While reactive nitrogen is important for life on this Planet, it can also react with the environment causing a cascade of negative impacts for both people and ecosystems. This report by Jessica Shade, PhD, The Organic Center, takes a deep dive into the impacts of nitrogen misuse and how organic practices can help prevent the global accumulation of an overabundance of reactive nitrogen
While most climate change media coverage focuses on carbon emissions, nitrogen is a critical – if often overlooked – part of the equation. Nitrous oxide (N2O) is a potent greenhouse gas, with over 300 times the global warming potential of carbon dioxide.
Agriculture is the largest source of N2O, contributing over two thirds of N2O emissions. Synthetic fertiliser application on conventional crops (especially corn and soybeans) is one of the leading sources of N2O emissions in agriculture, because excess application of nitrogen fertilizer can lead to increased nitrification and denitrification, which creates N2O as a byproduct and also leads to nitrate leaching into groundwater.
Nitrogen pollution contributes to smog, acid rain, forest dieback, coastal ‘dead zones,’ biodiversity loss, stratospheric ozone depletion, and climate change. These environmental problems are the result of an overabundance of reactive nitrogen due to increased human-caused nitrogen fixation. Fixing of nitrogen by humans has by far surpassed natural nitrogen fixation, and is currently seven times higher than reactive nitrogen inputs from natural sources.
What does organic have to do with nitrogen pollution?
Organic does not use synthetic fertiliser. All the nitrogen on organic farms comes from recycled sources like compost/manure, or a small amount of new reactive nitrogen from nitrogen-fixing bacteria in the roots of cover crops or other legumes. Those same sources also build a complex and rich soil able to hold onto nitrogen longer rather than just allowing it to run off the field.
Organic farming practices range in their impact on nitrogen pollution, but many common organic strategies, including crop rotations, composting of plant and animal materials, and the prohibition of synthetic substance application to crops, result in decreases in nitrogen pollution. Crop rotations reduce N pollution by preventing nutrient stripping from soil, and composting plant and animal materials improves N recycling, reducing the introduction of new N pollution to the environment with fertilizer usage.
The use of organic soil amendments rather than synthetic fertilisers provide crops with molecularly complex N sources that are slow to release N and therefore limit the loss of N from the field. While these organic management practices decrease the loss of reactive N into the environment, organic farms also use green manure and leguminous cover crops, both of which have the potential to result in increased amount of nitrogen lost to the environment.
Leguminous cover crops protect soil and reduce nutrient runoff and soil erosion, but they also fix atmospheric nitrogen. These cover crops are often tilled into the soil as green manure, which adds further nitrogen into the soil. Moreover, organic practices may result in crop yields that are lower than those of conventional practices because of lower nitrogen availability and greater pest pressure.
What does the latest research say about the benefits of organic?
A recent study out of the University of Virginia shows that the biggest difference between organic and conventional production is that organic farming helps reduce nitrogen pollution by relying on recycled nitrogen sources such as compost and other natural soil amendments. Across all food groups, organic production releases around 50 per cent less new reactive nitrogen to the environment.
Although organic and conventional farms have comparable on-farm nitrogen losses for crops systems, organic farming is recycling reactive nitrogen in its practices instead of adding to the global pool of reactive nitrogen in the environment. This means that not only is organic adding significantly less to the global pool of reactive nitrogen, it can also help put nitrogen waste that could otherwise contribute to nitrogen pollution back into food production. By using manure and food waste as fertiliser, organic helps keep nitrogen losses from other farm systems from entering the environment.
Organic farming:
- Helps reduce nitrogen pollution
- Releases 50 per cent less new reactive nitrogen pollution to the environment than conventional farming practices
- Recycles reactive nitrogen through the use of compost and manure.
Future research needs
While organic has a clear advantage when it comes to preventing new reactive nitrogen from being added to the global pool, the study also highlighted areas where additional research is needed to improve the environmental impact of organic, especially on the local nitrogen loss scale.
With the exception of beef, virtual reactive nitrogen losses in organic crop systems in the US are comparable to those in conventional crop systems. However, the pathway for organic reactive nitrogen losses differs from conventional pathways, with distinctive opportunities for intervening to reduce nitrogen pollution.
Organic has several advantages, including the use of recycled nitrogen, improved ability to access nutrients in the soil, and higher residue recycling rates. However, organic agriculture typically also has lower yields, resulting in reduced crop nitrogen uptake factors. Despite the benefits of organic systems, lower organic yields reduce calculations of crop nitrogen uptake, i.e. the proportion of reactive nitrogen applied to the crop that is taken up by the plant, and thereby decreases the efficiency of the system.
Although organic yields are, on average, lower than conventional yields, they can under some circumstances almost reach the levels of conventional agriculture. Improving yields in organic production and addressing non-nitrogen-related factors that currently limit organic yields (e.g., pest outbreaks, or the lack of crop varieties adapted to organic systems) is thus very important for improving the nitrogen use efficiency of organic systems.
Additionally, while on a per unit land basis the lower density of organic animal production could lead to lower nitrogen loss at the farm scale, organic beef also has a lower nitrogen use efficiency than conventional beef. This is likely due to the low nitrogen use efficiency of pasture, as organic cattle are required to be primarily pasture-raised.
Future research could examine methods for improving nitrogen use efficiency in pasture-raised animals. While improving organic yields and addressing nitrogen use efficiency in pasture will have the most dramatic impact on increasing nitrogen use efficiency in organic systems, there are several leverage points that could shrink nitrogen losses throughout the nitrogen food pathway for both organic and conventional systems.
For example, increased recycling of processing waste beyond field residues could prevent nitrogen losses between the field and the consumed vegetable. Reducing and recycling edible food waste would also have a positive impact on decreasing nitrogen-loss post farm-gate, as well as being critical for addressing a lack of food security.
Another point that could enhance Nr recycling in organic agriculture would be to support the increased integration of crop and animal systems. In addition to matching nutrient flows, integrating crops with animal systems can lead to additional on-farm benefits, including reduced dependence on inputs, improved soil health, and diversified profit streams. However, it has a higher labour cost and may require additional equipment investment.
Source: organic-center.org
