Scientists make a splash with urine experiment

Scientists make a splash with urine experiment

Not long before dawn, at a dairy farm in Colombia’s Cauca Valley, locals line up to buy fresh milk. But two people in the queue are seeking a less palatable purchase: urine. Twenty litres of fresh, steaming cow urine, specifically.

“Early morning is a good time to visit,” explains Ngoni Chirinda, a CIAT soil scientist whose teammates were among those waiting patiently in line. “Around milking time, the cows are ready to go.”

Procuring several bottles of the prized potion, they seal them up and head back to base.

With the help of the urine, they’re hoping to find out something about nutrients, something about climate change, and something that could be a major new focus of agricultural research.

First, a bit of background.

It will come as little surprise that grazing cattle pee where they eat. And cow pee is high in nitrogen – a vital plant nutrient. That’s why grazing areas are often nitrogen hotspots.

But nitrogen is tricky.

Low doses can improve soil fertility and help crops grow. But high doses can be environmentally damaging. Soil microbes convert excess nitrogen – whether from urine, fertiliser or other sources – into nitrous oxide. It’s a greenhouse gas 300 times more potent than carbon dioxide and a lot worse than methane.

With booming fertiliser use, and livestock production increasing around the world, those microbes are busier than ever. It means N2O – also known as laughing gas – is creeping up on us with something more akin to an evil cackle.

And just to top it off, nitrogen that isn’t converted into N2O can leach as nitrate into rivers and lakes, contaminating drinking water and triggering algal blooms that choke aquatic life.

With so much nitrogen being lost, it’s no wonder Ngoni describes pee-soaked pastureland as a “leaky toilet.”

Fixing the leak – by trapping some of this nitrogen in the soil – could save farmers money on fertiliser, slash greenhouse gas emissions on farms, and improve the health of the landscape all at once.

It’s no easy task, but little by little, scientists are edging closer to the sweet spot.

One way involves the grass Brachiaria. If you’re a regular on the CIAT blog, you’ll be familiar with its ascendance as a star of sustainable livestock systems in Colombia, and its potential for boosting smallholder dairy farming in East Africa.

But the plant has an ace up its sleeve: its roots release brachialactone, a compound that prevents the soil microbes converting nitrogen into N2O. The process has a tongue-twister of a name – biological nitrification inhibition (BNI) – but it means the nitrogen gets trapped in the soil in a form that plants can use as food. Look who’s laughing now.

“We know that different varieties of Brachiaria produce different amounts of brachialactone, meaning they have different BNI capabilities,” says Jacobo Arango, a CIAT forage specialist. “But it’s important to know just how much they can help reduce N2O emissions from livestock farming.”

Enter Ngoni, the rest of CIAT team, and their bottles of pee.

“We wanted to simulate a urination event,” explains Ngoni earnestly, without even a flicker of schoolboy amusement.

First they mixed urine samples from different cows to replicate a real-life scenario in which several animals would be grazing and peeing together. Then they poured the concoction onto patches of two different kinds of Brachiaria, one with high- and the other with low-BNI capacity. They covered the patches with plastic chambers and periodically drew vapour samples to see what was happening to nitrogen emissions.

The patches of high-BNI Brachiaria emitted 60% less N2O than the low-BNI plants. Sure enough, the nitrogen was being trapped, through the activity of brachialactone. The results were published in the journal Soil Biology and Biochemistry.


BNI is probably the plant’s evolutionary response to low-nitrogen soils. Over time it found a way to hold onto whatever nitrogen was available and store it for later use, like putting food in the refrigerator. Now we know just how much urine-derived nitrogen Brachiaria can trap.

Ngoni Chirinda

Soil Scientist, International Center for Tropical Agriculture (CIAT)

This means a few cool things, including one really big cool thing.

Firstly, it’s another feather in the cap of Brachiaria as a potentially important component of climate-smart livestock systems. Also, the fact that BNI is a natural plant function means there’s no need to rely on synthetic nitrification inhibitors, which smallholders can’t afford anyway.

But here’s where the rubber really hits the road.

While brachialactone is unique to Brachiaria, BNI is not. The roots of sorghum and rice, for example, also release compounds that can limit nitrification. If that BNI capacity could be increased, or bred into different crops, it could help increase something called nitrogen use efficiency – the uptake of nitrogen by plants. At the moment NUE is notoriously low in the tropics, meaning that when farmers apply nitrogen as fertiliser a lot of it goes to waste, causing environmental damage.

“BNI could be one of the next big things in sustainable agriculture,” continues Jacobo, a co-author of the urine study.

“Understanding more about it could enable scientists to breed cereals whose roots contain nitrification inhibitors, or activate inhibitors that are normally dormant. That would enable us to make better use of nitrogen across the board and could help wean us off our addiction to nitrogen fertiliser.”

The emerging vision is one of a suite of BNI-enabled crops, helping fix leaks on farmland around the world. Given the potential, it’s research probably worth spending a penny on.


Call to Action

  1. Fund a more in-depth study into the BNI capability of Brachiaria humidicola, the high-BNI variety used in the urine study, and other forage species to see how the nitrogen in applied urine behaves over the course of several seasons, and establish how persistent the plants’ BNI capacity is.
  2. Fund studies to establish BNI capacity of Brachiaria in different types of soil to establish a more detailed picture of how soil type affects BNI function.
  3. Fund studies to build on CIAT’s expertise in Brachiaria research, to establish the economic benefits – and additional environmental benefits – of the the plant’s BNI capacity.
  4. Invest in Future Seeds – CIAT’s new genebank. Understanding more about the genetic basis of BNI in Brachiaria will be vital to realising the potential of BNI in other crops. By constructing a new, state-of-the-art genebank, CIAT will help improve, conserve, and study Brachiaria varieties and other important tropical forages that could be used in BNI breeding efforts. Follow the link to get involved:

The study was conducted at the International Center for Tropical Agriculture, under the LivestockPlus project, which was funded through the CGIAR Research Program on Climate Change, Agriculture and Food Security (CCAFS), with additional support from The United States Agency for International Development (USAID) and the University of California, Davis Research for Innovation Fellowship. It involved close collaboration of scientists from CIAT’s Soils and Landscape Restoration (SoiLS) Research Program, and Tropical Forages Research Program.

The team would like to acknowledge the close and ongoing support of Dr G.V. Subbarao of the Japan International Research Center for Agricultural Sciences (JIRCAS), and Dr. Meryl Richards, CCAFS, for providing valuable insights.

The paper, Biological nitrification inhibition by Brachiaria grasses mitigates soil nitrous oxide emissions from bovine urine patches is published in Soil Biology and Biochemistry, and co-authored by Ryan C. Byrnes, Jonathan Núñez, Laura Arenas, Idupulapati Rao, Catalina Trujillo, Carolina Álvarez, Jacobo Arango, Frank Rasche, and Ngonidzashe Chirinda.