Towards Understanding: Nitrogen

R2.3 billion (or $127 million) – this is the value of Nitrogen in the atmosphere above every hectare of soil…

In fact, something that most of you would remember from your primary school days is that 78% of the earth’s atmosphere comprises of nitrogen gas (N₂). Nitrogen is one of the most important plant nutrients and an increasingly expensive input, when applied in the usual way (as inorganic nitrogen fertiliser), as any farmer can attest. To appreciate why one of the most abundant elements on our planet cannot be directly used by plants, and often has to be bought at great expense, an understanding of the nitrogen cycle and the central role that the soil and its microbes play is needed.

Nitrogen Fixation

As nitrogen is a major component of chlorophyll, which is used in photosynthesis, it is essentially used by plants for vegetative growth. Nitrogen is also a major component of amino acids - the building blocks of proteins. These proteins are involved in the structure of plant cells and act as enzymes that allow for vital biochemical reactions. However, there are only two forms of nitrogen that plants can use in the chemical pathways of metabolism: ammonium (NH₄⁺) and nitrate (NO₃^-). Nitrogen, in any other form, needs to be converted to one of these compounds before it is available to a plant. In a natural system, the most direct ways in which this can occur are through nitrogen-fixing bacteria and lightning. Lightning provides enough energy for N₂ to bond with oxygen in the air producing nitrates, which then wash down into the soil. This is, however, a very minor source of plant-available nitrogen. The greatest part of the atmospheric nitrogen that becomes plant available in nature is mediated by nitrogen-fixing bacteria, which, through nitrogenase enzymes, convert N₂ to ammonium. These bacteria may be free-living in the soil, or they may be found in root nodules, predominantly on legumes, like the famous Rhizobium species.

Free-living nitrogen-fixing bacteria are typically not very dominant in most cultivated soils due to tillage and chemical soil disturbance. This army of nitrogen-fixers, however, has the ability to sequester up to 60kg of nitrogen per hectare per year within the plant’s root zone. At a value of almost R1 826 per hectare per season, it is worth promoting these bacterial families. Free-living N-fixing bacteria perform best when soil nitrogen is at its lowest, and host plant species, predominantly grasses, are present, but experiencing nitrogen deficiency. They are suppressed by easily available nitrogen in the soil, whether synthetic, or from legume N-fixation. For these reasons, a grass cover crop, or grass component to a cover, can be very effective at promoting free-living nitrogen fixers, especially when the nitrogen fertility of the soil is low.

Nodule-forming nitrogen fixers tend to be host-plant specific and quite “picky”, with each species having a pH, temperature and soil moisture range at which they are most effective. The tissues of host plant species associated with N-fixing bacteria are known to have a higher nitrogen content than species that must take up nitrogen directly from the soil, hence the use of leguminous plants and their seeds as a protein source. It must be noted that N-fixing bacteria only fix as much nitrogen as the plant cannot take up from the soil. When soil nitrogen levels are high, such as after the application of synthetic N fertiliser, no bacterial fixation takes place, even if the bacteria are present and the plant is one that can form an association with them. Fertilising a nitrogen fixing legume with excess nitrogen is essentially paying for something which is available for free and requires little extra effort on the part of the farmer.

Storing nitrogen in the soil

While the nodule-forming N-fixing bacteria and their host plants are alive, little to none of the nitrogen fixed is available to other plants. It is only after death and decomposition, when the former host plant has been converted to soil organic matter (“SOM”), that the process of making the nitrogen available can take place. SOM also contains nitrogen that has already been cycled through other parts of the system. Sources include dead non-nitrogen-fixing plants, animal manure and dead microbes.

The nitrogen in SOM, known as soil organic nitrogen, is stable in the form of amino acids and is not easily lost in undisturbed soil. SOM, therefore, serves as a sort of nitrogen bank. The amount of soil organic nitrogen that is present in the soil is intrinsically linked to the organic carbon stored in the soil, which makes up the bulk of SOM. Stable SOM has a carbon to nitrogen (C:N) ratio of approximately 10:1 in South African soils. As a consequence, the more stable organic carbon is present in the soil, the more stable nitrogen will be present. Soil organic carbon is tightly linked to primary production, which is highly rainfall dependent in undisturbed landscapes. More biomass will be produced in a higher rainfall area, which will cause more dead organic matter to enter the soil. This organic matter will be processed by soil microbes until it has stabilised at a C:N ratio of 10:1. This means that the higher the rainfall, the more stable SOM is produced and the more stable nitrogen is “banked” in the soil.

A prime example of the interplay of these factors is the case of the American prairies and the high rainfall grasslands of South Africa. Both these environments are known for exceptionally high primary productivity, or carbon fixation, which in their natural state, leads to very high soil organic carbon levels. It is well documented that when these grasslands were first cultivated, crops with a satisfactory yield level could be produced for many seasons without any external nitrogen source before a yield decline was noted. This implies that there must have been massive amounts of nitrogen, in the form of soil organic nitrogen, stored in the soil. It has been estimated that, in South Africa, on average 3 500 kg/ha of pure nitrogen was available in the top 20 cm of the soil alone when the grasslands were first ploughed up, with that figure likely higher in many of the eastern grasslands. When a virgin soil is first cultivated, there is a rapid decline in soil organic nitrogen in the first five years. Thereafter, a gradual decrease takes place, lasting approximately 30 years, after which the organic nitrogen level in the soil reaches an equilibrium low.

