The N cycle is complex and contains many processes including fixation, mineralization (ammonification), nitrification and denitrification.
Figure 1. The Nitrogen Cycle. (Carrow et al., 2001).
This process involves the conversion of inorganic nitrogen into organic nitrogen. Firstly nitrogen gas is converted into ammonium, because it is the only way the micro-organisms can obtain nitrogen directly from the atmosphere. The ammonia is usually then converted into organic nitrogen compounds by several mechanisms. (6) Certain bacteria such as Rhizobium spp. have the ability to carry out this process. (6) Some of these nitrogen fixing bacteria have a symbiotic relationship with the grass plant and fix nitrogen for the grass plant in exchange for carbohydrates from the plant. High energy natural events such as thunder storms can also convert atmospheric nitrogen into organic nitrogen.
The organic nitrogen is then converted into ammonia (NH3), followed by ammonium (NH4+) (inorganic nitrogen) by soil microorganisms, which break down the organic substances within the soil. (9) This process is carried out by numerous micro-organisms including bacteria, fungi, and actinomycetes.
The majority of the ammonia produced is then converted into nitrite (NO2) and subsequently nitrate (NO3-) as follows:
2NH4+ + 3O2 …Nitrosomonas..bacteria……2NO2 + 2H2O +4H+
2NO2 + O2 …...Nitrobacter..bacteria ….. 2NO3- (Crum, 2006, pers. comm.)
Nitrosomonas and Nitrobacter as called nitrifying bacteria and are required for nitrogen to be made available to the turfgrass plant.
Nitrate nitrogen is lost to the atmosphere as nitrogen gas through this process as follows
2NO3- --- NO2 --- NO --- N2O --- N2
Denitrification is an anaerobic process, which is carried out by denitrifying bacteria. (6) Once the nitrate is converted into nitrogen gas, it is quickly lost to the atmosphere.
Nitrogen can be lost to the plant through other processes including:
Ammonia is lost to the atmosphere through this process. Ammonia volatilisation from turf is extremely variable and depends upon several factors including N source and rate, temperature, thatch levels, irrigation, rainfall and soil pH. (5) In alkaline soils, ammonia losses can be high, especially from urea when the applied nitrogen is not watered in. (3) This is because as urea absorbs water, a free hydroxyl (OH-) is formed in close proximity to the NH4 and thus Ammonia gas (NH3) is formed. (5) Adequate irrigation after urea application on alkaline soils is usually recommended to reduce volatilisation, since the water may help to reduce the production of OH-. (5) Grass systems containiny thick thatch layers, may have high ammonia losses, since thatch contains high levels of urease, an enzyme that helps to convert nitrate into ammonia. (8)
Loss of nitrogen from turfgrass soils has been a topic of debate for many years. Nitrate (NO3-) is an anion and not held tightly by the soil, which allows the nitrate to move fairly freely through the soil profile. (5) Much concern has been expressed regarding water quality, with issues such as eutrophication frequently mentioned among environmental managers.
Managing Nitrogen Inputs
- Good cultivation practises are vital in ensuring that the rootzone contains adeqaute amounts of air, which may help to reduce denitrification losses.
- Keep thatch under control to reduce volatilisation losses.
- Irrigating after fertilisation to ensure that volatilisation losses are minimal, although care must be taken not to overwater and cause problems with leaching (particularly on sandy soils) and surface runoff.
- Careful use of nitrate nitrogen on sandy soils particularly during turfgrass establishment.
Tim Butler is currently studying for a Doctorate Degree in Sportsturf Science at both University College Dublin, Ireland and Michigan State University, U.S.A.
1. Anon. 2006. The Nitrogen Cycle http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/N/NitrogenCycle.html
2. Beard, J.B. 1973. Turfgrass: Science and Culture. Prentice-Hall, New Jersey. 658pp.
3. Bowman, DC., J.L. Paul, W.B. Davis, and S.H. Nelson. 1987. Reducing ammonia volatilisation from Kentucky bluegrass turf by irrigation. Hort. Sci. 22:84-87.
4. Carrow, R.N., Waddington, D.V., and Rieke, P.E. 2001. Turfgrass Soil Fertility and Chemical Problems. John Wiley & Sons, Hoboken, New Jersey. 400pp.
4. Horgan, B. 2004. Where did my nitrogen fertiliser go? Greenkeeper International. November. http://bigga.org.uk/magazine/back-issues/11-2004/2307/where-did-my-nitrogen-fertiliser-go.html
5. Harrison, J.A. 2003. "The Nitrogen Cycle: Of Microbes and Men," Visionlearning Vol. EAS-2 (4). www.visionlearning.com/library/module_viewer.php?mid=98
6. McCarty, L.B., Rodriquez, I.R., Bunnell, B.T., and Waltz, F.C. 2002. Fundamentals of Turfgrass and Agricultural Chemistry. John Wiley and Sons publishers, New Jersey. 375pp
7. Nelson, K.E., A.J. Turgeon, and J.R. Street. 1980. Thatch influence on mobility and transformation of nitrogen carriers applied to turf. Agron. J. 72:487-492.
8. Pidwirny, M. 2006. Fundamentals of Physical Geography, 2nd Edition. Date Viewed. http://www.physicalgeography.net/fundamentals/contents.html
9. Turgeon, A.J. 2002. Turfgrass Management - 6th edition. Prentice Hall Inc., Englewood Cliffs, New Jersey. 400 pp.