Billen, G., Garnier, J., Grossel, A. et al. Modeling indirect N2O emissions along the N cascade from cropland soils to rivers. Biogeochemistry 148, 207–221 (2020). https://doi.org/10.1007/s10533-020-00654-x
Countries need to keep track of their national greenhouse gas emissions in an effort to meet targets to combat climate change. Even though carbon dioxide gets most of the attention, nitrous oxide is the most potent of the top three greenhouse gases that contribute to current global warming, having 300 times the warming strength of carbon dioxide. In many countries agriculture is the largest source of nitrous oxide emissions. Keeping track of these emissions after they leave the farm fields via water pathways is difficult.
Whole farm modelling takes into account all areas on a farm that are impacted by nitrogen fertilizer application such as soil, groundwater, wetlands, and rivers, and assesses their interactions as a whole system. Models that draw upon high quality datasets are essential in developing practical strategies to ultimately reduce agricultural greenhouse gas emissions. Managing nitrous oxide emissions that incidentally occur as a result of agricultural practices is especially difficult, yet important, to know. A study by Gilles Billen and colleagues from Pierre and Marie Curie University assessed the contribution of nitrous oxide emissions from different areas within the large Seine agricultural watershed located in France. The team drew on field data collected in previous studies from different areas within the farm landscape in order to construct a model that calculates emissions in the entire catchment.
How does agriculture produce nitrous oxide emission?
Soil and water break down nitrogen, naturally occuring and that of fertilizer, into several compounds. These include our greenhouse gas, nitrous oxide. All crops and pastures need nitrogen to grow at their best, so nitrogen is readily applied to farm fields in the form of synthetic fertilizer for crops and animal manure deposited on pasture during grazing. Not all of this nitrogen is used by plants or stored in soil, so some of it is lost as gaseous nitrous oxide. As a consequence, this abundance of agricultural nitrogen produces higher amounts of nitrous oxide gas than we would otherwise observe in natural ecosystems.
Direct and Indirect emissions
Nitrous oxide emissions from agriculture and other industries are separated into direct and indirect emissions. Direct emissions are those produced on the farm, where soils contribute the largest amount of nitrous oxide. Indirect emissions are linked to agricultural practices but occur outside of the farm. Examples include nitrous oxide emissions from fertilizer nitrogen that finds its way into groundwater, field drains and ditches, wetlands, streams, rivers, and lakes. In the absence of local data, the Intergovernmental Panel on Climate Change (IPCC) provides default emission factors which can be applied to estimate indirect nitrous oxide emissions from ecosystems connected to agricultural runoff water. However, the magnitude of indirect nitrous oxide emissions from agricultural catchments can be different due to local factors that influence nitrous oxide production, e.g. soil type, land use, hydrology, climate, and the amount of fertilizer applied. Given this, it is no surprise that the largest source of uncertainty in regional agricultural nitrous oxide inventories stems from these “indirect” emissions.
Tracking agricultural nitrogen from farms to rivers
This study developed a framework to track the fate of agricultural nitrogen from farm soils to the river. Like all catchments, the landscape is an interconnected continuum of soils, vegetation, and water ecosystems. Past research has struggled to take into account the dynamic interactions between these systems. Here, the authors accounted for nitrous oxide production and emissions from watershed soils, riparian wetlands, and the river surface using data collected from their previous studies to develop their model. Once the model was developed, researchers had the necessary tools to upscale emission estimates across regional to national scales – in this case the Seine watershed (France).
Riparian wetlands are an important nitrous oxide source
Researchers used the model to calculate that indirect emissions represented 21% of total agricultural nitrous oxide emission over the entire Seine watershed. Riparian wetlands were an unproportionally high source of these emissions. Considering that riparian areas only make up 7% of the watershed land area, they contributed 17% of watershed wide nitrogen emissions . Another insight was that only 1-2% of nitrous oxide produced in the soil is collected by water pathways (e.g. groundwater and drainage systems) and emitted away from the farm. Therefore, riparian wetlands, often hailed as best management practices to intercept polluted farm water and remove its nitrogen, produce large amounts of nitrous oxide in doing so.
This study highlighted the importance of taking inventory of local agricultural nitrous oxide emissions to ensure national agricultural emissions are not over- or under-estimated using IPCC guidelines. In this case, indirect emissions were around the same compared to previous estimates calculated using IPCC default emission factors. Conversely, total direct emissions were 26% less than originally calculated by the IPCC, which meant an overall reduction of 2,137 ton-N2O per year using the more accurate model. In another recent study, researchers revealed that direct nitrous oxide emissions from agricultural activities in Ireland had also been overestimated by 11% based on new data on local manure and synthetic fertilizer nitrous oxide emissions. Why this variability? Fertilization practices play a huge part in the agricultural nitrous oxide budget and is an area with the greatest potential for cutting back emissions.
Local watershed nitrous oxide budgets have their own story
A key part of achieving sustainable agriculture requires continued efforts to reduce the greenhouse gas impact of the agricultural industry as a whole, and research that accurately tracks the sources of greenhouse gases will help identify target areas for mitigation action. Globally, food production farms are the single-largest source of manmade nitrous oxide emissions due to the nearly 10-times increase in the use of nitrogen fertilizer since 1961. The Ieading Intergovernmental Panel on Climate Change estimates that on a global scale, 1.75% of nitrogen applied to farms is emitted as nitrous oxide from both direct and indirect sources, with indirect sources representing ~43%. This study found that indirect emissions contribute only half as much, at 21% of combined emissions. This finding highlights that under local contexts, agricultural nitrous oxide budgets can have their own story and should be managed as such for the ultimate chance in reducing nitrous oxide emissions.