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

Article 1 Myths of organic farming. Article 3 Hang on to your nitrogen. Article 4 Saying no to too much nitrogen

In the previous article in this series [1] I drew attention to two of the myths associated with organic farming and gardening. In this one I will refer to two more such myths: firstly, that nitrogen losses in organic farming are necessarily much smaller than in conventional farming and, secondly, that organic farming solves the problem of nitrate leaching by using winter cover crops. To appreciate the misleading nature of these statements we need to look in detail at why nitrogen losses take place.

The main pre-condition for such losses is that the soil should be in a state of so-called nitrogen saturation. This has been defined as a state of the soil in which nitrification [2] exceeds immobilisation [3]. In other words, the supply of nitrate to the soil water exceeds microbial demand for it. This leaves an excess of inorganic nitrate in the soil and makes nitrogen loss very likely, unless the nitrogen is removed from the soil by some other agent, such as plants. But for plants to do that effectively, they need to have a relatively dense root system and to be actively growing and photosynthesising.

Nature versus nurture

The above-mentioned state of nitrogen saturation hardly ever exists in nature. It is more characteristic of arable farming, where it is deliberately created by the application of fertilisers (chemical or organic) to the soil. The reason for this is to ensure a plentiful supply of nitrate to the crops and hence high yields. So, while in nature (apart from a very low background level of nitrous oxide emission [4]) there is practically no loss of nitrogen from the soil, in farming and growing (whether conventional or organic) the loss of nitrogen can be massive (up to 30% of the nitrogen applied as fertiliser).

The main causes of nitrogen loss from soil are nitrate leaching [5] and nitrous oxide emission. Both processes are favoured by the above-mentioned state of nitrogen saturation, as well as by an ample supply of water. The water is required by the microorganisms that carry out the reactions, so in a completely dry soil the nitrogen losses should theoretically be nil. Water is also necessary (in the case of leaching) to transport the nitrate downwards and (in the case of nitrous oxide production by denitrification [6]) to fill the soil pores and create the necessary anaerobic conditions for the denitrifying bacteria.

Green manuring versus fertiliser

It is clear, from the above considerations, that any agricultural operation that produces nitrogen saturation, in the absence of plants that can take up the excess nitrate, will give rise to relatively large nitrogen losses from the soil. One of the worst operations in this respect is the incorporation into the soil (by ploughing or digging) of cover crops and green manures, particularly when the soil is warm and wet and when the plants are fresh and young. These conditions will give rise to a huge release of nitrate into the soil and hence to a potentially substantial loss of nitrogen through leaching and nitrous oxide emission. The situation is exacerbated by the fact that, at the time of the incorporation, the soil is inevitably bare, so there are no plants to take up the excess nitrate, and there will be no such plants for several weeks – until the following crop has been sown and well established. The incorporation of green manures into the soil in this way is almost universally practised in organic farming and is, in fact, the main method used to fertilise the soil.

In conventional farming, on the other hand, nitrogenous fertiliser is nowadays applied progressively in two or three doses, mostly at times when the crop is growing fast and hence is capable of taking up all the nitrate that it can get access to. Of course there will still be some nitrate that will escape, not being taken up either by crops or microorganisms, but the nitrogen losses may sometimes be considerably less than in the case of organic farming.

Leguminous green manures are potentially an even more serious source of nitrogen losses. They do not even need to be incorporated into the soil to cause nitrate leaching and nitrous oxide emission. Even while the crop is growing, nitrogen loss may be taking place. This is because the soil under a leguminous green manure crop, especially if it is being grown as a pure stand or with a relatively small admixture of grass in it, may be replete with nitrate, ready to be leached or denitrified. (This will only be the case, however, when the bacteria on the plant roots are actively fixing nitrogen, so not in winter.) Also, many nitrogen-fixing bacteria, especially the Rhizobium species, also carry out denitrification when free-living in the soil, i.e. when they are not contained in root nodules on the legume plants. This will increase the nitrous oxide emissions from the soil.

