By David Nutt
It might not be a pleasant image, but recycling all the human and livestock feces and urine on the planet would contribute substantially to meeting the nutrient supply for all crops worldwide, thereby reducing the need to mine fertilizers such as phosphorus and dramatically reducing the dependency on fossil fuels, according to a global analysis of nutrient recycling published in Nature Sustainability.
"We have to find ways to recycle the nutrients that are now poorly utilized, and our data shows that there is a lot of it: Many countries could become self-sufficient at current fertilizer use if they would recycle excreta to agriculture," said Johannes Lehmann, the Liberty Hyde Bailey Professor in the School of Integrative Plant Science, in the College of Agriculture and Life Sciences (CALS), and the study's senior author.
The lead author is doctoral student Mariana Devault, and Dominic Woolf, a senior research associate in the School of Integrative Plant Science Soil and Crop Sciences Section (CALS) is a co-author.
The researchers analyzed a large array of datasets retrieved from various databases, including the United Nations Food and Agricultural Organization's FAOSTAT and the International Fertilizer Association's STAT, as well as satellite-based maps, to identify the locations of crops and livestock, and learn which fertilizers, and how much of them, are being used in as many as 146 countries.
"A major aspect of our study is estimating these nutrient flows at the subnational scale in every country prior to making conclusions at the national and global scales," Devault said. "Knowing that the cost of transportation is a barrier for an agronomically sound use of livestock excreta in many areas, we wanted to estimate how much of these nutrients has been poorly used simply to reduce operational costs at the farms. Then, we can imagine alternative ways to better manage the use of livestock excreta locally, which can help improve nutrient circularity nationally and worldwide."
After calculating the locations and quantities of nutrients accruing in excreta from humans and livestock, the team modeled how much of this waste, if recycled, would be needed to satisfy crop and grassland production systems worldwide.
The analysis showed that the global amounts found in human and poorly utilized livestock excreta represent 13% of crop and grassland needs for major nutrients. National recycling of those nutrients could reduce global net imports of mineral fertilizers by 41% for nitrogen, 3% for phosphorus and 36% for potassium.
The use of recycled excreta, Lehmann said, would have additional benefits, such as diverting waste nutrient runoff from entering local water sources, where it becomes a pollutant—for example, the harmful algal blooms found in the Finger Lakes. Nutrient recycling could also help establish a circular economy between food consumption and agriculture.
"It doesn't make any sense to pollute our environment, especially our waters and soils, and then have not enough fertilizer for agriculture," Lehmann said. "We need to close the loop from poorly utilized nutrients, wherever they come from, and in this paper, we show that taking only two of these feedstock types, animal excreta and human excreta, we could theoretically satisfy all our fertilizer use at present."
Lehmann sees the urgency of meeting global fertilizer needs as a geopolitical issue comparable to that of oil, with the vast majority of phosphorus, a nonrenewable resource, mined in very few countries. Nitrogen, similarly, is expensive and requires a great deal of energy to commercially produce, creating a large greenhouse-gas footprint.
"The basic fact is that any nutrient that we remove in agriculture, and we obviously remove a lot, we have to replenish," he said. "There's no free lunch."
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