Stable nitrogen stored in the soil has very real benefits for farmers. It is used by soil microbes which improve the soil structure by forming aggregates; it encourages the population of microbes which suppress plant diseases and it serves as a sort of insurance policy for times when the applied nitrogen fertiliser or legume nitrogen cannot meet the needs of the crop. For these reasons, soil organic nitrogen must be preserved and built up wherever possible and should be replenished when it is depleted by a crop which was grown with insufficient applied nitrogen.

Nitrogen Mineralisation

Stable soil organic nitrogen becomes available to plants through a process known as mineralisation. This is the bacteria-mediated conversion of amino acids to plant available ammonium. In an undisturbed soil, the rate of organic nitrogen mineralisation depends on the amount of ammonium in the soil solution. As ammonium is depleted, more ammonium is released. Ammonium may also be converted back to the stable amino acid form through the process of immobilisation. This occurs when organic material with a high C:N ratio is being decomposed. Ammonium will continue to be removed from the soil solution until the material is turned into stable SOM with a 10:1 C:N ratio. If insufficient nitrogen is present in the solution for both plant uptake and decomposition, plants will show a deficiency, as the bacteria involved in decomposition take up nitrogen faster than plants can. This will continue until all the dead material has reached a stable state, and ammonium is once again being released into the soil solution. When high C:N ratio material, e.g. wheat straw, is incorporated into the soil, this effect can be particularly dramatic, causing a crop planted soon after incorporation to show heavy deficiency symptoms, if not enough extra N is provided for both the crop and the decomposers. This period of decomposition, when nitrogen is taken up from the soil by decomposers, is known as the “nitrogen negative period”.

Free ammonium, which has a positive charge, may also be immobilised by adsorbtion onto the negatively charged surfaces of 2:1 clay minerals (sticking clays) generally found in unweathered, “younger” soils. This means that certain soils with a high fraction of these minerals have the potential to store more nitrogen.

Unproductive Nitrogen Loss

Using nitrogen in a productive cycle is an investment. Losing nitrogen without productivity gains is an expense. It is estimated that at least 30% of applied fertilisers are lost without ever being used by the crop.

Soil cultivation has the potential to cause the rapid loss of soil organic nitrogen. When a soil is cultivated, the soil is heavily aerated. This favours the aerobic, or oxygen-loving, nitrifier bacteria, which break down stable SOM in a process called nitrification. Nitrification is the conversion of ammonium to nitrite, and then to nitrate, essentially “mining” the soil for nitrogen. Nitrates are more unstable and mobile in the soil than ammonium, and may be leached below the root zone, or in the worst case, into the ground water, or a surface water source.

Nitrogen may also be lost from the soil during a water-logging event. Such conditions favour anaerobic denitrifier bacteria that convert nitrates, through various intermediate compounds, into nitrogen gas, which is released back into the atmosphere. Some of the intermediate compounds, such as nitrous oxide (N₂O), are potent greenhouse gases, and may enter the atmosphere before conversion to N₂. Low-lying, or badly drained, areas in conventionally tilled fields in summer rainfall areas provide ideal conditions for denitrification.

Note: Loss of nitrates from the soil through leaching or denitrification is a significant cause of soil acidity. When ammonium is converted to nitrates through the nitrification process, a hydrogen atom (H⁺) is released for every ammonium molecule converted. When plants take up a nitrate molecule, they release a hydroxide ion (OH^-), this balances the soil pH. However, if the nitrate is lost before plant uptake, the H⁺ is not balanced, and acidity builds up. A leading cause of this problem is the addition of more ammonium fertiliser into an aerated soil than the plant can take up quickly enough before the resulting nitrates are lost.

Nitrogen may also be lost to the air through volatilisation. This process occurs under specific conditions near the surface of the soil. When synthetic urea or animal manure is applied, it must be converted to ammonium in the soil before plants can use it. An intermediate product is ammonia (NH₃), which is volatile, and easily lost to the air when the process takes place close to, or on, the soil surface. Volatilisation of urea to ammonia accounts for the chemical smell of feedlots and heavily used kraals after rain.

The Crux of the Matter

To summarise, contrary to popular belief, the soil does have the ability to store significant quantities of nitrogen in a stable state. This nitrogen is contained in the stable organic matter fraction of the soil, and is, therefore, determined by the organic carbon content of the soil, which has a constant C:N ratio. This nitrogen needs to be managed by preventing aeration of the soil, that is, heavy tillage, which causes the conversion of stable nitrogen forms to highly mobile nitrates, which are easily leached or denitrified when waterlogging occurs. The organic nitrogen also needs to be replenished after crop uptake by, either adding enough synthetic nitrogen and lignified organic matter, or planting a nitrogen fixing crop which is not harvested. Free-living N-fixing bacteria should also be encouraged, as they add nitrogen to the soil for, arguably, the least cost to the farmer.