Balancing nitrogen between food production and climate change
Soil health and its pivotal role in growing food has recently met with the need to address another global challenge: climate change.
Providing food is a major source of greenhouse gases, emitting almost 40 percent as much as anthropogenic emissions from fossil fuels in areas not related to food production. Greenhouse gas emissions from agricultural soils are a major factor in this, accounting for 60 percent of agricultural emissions. Only a few years ago, at the UN climate convention in Paris, it dawned on the global community that soils not only harbor the opportunity to reduce emissions but also to sequester carbon. A fact well known among Earth scientists is that several times more carbon resides in soils on a global scale than in all vegetation and the atmosphere combined. Increasing organic carbon in soil by a relatively small proportion would thereby make a large difference to global climate. A much-needed global agenda was born that included not only scientists but particularly policy makers, industry and the farm and ranch community to promote carbon farming.
However, we are still lagging on the climate impact of nitrogen and its greenhouse gas nitrous oxide. Not that we forgot about it, as nitrous oxide is responsible for over a third of greenhouse gas emissions in agriculture that emits over 80 percent of all anthropogenic nitrous oxide. But no coordinated activity focuses on nitrogen similar to what already now exists for soil organic carbon. We therefore suggest the formation of a platform that addresses the climate impacts of nitrogen head-on.
Such an initiative is important as significant synergies and trade-offs exist between food production and climate change mitigation on the one hand, and soil organic carbon sequestration and nitrogen management on the other. Having more soil organic carbon typically improves crop yields, thereby promoting food security and climate change mitigation. This cannot necessarily be assumed to be the case for soil nitrogen management: nitrogen additions are needed to maximize crop yields. But, at the same time soils convert some of that nitrogen to nitrous oxide, a much more potent greenhouse gas than carbon dioxide. And increasing soil organic carbon to improve food production may in itself also increase nitrous oxide emissions, another trade-off involving nitrogen.
We should not be in the position where we are forced to choose between food security and mitigating climate change. In much of the industrialized world there is an excess of nitrogen from over-fertilization, which in addition to being responsible for massive nitrous oxide emissions, leaches into aquatic systems. In California for example, now that policies have successfully limited transportation sources of nitrogen oxide pollution, agricultural soils are now a primary source. In fact, in some places, nitrogen use efficiency is still decreasing, meaning that less and less of the applied fertilizer is actually used by crop plants, and more is entering waterways and the atmosphere. Some estimate that less than one tenth of the nitrogen we add to soils is consumed as food. Yet other regions, especially sub-Saharan Africa, still experience low yields due to insufficient nitrogen supply to crops. By some projections, it would take a 30 percent increase in global nitrogen additions to close this yield gap, which would increase rather than decrease nitrous oxide emissions and exacerbate climate change.
To end hunger, we have to supply nitrogen to crops, and to mitigate climate change, we have to reduce nitrous oxide emissions while increasing soil organic carbon. Are these irreconcilable goals? We do not think so. But they require a similar effort as is now globally being directed to support soil organic carbon sequestration. This requires a concerted effort by science and implementation as laid out for carbon in an article in last week’s issue of Nature Communications: site-specific trade-offs have to be accounted for, which demands a localized soil information system as is already being developed for increasing nitrogen use for crops as part of efforts in digital agriculture, but lacks consideration of nitrous oxide emissions.
The opportunities are largely untapped: IPCC’s Special Report on the impacts of global warming of 1.5 degrees Celsius from 2018 only offers a 20 percent decrease in nitrous oxide from agriculture over the next 80 years. Over the same time horizon, the report highlights much larger options, for example, for decreasing methane emissions from agriculture by over 50 percent. Despite this relatively low ambition on reducing nitrous oxide compared to other greenhouse gases there are many known points of leverage that could be combined to transform nitrogen management. Greater reliance on organic nitrogen management within regenerative agriculture and legumes for biological nitrogen fixation, low-emission amendments, precision agriculture, plant breeding for enhanced nitrogen-use efficiency and nitrogen-fixing crops, and a switch to alternative protein sources, including microbial meat, should all be part of a comprehensive management strategy.
It is very clear that only a joint view of organic carbon and nitrogen management will provide the synergies needed to address both food security and climate change. These goals can only be delivered in an effective way by working from the ground up, starting with those who manage the land and providing the site-specific and dynamic information systems that allow a systems view of soil health management. In our opinion, only a joint platform to address the carbon-nitrogen nexus in soil health management will accomplish that.
Johannes Lehmann is a professor at the School of Integrative Plant Science and at the Atkinson Center for Sustainability at Cornell University.
Deborah A. Bossio is the lead soil scientist at the Nature Conservancy.
Dominic Woolf is a senior research associate at the School of Integrative Plant Science and at the Atkinson Center for Sustainability at Cornell University.