The loss of nitrogen, as either nitrate, ammonia or nitrous oxide, has become a matter of increasing concern from all soils.
The higher the soil is loaded with nitrogenous compounds and organic matter, the higher the risk of greater losses occurring.
Firstly, the consequences. Loss of nitrogen compounds is a loss of costly nutrient from land and its presence in air or water represents pollution or a greenhouse gas (GHG).
Nitrate: The loss of nitrogen as nitrate (NO3), or the highly plant available form, is mainly a water quality issue, as it is not a gas, but it is soluble in water. It can move into water bodies by either:
Whatever the route, there are limits set for the maximum concentrations of nitrate allowable in rivers, estuaries and ground water.
Nitrous oxide: The loss as nitrous oxide (N2O) is a different matter, because this is a gas which is a very strong GHG, with an estimated 280 times the warming potential of carbon dioxide (CO2).
It is a by-product of the natural nitrogen cycle and the amount produced is a direct function of the level of reactive nitrogen in the soil system. Agriculture remains a major cause of its production.
Ammonia: This is also a gas, but it is not a GHG. Its concentration is controlled by air quality legislation and agriculture is the primary cause.
This gas is readily degraded in the atmosphere and it can revert back to nitrogenous compounds and be returned in rain. The difficulty is that we cannot control where it falls and such nitrogen can damage environmentally sensitive areas and cause ecosystem malfunction.
How are they produced
Put simply, nitrogen is not a stable element in the soil and it is constantly being transformed from one form to another by soil microorganisms.
These reactions can occur in the nitrogen cycle, which is a natural phenomenon that helps to recycle the nutrient in the soil from organic matter, etc. The conditions in the soil are a primary driver as to what microorganisms predominate and what substances are produced.
At a recent International Fertiliser Society conference in Cambridge, Nick Cowan from the Centre for Ecology and Hydrology in the UK explained some of the background behind soil functioning and the production of N2O in particular.
N2O and global warming
To begin with, he explained that N2O contributes an estimated 6% to global warming and it also contributes to ozone depletion in the stratosphere. So, it is a relatively nasty molecule, as well as being a source of nitrogen loss for farmers.
He commented that nitrous oxide levels ran at around 265 parts per billion (ppb) for decades, but the level has increased to 332 ppb in recent times and is still rising. That is roughly a 25% increase.
Nick commented that N2O emissions from industry in the UK have decreased by 96% since 1990, but agricultural emissions have remained the same.
What must be remembered here is that almost all N2O emissions originate from microbial activity in the soil so they are a natural phenomenon.
Nick explained that the problem is more likely to get worse rather than better as time progresses. While efforts to make agriculture a net-zero carbon activity seem likely to meet with a high level of success, they will only tackle carbon.
Most of these measures will not address N2O, so it is likely that it will become a bigger proportion of total emissions as time progresses.
Reducing the risk
So what, if anything, can be done in practice to help reduce nitrogen loss as nitrous oxide? Nick explained that oxygen availability in the soil determines whether microbes engage in nitrification or denitrification.
Nitrification is the process by which ammonia is converted to nitrites and then to nitrate (NO3). This is done by specialised soil microorganisms in oxygen-rich environments.
Denitrification converts nitrate to a range of gasses such as NO, N2O and N2 when oxygen is poorly available, ie waterlogged or compacted soils. So conditions such as compaction or waterlogging influence which process occurs.
Options to reduce the risk of nitrous oxide production are limited in their ability. Nick indicated three major mechanisms, but concluded that most offer limited practical benefit.
Field factors such as compaction or loosening are features of different land use, for example grassland versus tillage.
Compaction decreases oxygen concentration in an area, which makes it more likely to push towards denitrification, which could lead to increased N2O emissions.
This may be an issue for evolving tillage systems and min-till was found to be associated with slightly higher N2O emissions in recent research.
It seems likely that direct drilling could also increase N2O production, but it must still be stated that the overall amounts may still be very low compared with grassland situations.
However, one might wonder if establishment systems that avoid or minimise cultivation could be at even greater risk of N2O emissions over time where organic materials and manures are constantly being left on the surface?
This is where earthworm activity would become very important to help decrease the concentration of organic matter and reactive nitrogen close to the surface and deposit it deeper into the soil profile.
Nick commented that increasing soil pH helps to decrease N2O emissions, as can the source of nitrogen fertiliser used. But research tends to be less than clear as to the impact of these husbandry options on land in tillage.
Ultimately, having less surplus nitrogen in the soil system remains an important, and probably the most effective, way to limit all gaseous losses and especially N2O.
Nick pointed out that the field factors that drive these reactions can be very local. For example, compaction can be confined to wheel tracks.
Specific areas of very high risk are areas around fodder feeders or meal troughs in fields. Such area would have high nitrogen loading through excrement, have poor soil structure and low oxygen concentration. He commented that these ‘hot spots’ can produce exponentially higher N2O emissions.
While tillage land is not generally associated with N2O losses, we have to be conscious that it can occur, but, as in many other instances, good soil structure, health and fertility tend to be factors that both increase productivity and decrease the risk of N2O loss.