The coastal ocean is a critical resource supporting economies and communities around the globe. These marine areas are impacted by human activities on land. With a population of 7.6 billion and expanding, humans are levying increasing demand on the global food system. The development of nitrogen-based fertilizers and other agronomic advances of the 20th century have improved agricultural productivity worldwide. However, global cropland budgets suggest that only 40-50% of nitrogen inputs (e.g., fertilizers) are recovered in crop yields meaning that roughly half remains in the environment. Excess or surplus nitrogen in the environment leads to a number of negative impacts including degraded air, soil and water quality, increased greenhouse gas emissions and biodiversity loss. Further, approximately 30% of reactive nitrogen introduced by humans is exported by rivers to lakes or the coastal ocean (Billen et al. 2013), which can enhance coastal eutrophication (Figure 1).
In previous posts, we illustrated how our understanding of the changing chemistry, biology and physics of the ocean has improved in recent decades, primarily through the lens of ocean acidification. The oceans are subject to a suite of many interacting stressors, including nutrient input at the oceans’ margins. The key to addressing co-occurring stressors together is to identify commonalities and develop practical solutions. To achieve goal oriented solutions, close coordination is needed among the ocean science community, other natural and social science disciplines, policymakers and the public. Mueller et al. 2017 propose that efficient cropland spatial allocation of nitrogen resources could substantially increase global nitrogen use efficiency (NUE or the ratio of nitrogen crop yield to nitrogen input) and reduce excess nitrogen in the environment.
How can we reduce excess nitrogen in the environment?
Cropping systems follow the law of diminishing returns with respect to agricultural yields and nitrogen application. For a given system, agricultural yield benefits or responses increase with increasing rates of nitrogen application (higher NUE, more linear response), but at some point yields begin to level-off despite the rate of fertilizer application (thus decreasing NUE at high rates of nitrogen input). There are three conceptual strategies that can increase NUE:
- First, a decrease in nitrogen inputs will increase NUE; however, on a large scales such as regional or global, an increase in cropland would be necessary to satisfy rising global food demand.
- Second, changes to agronomic practices (e.g., better crop management practices and application of fertilizer in the right space and time) could ‘shift’ the nitrogen input yield curve such that a higher level of productivity is achieved for a given nitrogen input.
- Third, distributing nitrogen across regions (e.g., from an area with high nitrogen inputs to an area with lower nitrogen inputs) in order to maximize productivity would result in a more efficient spatial allocation of nitrogen resources.
Where do we currently stand?
Mueller at al. (2017) use regional time-series nitrogen budgets across twelve world regions from 1961-2009 to examine the efficiency of nitrogen use through time. In most world regions, agronomic improvements have led to increased nitrogen yields, particularly in the Former Soviet Union, North America, Europe, South American Soy, and Southeast Asia since the 1960’s. Despite the improvements, global NUE has decreased from an average of 54% (1961-1977) to 47% (1991-2009). The reduction in NUE is related to a decrease in spatially optimal nitrogen application. The decline in NUE creates abundant opportunities for more efficient nitrogen allocation that could both decrease nitrogen inputs and increase nitrogen yields.
There are regional differences related to nitrogen input yields. For example, areas such as the Former Soviet Union and Sub-Saharan Africa, remain on the lower, more linear, end of their respective nitrogen input yield curves (i.e., higher nitrogen yield response to nitrogen input) while East Asia (China and Japan) uses a high rate of fertilizer application with lower marginal returns despite the additional nitrogen input. Optimization of spatial allocation while maintaining global productivity (increasing NUE) would shift a large amount of nitrogen allocation from East Asia and a moderate amount from the Middle East/North Africa, and Central and South America to the Former Soviet Union, Sub-Saharan Africa and North America.
While the math is relatively simple, there are many challenges associated with optimizing or even increasing global NUE. It is not realistic to expect diverse regions with diverse farming systems, overseen by national governments with varying social, economic and political objectives, to fully orchestrate the use of nitrogen. However, net environmental and public health benefits could be leveraged to move toward a greater NUE on a regional or wider basis.
There is ongoing awareness and progress towards a more efficient use of nitrogen on a regional level. Agricultural scientists have shown that nitrogen inputs could be reduced with little yield loss in China. Infrastructure investment and access to credit for farmers is increasing nitrogen use in Sub-Saharan Africa. In the US, state by state and within-state recommendations exist for efficient nitrogen use, with incentives for precision agricultural technologies.
Despite regional progress, it is not known how to align policies to improve NUE across scales. These regional and global challenges require more research and present an opportunity for transdisciplinary science approaches focusing on spatial efficiency and governance of nitrogen inputs across various geographic scales.
Primary Citation: Mueller, N.D., L. Lassaletta, B.C. Runck, G. Billen, J. Garnier and J.S. Gerber. 2017. Declining spatial efficiency of global cropland nitrogen allocation. Global Biogeochemical Cycles 31(2) 245-257. https://doi.org/10.1002/2016GB005515.
Additional Citation: Billen, G., J. Garnier and L. Lassaletta. 2013. The nitrogen cascade from agricultural soils to the sea: modeling nitrogen transfers at regional watershed and global levels. Philosophical Transactions of the Royal Society B 368:20130123. http://dx.doi.org/10.1098/rstb.2013.0123.
Other Envirobites contributions related to nitrogen pollution in coastal waters: