There are a number of sustainability issues getting a fair amount of attention these days: climate change, regional and local food systems, and soil health, to name a few. While this is obviously good, there are also issues that may be getting somewhat less attention than they deserve. And closing the nutrient loop is one of these.
Our current agricultural system, reliant on large amounts of nitrogen and phosphorus fertilizer, raises serious global sustainability issues. Nearly all the phosphorus used in intensive agriculture globally is mined from a finite number of commercially-exploitable rock phosphate deposits. With projected increases in demand of 50-100% by 2050, it has been estimated that global phosphorus reserves may last only 50-100 years.
Meanwhile, while nitrogen fertilizer is created from plentiful non-reactive nitrogen gas, the industrial process that is used (Haber-Bosch) requires lots of energy and has negative climate impacts. And although organic farming doesn’t use synthetic forms of nitrogen, much of the nitrogen used in organic farming comes from animal manures, and thus is ultimately derived from synthetic N, as described in Andy McGuire’s post here.
The widespread use of nitrogen and phosphorus in intensive agriculture has also altered global cycling of these nutrients in ways that can contribute to severe environmental issues. Particularly when applied in excess of plant needs, nutrients can escape from farm fields to surrounding soils, air, and waterways. And even the portion that is harvested in crops and consumed is generally concentrated in the locations where humans and animals reside, with the majority of the nutrient excreted along with wastes.
Closing the nutrient loop includes a wide range of ongoing efforts to make sure that nutrients are applied at times and places that match up well with plant needs. It also includes efforts to recover nutrients in usable form from places in the food system where nutrients concentrate – including wastewater treatment plants, livestock production facilities, compost operations, and food processing plants – and recycle them to cropping systems. By recovering nutrients in more easily used forms than manure, the logic is that we can increase the amount of nutrients recycled, creating a more “closed” system.
Nutrient recovery technologies are still in the process of development, with several different technological approaches in various stages of commercialization. A new factsheet “The Rationale for Recovery of Phosphorus and Nitrogen from Dairy Manure,” from the Center for Sustaining Agriculture and Natural Resources describes in more detail the rationale behind emerging nutrient recovery technologies for diaries. It also provides a broad overview of nutrient recovery and its status. Technologies differ in that some are most appropriately used on specific forms of untreated dairy manure (e.g. scrape, flush), while others are more appropriate when combined with anaerobic digestion (see the figure below). Lastly, the factsheet summarizes some of the major benefits of these new technologies, and challenges that will need to be addressed.
On their own, these technologies will not solve all problems related to nutrients. Nutrients can still be lost to the environment from nutrient recovery products, or from the wastewater that results from most technologies, especially if these are applied to croplands with improper application rates or timing. However, if used as part of an improved nutrient-management strategy, these technologies have the potential to greatly enhance nutrient recycling, by recovering nutrients in more homogenous, predictable, concentrated, and transportable forms.
 Cordell, D., Drangert, J.O., and White, S. 2009. The story of phosphorus: Global food security and food for thought. Global Environmental Change 19:292-305.
 12 Kwh and 1.4-2.6 kg CO2e for every kg N, as estimated by these two sources:
Sutton, M.A., Reis, S., and Bahl, K.B. 2009. Reactive nitrogen in agroecosystems: Integration with greenhouse gas interactions. Agriculture, Ecosystems & Environment, 133(3), 135-138.
Wood, S., and Cowie, A. 2004. A review of greenhouse gas emission factors for fertiliser production. In IEA bioenergy task (Vol. 38, p. 20).