The conversation between plants and soil
To a child, soil is just dirt – a home for worms. To a gardener, soil is a collection of organic matter and nutrients. But to plants, soil is a hotbed of chemical activity. And plants don't just observe, they actively participate in this activity.
Plants release chemicals into the soil, called exudates, that tell microbes to turn on or turn off certain chemical processes. Scientists are beginning to understand these signals and are hoping to exploit them to improve the efficiency, sustainability and environmental impact of the trillion dollar agricultural industry.
The three main nutrients plants need to grow are carbon, nitrogen and phosphorus. A plant's carbon needs come from air in the form of carbon dioxide, but the nitrogen and phosphorus needs come from the soil, and often nitrogen is the element in shortest supply naturally – and so to increase their yields, farmers add nitrogen to the soil.
Artificial nitrogen fertiliser is an essential component of an agricultural system that feeds more than 7 billion people, but it comes with a huge environmental cost. As the population grows, and as feeding habits change to more meat-based diets, nitrogen pollution looks set to become even more of a problem.
Combatting nitrogen pollution
"The extent to which we've meddled with the nitrogen cycle globally is astonishing," says Professor Herbert Kronzucker, head of the University of Melbourne's School of BioSciences.
"210 million tonnes of nitrogen per year are taken from the atmosphere and turned into a solid form of nitrogen through human activities. And most of this ends up as fertiliser in agricultural soil.
"But less than half of this can actually be captured by plants. The rest is lost to the atmosphere as nitrogen gas or the greenhouse gas nitrous oxide, or leaches into waterways where it is a major pollutant.
"In the US, more than half of all lakes are badly impacted by too much nitrogen or phosphorus."
But what if instead of adding more and more nitrogen to the soil, we help plants to better use the nitrogen that's already there?
Professor Kronzucker and his colleagues at the University of Toronto, Laval University and the Chinese Academy of Sciences are looking for crops that communicate with the soil in a way that reduces their nitrogen requirements.
"We became interested in the relationship between chemicals from plants and the impact these have on soil microbes," says Professor Kronzucker.
Nitrogen takes many chemical forms. The most useful forms for plant growth are nitrate (NO3-) and ammonia (NH3). Chemical processes in the soil transform nitrogen between these and other forms.
One example is a process called nitrification, which turns ammonia into nitrate. Nitrate is problematic in the soil because, while plants love it, it doesn't stick around like ammonia does. It tends to dissolve in water and is washed out of the soil through rain and groundwater. Also, soil microbes turn nitrate into nitrogen gas, which is useless to plants.
All of these processes are reversible, and eventually nitrogen gas is returned to the soil through another microbial process called nitrogen fixation, but this process is too slow for industrial agricultural systems. Farmers have to keep adding more and more nitrogen, and usually this is in the form of artificial nitrogen fertilisers.
These fertilisers are produced using an energy intensive industrial process that 'fixes' nitrogen gas by turning it into ammonia. This process, called the Haber-Bosch process, was a major factor in the Green Revolution that began in the 1960s and now provides food for more than 7 billion people.
In recent years, human activity has more than doubled the amount of nitrogen entering the Earth's soil. And half of this additional nitrogen is wasted. But Professor Kronzucker says it doesn't have to be this way.
Plants that grow in areas with low or intermittent nitrogen availability produce exudates that can block or enhance soil nitrogen transformations to improve nitrogen take-up when soil nitrogen availability is low.