Nitrogen sustainability - a race against time

Take too much fertiliser, an inefficient crop, not enough land into the future to feed the world and more than a pinch of economics - and you have a recipe for disaster. Thankfully, Brent Kaiser* is quietly confident that scientists like him can contribute to a more sustainable future.

“Houston, we have a problem” … it is called nitrogen and most of it is ending up down the drain. The nitrogen in question comes from the excessive fertiliser used worldwide to feed the plants that feed us, and the problem has arisen because, like many other things involving humans in our post-modern, post-industrial and post-sanity world, the whole natural balance is out of whack.

A costly problem

In an ideal agricultural world, plants would get all the nitrogen they need from the soil, helped by the host of clever bacteria that can ‘fix’ nitrogen from the atmosphere or convert it into a plant-friendly form from decomposing plant and animal matter. However, over the last 60 years or so of humans striving for stronger plants and higher-yield cropping, the ideal has faded in terms of the natural nitrogen cycle.

We have bred nutrient-hungry monoculture crops like maize, wheat, rice and canola to be increasingly dependent on an organically rich soil that exists quite rarely in nature. To thrive and produce the food we require, these crops now depend almost entirely on what we put into the soil.

These higher-input requirements ultimately drive an increasing demand for inorganic nitrogen-based fertilisers that are cheap to produce and easy to use. Unfortunately, the uptake and use of such fertilisers by most crop plants is inherently inefficient due to a combination of agronomy-, soil- and plant-dependent factors, resulting in higher amounts needed more often in an ongoing and vicious cycle of cost and waste.

Indeed, after seed, nitrogen fertiliser is the single-biggest dollar cost to crop farmers - increasing the efficiency of nitrogen use by plants, even by 10%, would save farmers a ‘ton’ of money.

The environmental costs associated with our ever-increasing fertiliser use are also large, although harder to quantify, as Associate Professor Brent Kaiser from the University of Adelaide attests.

“The numbers are very scary on how much nitrogen we use to grow what we eat,” Kaiser said. “Every year that we put nitrogen fertilisers into the ground (~120 million tonnes per annum globally), we are putting more and more reactive nitrogen into the environment.”

The problem is that a typical cereal crop will take up only 30-50% of the nitrogen supplied in fertiliser. While soil microbes compete with some of the remaining nitrogen, a significant amount is either released into the atmosphere as nitrous oxide, a major greenhouse gas, or simply leached into the groundwater in the form of nitrates. From there it seeps into the rivers and streams, and eventually into the sea.

“Of course, other organisms down the chain can use this extra nitrogen. However, very efficient and competitive users like algae thrive on it, leading to eutrophication of aquatic ecosystems. Combined with extra phosphorous in the soil solution that is also coming out into the groundwater, the conditions can result in huge algal blooms that eventually make the water so hypoxic that fish and other sea life struggle to survive,” explained Kaiser.

The annual release of agriculturally derived nitrogen, combined with other forms of post-industrial nitrogen pollutants (nitrogen oxides) caused by the burning of fossil fuels, has the potential to create significant long-term environmental havoc.

Managing a nitrogen diet

For the last 10 years Kaiser and his team have focused on nitrogen transport and defining the biological mechanisms by which plants take up nitrogen and move it around between plant cells and tissues as they grow and produce seed. Basically, they are trying to make agricultural crops better managers of their own nitrogen diet.

“We have a basic idea of how nitrogen gets into the root and is distributed within the plant," Kaiser said. "However, we have a poor understanding of how this process is regulated at the molecular level and where better efficiencies in nitrogen use can be achieved as the plant grows, senesces and ultimately produces its final yield.”

According to Kaiser, the post-genomic era has revealed a lot more than we used to know about plant genes and systems, particularly with respect to how plants use nutrients. It has also provided us with tools to investigate quite complex plant regulatory systems.

“However, much of the work was and is done in model non-agricultural plant systems, and taking those results to an agriculturally important crop grown in the field has told us that we actually know very little. Many things change when genetic, environmental and management interactions are evaluated together.”

