1 Knowledgeable Management is the Key to Reducing Nitrogen Leaching

Much has been written over recent years about the overall influence nitrogen leaching has on our environment. The formation of blue-green algae on waterways and the increased concentration of nitrogen sources in ground water can often be attributed to poor nitrogen use. Agricultural run off and the lack of appropriate effluent treatment plants near rivers and waterways are prime sources of nitrogen contamination.

Turf managers will continue to be placed under greater scrutiny from the perspective of fertiliser and pesticide applications to the property they are responsible for and surrounding areas that could also be affected by their use. There is already quite detailed tasks that have to be followed by the users of effluent water sources in turf situations whereby a combination of soil, surface and sub surface water samples have to be collected to monitor any changes to the nutrient status over time. It is probably only a matter of time before this monitoring method becomes mandatory along with the necessary recording of this data for inspection of the relevant authorities.

Nitrogen products that have a controlled release pattern when compared to soluble nitrogen sources have been popular for many years now in the turf market. There is generally a lack of locally produced evidence that compares the various forms of controlled release nitrogen carriers and so the decision on what type to use, if in fact it is seen as an advantage to use any, is based on a fair degree of guess work and information from distributors.

THE NITRIFICATION PROCESS

Ammonium (NH 4+) added to soils or formed by decomposition of organic N compounds is oxidised by bacteria in the soil to nitrate nitrogen (NO3) in a process termed nitrification. Nitrate is normally the form of N taken up by plants; however, most plants can also assimilate ammonium. In most soils, nitrification of applied ammonium is rapid (2-3 weeks), but nitrification rates are greatly reduced by cool soil temperature (10 C), low pH (5.5) and waterlogged conditions.

N LOSS FROM SOILS

Leaching is important in sandy soils. Nitrate may be leached from naturally well-drained soils by percolating water. Thus, during periods of excess rainfall, leaching may move nitrate out of the effective rooting zone of plants. Denitrification (the microbiological conversion of nitrate to gaseous forms of N) is the major pathway of N loss from most fine-textured soils. It normally occurs in soils that become waterlogged by excessive rainfall or irrigation. Denitrification occurs at maximum rates when soils are warm, pH values are high, nitrate is plentiful, and an energy source (carbon) is available. In waterlogged soils, more than 100 kg. of nitrate N per hectare can be denitrified within a 5-day period. However, in cold soils or soils with low pH values, denitrification rates are slow.

WESTERN AUSTRALIAN WORK

Recent work presented by Dr. Louise Barton, Australia, showed little difference in the leachate derived from a controlled release nitrogen source and a soluble form of nitrogen fertiliser. This was even more remarkable as this experiment was conducted on sand with a low cation exchange capacity and a high drainage rate. The most important factor in this work was the use of soil moisture sensors that were used to determine the frequency at which irrigation was applied; 2 irrigation quantities were used. Sensors limited the potential leaching but of course have little effect if persistent rainfall occurs. I have seen significant nutrient loss from a sand profile in an experiment conducted on a sand based putting green in a matter of a couple of hours.

The big message from the work conducted by Dr. Barton is to start using appropriate soil moisture sensors, a similar message presented by Dr. Bob Carrow at a previous Australian Golf Course Superintendents Conference.

UREASE AND NITRIFICATION INHIBITORS

One of the more recently available sources of controlled release nitrogen carriers are the inhibitor types. Nitrogen (N) is an essential element for plant growth and reproduction. Today an average 25% of plant-available N in soils (ammonium and nitrate) originates from the decomposition (mineralisation) of organic N compounds in humus, plant and animal residues, and organic fertilisers, 5% from N in rainfall, and 70% from applied inorganic N fertilisers. In soils, organic N is converted to ammonium through microbial decomposition. Ammonium formed in soil, added as fertiliser, or in precipitation is rapidly oxidised to nitrate in the nitrification process. Nitrification results in the production of nitrate, a form of plant-available N which is readily lost from soils and particularly sands that are frequently used in turf situations. Sands are also usually deficient in organic matter of all types and are therefore more reliant on applications of fertilisers for adequate plant health.

Nitrification inhibitors (NI) are chemicals that reduce the rate at which ammonium is converted to nitrate by killing or interfering with the metabolism of Nitrosomonas bacteria. The loss of N from the root zone can be minimised by maintaining applied N in the ammonium form during periods of excess rainfall prior to rapid N uptake by plants. A number of compounds have been shown to inhibit nitrification in laboratory and field studies such as Nitrapyrin (chemical name, 2-chloro-6 (trichloromethyl) pyridine; trade name N-Serve®; dicyandiamide (DCD) (chemical name, dicyandiamide; international trade names: Alzon, Didin and Ensan) and more recently DMPP (chemical name, 3,4 dimethylpyrazole-phosphate). Nitrification inhibitors are chemicals that slow down or delay the nitrification process, thereby decreasing the possibility that large losses of nitrate will occur before the fertiliser nitrogen is taken up by plants.

UREASE INHIBITORS

There is considerable evidence to show that fertiliser urea can be less efficient (i.e. lower plant yield per unit N applied) than nitrate-type fertilisers. The major reason for this is that the soil pH in the vicinity of urea granules increases as a result of hydrolysis, resulting in the loss of ammonia to the atmosphere. Typical losses range from 5-20% of the total N applied, but the results are extremely variable and can be up to 50% in extreme conditions. The most important influences on these losses are soil pH, temperature, moisture and rainfall. In brief, ammonia volatilisation, from urea, is greater under conditions of high soil pH, coupled with warm, moist soils under windy conditions.

Not surprisingly, the measured benefits of treating urea with urease inhibitors (most of the research has been done with nBPT, N-(n-butyl)thiophosphoric triamide; trade name Agrotain) are also variable, depending on the same variables that control ammonia volatilisation.

Urea can damage seedlings and inhibit germination. By slowing the rate of hydrolysis,
nBPT can reduce this effect. There is also evidence of phytotoxicity associated with the use of nBPT and this is caused by the uptake of urea by plants, which causes leaf-tip scorch. It is not known whether this is a direct toxicity of urea or an indirect effect, however, it is transitory and occurs in situations where high rates of urea and the inhibitor are used.

These additional materials offer alternative methods for reducing nitrogen loses for the turf manager. To extend the work of Dr. Barton in order to evaluate various controlled nitrogen sources would seem a worthwhile exercise. The importance irrigation plays in the overall management of nitrogen leaching cannot be overstated and there is still a lack of expertise in this field within turfgrass management.

David Nickson
Melbourne
Evergreen Turf

Article supplied by the authors and the NZ Turf Management Journal.

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