beneficial for long-term soil health and fertility, it also means that some nitrogen inputs are locked in stable organic forms and are not immediately available for plant uptake. The process of microbial immobilization is governed by the carbon-to-nitrogen (C:N) ratio, meaning that soils high in carbon but low in nitrogen will experience increased nitrogen immobilization by microbes. As a result, additional nitrogen fertilizers may be required to offset the extra nitrogen that is immobilized while SOM is increasing in soils. (Karimi et al., 2020)

However, it is crucial to avoid over-application of nitrogen, as excess fertilization can decrease carbon sequestration rather than enhance it further by accelerating microbial decomposition of SOM. A key challenge in carbon sequestration is the dynamic balance between carbon inputs and the decomposition of SOM. Microbial activity, which is influenced by nitrogen availability, plays a central role in determining whether soil gains or loses carbon over time. Nitrogen fertilization enhances microbial activity, leading to the accelerated decomposition of SOM and the release of CO2. While this process releases plant-available nutrients that support growth and further carbon input, excessive microbial breakdown of SOM can counteract sequestration efforts by increasing carbon losses from the soil system. (Khan et al., 2007)

The balance between carbon inputs and decomposition is crucial in determining changes in soil carbon content. When nitrogen fertilization is applied in appropriate amounts that match plant demand, it fosters plant growth and organic matter accumulation without significantly accelerating SOM decomposition. However, when nitrogen is supplied in excess, it stimulates microbial decomposition beyond the rate at which carbon is replenished, leading to net losses of soil carbon. Excess nitrogen can tip the balance toward decomposition rather than accumulation, thereby reducing the effectiveness of soil as a carbon sink.

To optimize carbon sequestration, nitrogen application should be carefully managed to match spatial and temporal plant requirements. Precision nitrogen management with SWAT MAPS—applying the right amount of nitrogen to each SWAT zone within a field—ensures that plant growth is not limited by nitrogen deficiencies while also preventing excessive nitrogen levels that would accelerate SOM decomposition. Areas within fields that are nitrogen deficient will experience reduced plant growth and, consequently, lower carbon inputs as well as increased immobilization as the C:N ratio of the soil rises. Conversely, areas within fields with consistently excessive nitrogen levels will exhibit heightened microbial activity, leading to SOM depletion and increased CO2 emissions. By optimizing nitrogen applications to match plant demand and soil conditions, it is possible to sustain agricultural productivity while maximizing carbon sequestration potential in soils.

The SWAT ECOSYSTEM allows for fine-tuning nitrogen application rates and distribution. It can therefore enhance soil carbon sequestration while maintaining or even improving crop yields. Technologies such as soil testing, moisture monitoring, remote sensing, yield monitoring, and variable-rate fertilization enable more precise nitrogen management, reducing the risks associated with both nitrogen shortages and surpluses. Ultimately, managing nitrogen effectively in agricultural systems requires a strategic approach that optimizes fertilization withing zones of varying capabilities for both productivity and long-term soil carbon storage. And long-term carbon storage benefits everyone. 

Karimi, R., Pogue, S.J., Kröbel, R, Beauchemin, K.A., Schwinghamer, T. & Janzen, H.H. (2020) An updated nitrogen budget for Canadian agroecosystems. Agriculture, Ecosystems & Environment. 304: 107046. https://doi.org/10.1016/j.agee.2020.107046 

Khan, S.A., Mulvaney, R.L., Ellsworth, T.R., & Boast, C.W. (2007) The myth of nitrogen fertilization for soil carbon sequestration. J. Environ. Qual. 36:1821–1832 (2007). doi:10.2134/jeq2007.0099

Tiefenbacher, A., Sandén, T., Haslmayr, H.-P., Miloczki, J., Wenzel, W., & Spiegel, H. (2021). Optimizing Carbon Sequestration in Croplands: A Synthesis. Agronomy, 11(5), 882. https://doi.org/10.3390/agronomy11050882