Using a combination of approaches, including traditional gene and protein profiling in plant tissues, expression of cloned transport proteins and whole-plant physiological analysis, Kaiser’s team have embraced maize as their model of choice to focus on improving nitrogen use in agricultural crops.

“As maize is one of the biggest users of fertilisers globally, there is hope this research will contribute to direct improvements at the farm level that improves production while reducing nitrogen use and its potential loss to the environment.”

Mapping the maize nitrogen network

The team is assembling interesting results and ideas about how plants use nitrogen in what they call the nitrogen transport network.

“Our goal is to develop a road map of key genes and proteins involved in nitrogen transport and its regulation - focusing on where they are, when they are off, when they are on across the developmental growth cycle. Then, once we have that road map for our experimental plant, we can start to look at natural variation among maize inbreds and hybrids and how those networks interact and respond to different environmental cues," Kaiser explained.

"Ideally we would like to work out the key genes or pathways associated with the best plant vigour from a low nitrogen input context. So, if we could, for example, identify key genes that are on at a certain stage of maize development or when maize nitrogen demands are high, then we can start to manipulate the nitrogen network to improve uptake, storage and/or remobilisation.”

Simple models have their place

Kaiser also uses model systems such as Xenopus laevis oocytes to work out the transport properties and exact functions of individual genes and proteins involved in moving nitrogen in and out of plant cells.

“This is one aspect that is rather lacking in the field … knowing at a system level what things actually do or can do,” he added. “There is a real efficiency to be gained in taking the systems biology findings back to the protein components to identify functionality.”

For the last seven years, Kaiser and colleagues at The Australian Centre for Plant Functional Genomics, have worked in collaboration with large US seed company DuPont Pioneer whose major interest is finding a trait or traits in maize that will reduce fertiliser requirements by up to 30% while producing the same or increased yields for the grower.

This means less cost for the producer, potentially less pollution and a competitive edge for the company.

Take-home message

“In summary, we are looking for conserved genes that can become a representative signal of nitrogen use in the plant,” said Kaiser.

In this context, Kaiser and his team hope to establish maize as a proper model system for use by others. He is also passionate about convincing other plants scientists that working on a crop that is of direct application is the way to go.

“Experimental biology such as ours can then be immediately and directly applied to a crop that you put in the soil for food production.”

In terms of the original problem, Kaiser is sure that the breeders and basic scientists working closely together will get there in producing more nitrogen-efficient crops. He does remain a little worried, however, when the talk turns to the staggering rise in global population expected within the next 30-50 years … and suggests that, perhaps, it is time to start loving those nitrogen-sustainable crops including chickpeas and algae a whole lot more.

*Associate Professor Brent N Kaiser heads a small research team in the School of Agriculture Food and Wine at the Waite Campus of the University of Adelaide. He teaches viticulture and plant biology and is program coordinator for the wine science degrees within the school.  He completed a PhD in plant biology at the Australian National University followed on with short postdoc placements at the University of Nice-Sophia Antipolis, University of British Columbia and the ANU. His current research interests focus on the sustainable use of nitrogen by crop plants and the symbiotic relationship between legumes and soil bacteria.

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Contributing to global warming

Nitrogen fertilisers contribute to global warming by being a source of greenhouse gases - they increase nitrous oxide emissions from the soil into the air.

This accumulation of nitrous oxide will only decline slowly because of its long half-life (150 years). Nitrogen fertilisers also add to methane emissions by reducing the capacity of soil microorganisms to absorb methane. And they contaminate groundwater.

In addition to this, the production process itself for inorganic nitrogen fertilisers is also a global environmental issue. It is energy intensive and adds to global warming through carbon dioxide emissions.

“Converting atmospheric nitrogen into a form usable by plants is a highly energy-intensive process that has traditionally used fossil fuels as an energy source. So, not only are we using a non-renewable resource that is itself associated with greenhouse gas generation for fertiliser production, we are also generating a humungous amount of greenhouse gases in the process. On the other hand, we need this nitrogen to grow plants to eat!! So we are sort of caught in a not-so-desirable circle.”

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