A global analysis finds that nitrogen fertilizers could be used more efficiently if their international distribution across croplands was altered — a measure that would also decrease nitrogen pollution.
Global crop production must double from 2005 levels by 2050 to meet the growing demand for food and biofuels1. Nitrogen is the main nutrient required for crop growth, and therefore needs to be added to boost crop yield — usually in the form of fertilizers and manure, but in some cases through nitrogen fixation by leguminous crops. However, on average, only about 42–47% of the nitrogen added to croplands globally is harvested as crop product2,3. Most of the rest is lost to the environment, where it poses threats to human and ecosystem health on local to global scales. Writing in Global Biogeochemical Cycles, Mueller et al.3 assess regional differences in nitrogen use around the world, and propose that reallocation of nitrogen resources would help to increase global nitrogen-use efficiency.
Crop yields respond to nitrogen addition differently in different regions2. In areas where farmers add little nitrogen to croplands — sub-Saharan Africa, for example (Fig. 1) — yield is limited by nitrogen availability, which means that most of the nitrogen added to croplands is converted to crop products (Fig. 2a). But in regions that have high nitrogen-fertilization rates, such as China, yield is no longer limited by this nutrient. As such, additional input has a limited, or even negative, impact on crop yield (Fig. 2b); most of the added nitrogen is lost as air and water pollution. Other regions, such as Brazil and the United States, have favourable climate and soil conditions, or farms that use advanced technologies and management practices. In these regions, a high percentage of added nitrogen is converted to crop products even at relatively high nitrogen-fertilization rates (Fig. 2c) — although the fraction lost to the environment is often still sufficient to have undesirable effects on air and water quality.
Using the historical record of nitrogen use for crop production from 1961 to 2009, Mueller et al. quantify the yield response to nitrogen input for 12 major world regions, and identify the optimal nitrogen allocation among regions that would maximize nitrogen-use efficiency for a given production target. The authors find that the spatial allocation of nitrogen became less efficient over the study period, and that nitrogen lost to the environment could be mitigated by 41% if the allocation among regions was optimized. Even greater mitigation is possible if nitrogen resources can be efficiently reallocated on a subregional scale.
But is internationally coordinated nitrogen use a realistic proposition? Could, say, Chinese farmers be convinced to apply less nitrogen to croplands, and the amount saved then be used in sub-Saharan Africa or Latin America? No intergovernmental mechanisms have been developed or are under consideration for this goal, but the increasingly interconnected global market might provide opportunities to promote the efficient allocation of nitrogen for crop production. For example, China imports about 70% of its soya beans from the United States and Brazil (according to data from the Food and Agriculture Organization of the United Nations; http://faostat.fao.org). The latter two countries produce this crop at much higher yield and nitrogen-use efficiency than does China itself2. Much more nitrogen and cropland would be needed for China to produce all of its own soya beans.
But although international trade might enable a more efficient allocation of resources across national boundaries, it could have unintended environmental consequences. China's importation of soya beans, for instance, has contributed to deforestation in the Brazilian Amazon4. Moreover, the main use for soya beans in China is in animal feed, so most of the nitrogen contained in the imported beans ends up in manure, adding to this country's nitrogen pollution.
A heavy reliance on foreign crop production can also expose importing countries to other risks. For example, the Middle East relies on wheat imported from Russia, but, in 2010, a heatwave in Russia led to a drop in wheat production and restrictions on the export of this crop. This increased the market price of wheat, which probably helped to spark the Arab Spring5. So the reallocation of crop production — and the associated nitrogen use — through international trade can affect food and national security. It could even affect energy security if the increasing use of crops to produce biofuels continues.
In theory, international trade policy could discourage fertilizer subsidies in countries that overuse them and encourage such subsidies where farmers currently cannot afford fertilizer. It might also be possible for international policies to concentrate crop production in exporting countries that use nutrients efficiently, but the complex consequences of this would be difficult to predict using available knowledge. In all cases, it will be crucial to evaluate how international trade affects the efficiency, efficacy and resilience of agricultural production.
Coordinating nitrogen use on sub-national or finer scales seems practical, but the rate at which nitrogen is applied to croplands is usually determined to maximize profit, rather than nitrogen-use efficiency6. Financial incentives and regulatory policies7, as well as outreach efforts by governments and scientists, can persuade or require farmers to consider how their management decisions affect downstream and downwind ecosystems through nitrogen pollution, and thus to optimize nitrogen management for both profit and efficient nitrogen use8.
A benefit of Mueller and colleagues' proposal for the efficient reallocation of nitrogen is that it would not require any additional development, or adoption of farming technologies and management practices. The opportunity they present therefore seems too good to miss. But to truly optimize environmental and socio-economic benefits, nitrogen reallocation would need to be accompanied by the allocation of other resources, such as phosphorus and water, and by the adoption and development of new technology9 (Fig. 2d). A holistic approach that considers how all of these changes would affect the environment, the economy, society and the resilience of the food-supply system is therefore needed to develop nitrogen reallocation as a strategy for sustainable agricultural production.
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