A recent European study that analysed the nitrogen (N) cycle across 28 EU member states estimated fluxes within the cycle across natural, intentional anthropogenic, and unintentional anthropogenic processes and activities.
In the natural processes and activities, some N is cycled through fixation by leguminous plants and soil microbes and then released again into the atmosphere by fires burning plant material.
A quantity of N also leaches out of soils, moving into rivers and oceans.
Dr Alan Manson, a soil scientist with the KwaZulu-Natal Department of Agriculture and Rural Development, says the researchers found that before the advent of modern agricultural practices in the EU, an average of 0,2 teragrams (200 000t) of N was released back into the atmosphere annually as a result of denitrification in the EU’s total area.
“Since the advent of modern agricultural practices in the 1900s, especially the manufacture and use of chemical fertilisers since World War II, the quantities of N being put into soils in Europe and elsewhere in the world have multiplied hugely compared to before the 20th century.
“There’s a huge N-cycle now going through both crop and livestock farming across the planet, with much of this N being tapped off for human nutrition,” Manson explains.
The EU’s status as a net importer of food and animal feed also has an impact on N-cycling.
The N imported into the EU via these products adds to volumes of N already present through natural, intentional anthropogenic and unintentional anthropogenic processes and activities.
Manson explains that while N is essential for life in almost all its forms, too much of the element in a given area may result in negative consequences for life.
The EU researchers found that volumes of N being released into the atmosphere through denitrification in the region had increased to an average of 3,8 teragrams (3,8 million tons) a year.
Some of this N is in the form of nitrous oxide (N2O), a greenhouse gas that remains in the atmosphere for 100 years before breaking down into beneficial N and oxygen.
“Nitrous oxide has been found to be responsible for 10% of the total global anthropogenic radiative forcing that’s contributing to climate change,” says Manson.
“Since about 2005, it has also been the largest man-made contributor to depletion of the Earth’s protective ozone layer, now that CFC [chlorofluorocarbon] production has been restricted.”
High concentrations of N compounds in agricultural soils also result in nitrate (NO3-) leaching out into waterbodies such as rivers and oceans.
Algae in these waterbodies thrive on elevated nitrate levels, resulting in algal blooms that can deplete oxygen in the water to the point where much of the local aquatic or marine life dies.
Some algal blooms are also toxic, poisoning life below and above the water surface.
So bad has N pollution become in Europe that the European Parliament has enacted legislation that strictly regulates the sources and quantities of N that farmers and other users there may apply. Transgressors of this legislation face huge fines that could cripple their businesses.
Farmers’ role in controlling nitrogen
The world’s farmers can play a major role in helping reduce N pollution. The more efficiently that farmers use N products to produce food and fibre, the less excess N escapes into terrestrial, aquatic, marine and atmospheric environments.
In agriculture, the agronomic efficiency of applied N (AEN) is the index most commonly used by agronomists to calculate how effectively N is being applied by farmers and then utilised by their crops.
The higher the AEN reading, the higher the economic yield per unit N fertiliser applied. For an accurate calculation of an AEN result, a control plot where no N is applied to the soil and growing crop is required for comparison with the N-treated croplands.
A study conducted between 2003 and 2010 by one of Manson’s colleagues, Guy Thibaud, compared maize grain yields in response to equal N fertiliser applications across conventional tillage plots, no-till plots, and plots that replicated a cycle of conventional tillage for one year, followed by four consecutive years of no-till.
“In the 2009/2010 season, the AEN for an N application of 200kg/ha was highest for the no-till plots, which yielded an average of 13t/ha of maize grain,” says Manson.
“The AEN was second-highest in the one-year conventional tillage/four years no-till plots, which achieved an average of 12,5t/ ha of maize grain from 200kg/ha of N. At the same N application rate, the conventionally tilled plots achieved only about 10t/ha, and the AEN was much lower.
“For the conventionally tilled plots in this season, a high AEN was achieved with 125kg/ha of N, with a yield of 11t/ha of maize grain. At higher rates of N, N-use efficiency was much lower because additional N did not increase yield in these conventionally tilled plots.”
Manson points out that for the control plots where no N was applied, the no-till plots yielded an average of 6,5t/ha of maize grain, and the conventional tillage and one-year conventional tillage/four years no-till plots each yielded an average of 7,75t/ha of maize grain.
“Over the years, this trial, along with soil nitrate analyses of soils from a number of South African farms, has shown that the N application rates used by many commercial farmers are excessive. Much of the N applied is wasted and finds its way into the natural environment, where it can be considered to be a pollutant.”
Fortunately, farmers can now use one or more of a number of tools to guide their choice of N application rates. These include N budgeting based on land history, top-dressing based on high-N test plots in their lands, and pre-side-dress soil nitrate (PSNT) tests.
What Manson and Thibaud found particularly interesting about Thibaud’s maize response to the N field trial was the 6,5t/ ha to 7,75t/ha average maize grain yields achieved across all of the control plots that did not receive any applied N over the six years of the trial.
They realised that the source of the N in these control plots, which was being utilised by the maize crops growing in them, was the soil organic matter (SOM). This had built up as a result of the no-till practices on the lands over the preceding decade.
“Soil organic matter basically consists, to varying degrees, of living organisms, easily decomposed dead organisms, difficult-to-decompose dead organisms, and black carbon (C), or tiny bits of charcoal, that decomposes exceptionally slowly,” explains Manson.
“The diversity and numbers of these entities in a soil micro-aggregate from a soil high in soil organic matter is astonishing.”
He adds that SOM is typically unevenly distributed through soils, but concentrations of the living component are almost always found around plant roots growing in the soil.
This is largely because many soil microorganisms live on either dead cells shed by the plant from the outer layers of roots, or on root exudates released by the roots.
Other organisms feed on them, with each cycle of decomposition releasing some nutrients, such as ammonium (NH4+), nitrate, phosphate (PO43-) and sulphate (SO42-), in forms that are again available to plants. At the same time, some nutrients remain tied to C and contribute to a relatively stable pool of ‘recalcitrant’ SOM.
The benefits of soil organic matter
Among its benefits, SOM holds soil particles together to form aggregates between which air, moisture and fertilisers can move freely for the benefit of plants growing in these soils.
SOM also has an effect on plants’ uptake of N from the soil. While it has long been known that plants take up N in the form of nitrates and ammonium, more recently it has been determined that plants also take N up in the forms of amino acids and other organic N sources.
Manson points out that, in addition, scientists have recently shown that the roots of many plant species can promote the breakdown of stabilised SOM close to the root surface by breaking up micro-aggregates using organic acids.
When oxalic or citric acid is exuded from the root, it binds to cations such as calcium (Ca2+) and aluminium (Al3+), which previously bound stabilised SOM into micro-aggregates.
This released organic matter becomes food for bacteria and fungi, which, in turn, release nutrients precisely where the plant needs them: close to the plant root. This makes SOM a highly efficient source of N for plants as the N is mineralised near the root and is rapidly taken up by the plant.
Moreover, there is little risk of N loss through the leaching of nitrate, or as gaseous dinitrogen (N2) or nitrous oxide.
“By promoting increased soil organic matter in crop soils, farmers will not only help prevent N pollution but save on the costs of having to apply unnecessarily high rates of supplementary inorganic N. They’ll also benefit from improved crop yields,” says Manson.
Phone Dr Alan Manson on 033 355 9100, or email him at [email protected].
This presentation was given at the 2018 Autumn/Winter Symposium of the Intensive Growers’ Association, held on 15 March 2018 at the KwaZulu-Natal Department of Agriculture and Rural Development’s Cedara Agricultural Research Centre.