Tuesday, 9 October 2012

First, feed your food

I want start this blog properly by discussing crop nutrition – if we want to feed ourselves, we first need to feed our crops. Improving crop nutrition has been fundamental to increasing crop yields over the last century but is reliant on finite resources. Plants require at least 17 elements to grow and a lack of any of them can cause growth to decrease. In this post though I'm only going to discuss nitrogen, as it is the main nutrient that plants require.

Nitrogen is abundant on earth (78% of the atmosphere) but it is useless to plants as a gas and they need help to 'fix' it into forms that they can use. Biologically, this can only be performed by microbes which are either free-living in the soil or live inside specialised structures in plant roots. To achieve maximum yields however, crops need supplementary nitrogen. We began to supply this two centuries ago, when deposits of guano were found on tropical islands. This was inherently unsustainable since guano is a fossil resource, ultimately having come from life that lived thousands of years ago. A century later, the Haber process was invented, allowing us to fix nitrogen chemically. In the last 50 years, applications of N fertilisers have increased 20 times to 100 million tonnes a year, or 15 kg for every person on Earth. This makes it a major source of fixed nitrogen on Earth as the diagram below shows. The massive increase in the supply of nitrogen has allowed the population to blossom.

The nitrogen and carbon cycles. The orange part of this diagram represents the fixed nitrogen that we add to the world each year and what happens to it. The blue in the left half is what occurs naturally. The right hand side shows how the carbon and nitrogen cycles are linked. Borrowed from Gruber and Galloway 2008

This has come at a considerable cost however. The Haber process is extremely energy intensive and as a result the price of fertiliser is sensitive to changes in energy prices. One tonne of ammonium nitrate now costs around 3 times what it did a decade ago and this price pressure is not going away. Further, much (as much as 80%, often 40%) of the nitrogen applied to fields never ends up in crops. Rain washes it into the water table polluting our water supply and causing eutrophication of rivers, lakes and even oceans. In Cambridgeshire, the tap water can a contain higher concentration of nitrates than are ever found in plants! Some soil microbes eat fixed nitrogen producing nitrous oxide, a powerful greenhouse gas that also promotes the formation of smog, which in turn is damaging to crops. When the nitrous oxide is eventually washed out of the atmosphere by rain it fertilises nutrient-poor habitats allowing grasses to replace heathland, resulting in the local extinction of species. Across the EU nitrogen pollution has been estimated to cost £55-£280 billion a year (£110-560 per person), of which agriculture is responsible for 70% . This means that the financial cost of nitrogen fertilisers is double the benefit gained from using them! We can carry on as usual, but there has to be a better way.

The visible effects of nitrogen pollution - most is much harder to see. Courtesy Sasha Trubetskoy

Organic agriculture eschews the use of chemical fertilisers and so has to rely on a combination of nitrogen-fixing plants (crops like beans and peas and cover crops like clover), manures, composts and fish meal. These all have their own problems too. Beans and peas only leave behind a relatively small amount of nitrogen in the soil after they are harvested. The nitrogen in composts and manures has to come from somewhere, meaning that a larger area of land needs to be farmed to supply the nutrients. Ultimately, it seems likely that a fair chunk of the nitrogen in organic systems does still come from the Haber process – if cows on one farm are fed food from conventional farms, then their manure can be used on organic farms. Fish meal is probably the most wasteful source however, since producing it requires the wholesale destruction of marine ecosystems. The extra nitrogen that agriculture relies on has to come from somewhere, and currently organic agriculture cannot supply it. Considering this, it's not surprising that some 'enterprising' people have produced supposedly organic fertilisers that in fact contain chemically fixed nitrogen.

One of the simplest ways to improve the situation would be to encourage the growth of free-living nitrogen-fixing bacteria in soils. They already fix a small amount of nitrogen without us doing anything to encourage them. At the moment we know very little about how we might go about this, but obviously any increase would be helpful. It is likely that we could breed crop varieties that stimulate the growth of the bacteria and also identify bacterial strains that fix more nitrogen. More radically, we can try to genetically modify cereals to form symbioses with bacteria in the same way that legumes do. This may not be as difficult as you might imagine, since cereal roots already form symbioses with fungi to supply phosphates, and it was this mechanism that was hijacked by nitrogen-fixing bacteria in legumes to allow the symbiosis to form. A group at the John Innes Centre have recently been awarded £6 million to research this. It will certainly be worth keeping an eye on their results.

A more complex solution is to make more efficient use of the nitrogen that we apply to crops and ensure that more is recycled rather than flowing straight through the system as it does now. We need to establish how to keep nitrogen in the portion of soil that roots can access for longer. Often, nitrogen only is applied at the beginning of the growing season, meaning some is leached by the time the crop needs it. Controlled release fertilisers have been around for decades and have been shown to decrease leaching and increase yields. Evidently the cost of these to the farmer often outweighs the benefits though, or more would use them. In the same way that we could improve nitrogen supply by encouraging bacteria that fix it, we could use nitrogen more efficiently if we were able to decrease populations of microbes that 'eat' nitrogen. How we can do that though is a mystery at the moment, but I'll eventually get round to discussing how we can try to manipulate soil microbes in a separate post.

Most radically, we will need to change the way we waste nitrogen every time we go to the toilet. I think this is so important that it'll deserves it's own post!

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