By Dave of Darlington The fourth of four articles about the nitrogen cycle.

Article 1 Myths of organic farming. Article 2 Not-so-nice nitrogen. Article 3 Hang on to your nitrogen

When I was a child, people often used to say “You can have too much of a good thing.” I have never heard this saying used in recent years, perhaps because it does not fit the hedonistic culture that is prevalent in England today. But you really can have too much of a good thing, particularly when the good thing is plant-available nitrogen [1] in the soil.

It may seem surprising to say that there can be too much available nitrogen in the soil, when it is well known that supplying more nitrogen to the crops increases their yields. But unfortunately the increase in yield is not proportionate to the increase in nitrogenous fertiliser applied. Doubling the nitrogen supply does not double the yield. The law of diminishing returns applies here. All other factors being equal, as we apply more and more fertiliser (inorganic or organic), the proportion of it taken up by the crop decreases. And at the same time the proportion that is lost from the soil to the wider environment increases. It is this lost nitrogen that is of particular concern. Some of it goes into the atmosphere as nitrogen-containing gases and some of it dissolves in water and is leached down into the subsoil, from where it may carried by the field-drains into ditches, ponds and watercourses. In ponds and other areas of still water the nitrogen can accumulate and lead to an over-feeding (or eutrophication) of algae and other small organisms in the water, which eventually damages the ecosystem of the pond.

Fertiliser frenzy

But it is not just a few ponds that are being eutrophied. In a sense the whole world is eutrophied. Everywhere there is an excess of nitrogen in the soil and in the water. Nitrogen enters the atmosphere mainly in centres of human population and intensive agriculture, but it is carried by winds all over the world. Some of the nitrogen-containing gases, like ammonia and nitrogen dioxide, are soluble in water and therefore dissolve in rain drops and are deposited on the soil when it rains. So even the remotest ecosystems, like the vast coniferous forests of the northernmost parts of Asia, Europe and America, are getting fertilised with nitrogen to an extent that they never did before human intervention. The global nitrogen cycle has been augmented to an enormous extent. In the last two hundred years, largely through the production of nitrogenous fertilisers and the growing of leguminous crops, the amount of atmospheric nitrogen being fixed [2] has more than doubled. A significant part of that extra fixed nitrogen gets carried into natural ecosystems.

It might not seem a bad thing that plants in nature are being fertilised. It should make them grow faster, you might think. But it has to be borne in mind that most natural plants are highly adapted to grow in a very restricted nitrogen supply, because in natural ecosystems nitrogen is very tightly controlled. So for such plants suddenly to get extra nitrogen is not necessarily helpful. On the contrary, it could be a source of stress for them. There is evidence, too, that the excess of nitrogen suppresses the growth of mycorrhizal fungi [3], on which most plants are highly dependent for their nutrient supply, especially for phosphorus. So, while the plant gets more nitrogen, it could suffer from deprivation of other important nutrients.

As well as harming individual plants, the extra nitrogen could also harm the ecosystem as a whole, because there are some species of wild plants that, like cultivated crops, can thrive on nitrogen. In a nitrogen-saturated ecosystem the growth of these nitrogen-loving species would be favoured at the expense of the rest and the whole balance of the ecosystem would be upset. The beginnings of this sort of ecological damage has already been detected in some of the colder parts of the world, where natural nitrogen cycling is slowest.

Acid pain

Cereal rye – a nitrogen absorber. Photo: Lukáš Patkan (sxc.hu/profile/patkisha)

Then there is the effect of inorganic nitrogen compounds on the pH (the acid/alkaline balance) of the soil. They tend to make the soil more acid and, if they accumulate in the soil, the acidity could reach a point where it seriously impairs plant growth. Below a certain pH the cycling of nutrients in the soil practically ceases, partly because most microorganisms will not function in very acid conditions. Aluminium and other metals that are toxic to plants can be mobilised at low pH, while non-metals that the plants need, like phosphorus, can become immobilised.

Finally, there is evidence that excess nitrogen in the soil de-activates the bacteria that oxidise methane. These bacteria play an important part in limiting the atmospheric concentration of methane, which is a powerful greenhouse gas. Any inhibition of the activity of the bacteria will increase that concentration.

So, all in all, too much nitrogen in the soil can do serious harm to the natural ecology of this planet. There is therefore a pressing need to reduce the quantity of inorganic nitrogen compounds in the environment (quite apart from the global warming effect of atmospheric nitrous oxide). Industry and motor transport are important sources of these nitrogen compounds, but agriculture plays a major part in the problem too, principally as a result of the emission of oxides of nitrogen from the soil. The only way to cut that down is to reduce the inorganic nitrogen concentration in the soil water by applying less nitrogenous fertilisers (including green manures) to the soil. However, that would cause a reduction in crop yields, which would not be popular, either with farmers who depend on high yields to make a living or with people who are concerned about world hunger.

In the short term the answer to the latter problem is vegan organic growing, since this would immediately make much more farm land available for feeding people, which would compensate for the lower crop yields. But in the longer term serious consideration has be given to reducing the demand for food by limiting or even reversing the growth in the world’s human population so that human demand for the earth’s resources is brought down to as sustainable level.

Notes

[1] The term plant-available nitrogen here denotes not only inorganic nitrogen (nitrate and ammonia) but also organic nitrogen that is readily mineralised into inorganic forms.

[2] Fixation of nitrogen means taking nitrogen from the air and combining it with other elements to form compounds such as ammonia or nitric oxide. The principal means by which this takes place in nature is through lightning or through the action of nitrogen-fixing bacteria, some of which are free-living in the soil and others living symbiotically on the roots of plants.

[3] Mycorrhizal fungi are those fungi that live in very close association (symbiosis) with plant roots, getting their carbohydrate from the plant while supplying the plant with phosphorus and minerals that they get from the soil. Most plants need the help of these fungi to survive, especially in poor soils, because fungi can scavenge scarce nutrients from the soil much more effectively than plant roots can and they can also digest more recalcitrant nutrient sources like woody residues and rock particles, which plants cannot do.

This article appeared in Growing Green International magazine Num 25 (Summer 2010), p12.