To cut pollution, use more recycled poo

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New research calculates how much animal poo increases crop growth and reduces pollution.

Putting animal waste onto soil has long been a way to re-use and recycle it, but just how effective it is at increasing plant growth compared to synthetic or human-made fertilizers has not been well understood on a large, agricultural scale.

Researchers analyzed 141 studies in which animal waste, including cattle, swine, and poultry manure, was used on crops to replace part or all of synthetic nitrogen fertilizers.

“Synthetic fertilizers are widely used because they supply plants with the available nitrogen they need to produce protein to grow, and to trap energy from the sun via photosynthesis,” says team leader Deli Chen, professor at the University of Melbourne, head of the Soil Research Group.

“But global annual production of nitrogen from livestock manure reached around 100 million [tons] currently, so there is a lot of potential for agriculture.”

The study found that crop yields can go up by 12.7 percent when animal waste replaces between half and three-quarters of synthetic fertilizer. And boosting yields is more important than ever given the Earth’s population is estimated to reach 9.7 billion on 2050.

But the substitution also reduced nitrogen-based pollution in the form of gases and run-off when compared with synthetic fertilizer.

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Using animal waste also led to better storage of carbon in the soils, which means less carbon is lost to the atmosphere as the greenhouse gas carbon dioxide.

Chen and his colleagues suggest that substituting manure for fertilizer may promote the microorganisms in the soil to better immobilize or “lock up” the nitrogen and carbon in the soil. This immobilized nitrogen would then be available across the growing season for the crop.

“We have all seen the images of the destruction from blue-green algae in our rivers. The cost to rectify this ecological disaster is enormous. Another well-known example is excessive nutrients from fertilizers running off into the Great Barrier Reef,” says Chen.

Around 50 percent of nitrogen applied to cropping farmland is lost to the environment, and that increases to around 70 percent in cattle feedlots. This creates one of the biggest issues in agriculture today—nitrogen pollution.

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These results, published in Environmental Science & Technology, emphasize the importance of adopting an appropriate substitution rate for producing higher yields with lower nitrogen pollution.

The team found that when manure is used instead of synthetic fertilizer, ammonia emission decreased by 27 percent, nitrogen leaching into groundwater decreased by 29 percent, and nitrogen runoff also decreased by 26 percent.

The manure also provides other nutrients such as phosphorous, potassium, and essential micronutrients for crops.

In addition to agronomic and environmental benefits, the use of animal waste to complement synthetic fertilizers is of huge financial benefit to food producers, says Chen.

But there are some restrictions. Some crops including grains and vegetables did not grow as well with a pure manure substitution, and Chen found that these crops may need a little “kick start” with synthetic nitrogen. But 75 percent of fertilizer can still be based on animal waste.

These results emphasize the importance of adopting the right manure substitution rate for producing higher yields with lower nitrogen pollution.

The next step for the team is to maximize the amount of nitrogen that can be recovered from animal waste.

In a previous study, they have already shown that adding brown coal to the animal waste reduces nitrogen loss in the form of ammonia. The results show that the nitrogen retained would have the fertilizer value of AUD$49 per cow per year. With an average feedlot having approximately 20,000 cattle, that would be equivalent to around one million Australian dollars each year.

The researchers will further test this technology by partnering with modern intensive dairies, piggeries, and chicken farms.

“Here we are applying the motto ‘use it or lose it.’ If we see animal waste as an industry resource, we can prevent it being lost to the environment,” says Chen.

Coauthors of the study are from the Chinese Academy of Sciences’ Institute of Soil Science.

Source: University of Melbourne