Tread lightly

Besides nitrogen saturation and wet conditions, anything that increases the anaerobic character of the soil will promote nitrous oxide production. A common example is compaction of the soil, due to tractor traffic (or even just footsteps in the case of a bare soil). Denitrification can also be stimulated by the cycles of repeated freezing and thawing that often take place in winter in soils in temperate zones. For this reason nitrous oxide emissions can be just as great in winter as in the other seasons of the year. Obviously these factors apply equally to organic and conventional farming.

As for cover crops, it needs to be pointed out first of all that they are by no means a monopoly of organic farming. In fact, farmers of all kinds use cover crops, as anyone who travels round the English countryside in winter will see. However, they are not as much in evidence as they used to be in the old days, when it was normal practice for farmers to leave the stubble over winter, which provided a feeding paradise for seed-eating birds like tree sparrows and yellow hammers, now unfortunately both in serious decline. It was usual then to grow a dual-purpose fodder/cover crop – commonly the socalled stubble turnips. Nowadays farmers in England often plough the stubble straight after harvest and immediately sow an overwintering crop. So the function of a cover crop is served by next year’s cereal or rape crop.

Mind the gap

The second point to understand about cover crops is that they do not completely prevent nitrate leaching. They just reduce it by, on average, about half. Their effectiveness depends when and how they are grown and what is done with them afterwards. The point at which the crop is dug or ploughed into the soil is, as explained above, crucial for the conservation or loss of soil nitrate. The period between crops is also very important from this point of view. The watchword of cover-crop growers should be “Mind the gap!”, because it is precisely in the gaps between harvest and the establishment of a cover crop and between the incorporation of the cover crop and the establishment of the following crop that the soil is most liable to leaching.

Food for thought

All this demonstrates that organic (including vegan-organic) farming and growing can be just as bad as, if not worse than, conventional, where nitrogen losses to the wider environment are concerned. Of course this does not mean that organic farming is inferior to conventional farming. There are many ways in which organic farming is clearly superior to conventional, not least in its much lower consumption of fossil fuels and consequent lower carbon dioxide emissions. It just happens that nitrogen loss is often a weak point in the organic system. In the next article in this series I will discuss some ways in which such losses can be minimised.

Notes

[1] Myths of organic farming

[2] Nitrification is the process by which bacteria oxidise ammonia (NH3) to nitrate (NO3). The process takes place in a series of stages, in one of which nitrous oxide (N2O) is produced as a byproduct and is emitted from the soil to the atmosphere. The bacteria involved are aerobic, that is, they need to breathe air.

[3] Immobilisation of nitrogen is the process by which soil microorganisms take up inorganic nitrogen from the soil (in the form of ammonia or nitrate) and incorporate it into their own bodies as organic nitrogen compounds such as proteins.

[4] The term nitrous oxide emission refers to the production of the gas nitrous oxide (N2O) in the soil and its upward percolation through the soil into the atmosphere (where it acts as a powerful agent of global warming, 240 times stronger than CO2). In well-oxygenated soils nitrous oxide comes mainly from the process of nitrification, as described in note 2 above, but denitrification is also a major source of nitrous oxide (note 6 below).

[5] Nitrate leaching takes place when water (from rain or irrigation) washes nitrate out of the topsoil into the subsoil, from where it can either percolate down into the groundwater or enter the field drains, which will carry it into ditches, ponds and water courses.

[6] Denitrification is the bacterial reduction of nitrate to nitrous oxide (N2O) and nitrogen (N2) gases. The bacteria in this case are anaerobic – they do not need air, because they get the oxygen they need from the nitrate itself (NO3). There are small airless pockets in the soil at all times, so, even in a well aerated soil, there is a constant stream of nitrous oxide being produced by denitrification as well as that from nitrification, but denitrification greatly increases, and becomes the main source of nitrous oxide, when the soil is predominantly anoxic, for example, when it is waterlogged after heavy rain.

Editors’ note: field scale growing as practised by experienced stockfree growers is by far the best system for reducing nitrogen loss. Dave expands on this aspect in his next article.

This article appeared in Growing Green International magazine Num 22 (Winter 2008), p14.