Technical Sheet - The Nitrogen Cycle
Review by Paul Storer (B.Sc., M.Sc.) Microbiologist, Western Mineral Fertilisers

Introduction:

All life requires nitrogen-compounds, e.g., proteins and nucleic acids. Air, which is 79% nitrogen gas (N2), is the major reservoir of nitrogen. However, most living organisms cannot use nitrogen in this form. Plants must obtain their nitrogen in "fixed" or "reactive" form, i.e. incorporated in compounds such as:

• nitrate ions (NO3-)
• ammonium (NH4+)
• urea (NH2)2CO.

Animals generally obtain their reactive nitrogen (& all other) compounds from plants (or animals that have eaten plants).

Four processes participate in the cycling of nitrogen through the environment:

• nitrogen fixation (biological fixation by certain microbes - alone or in nodules forming a symbiotic relationship with plants),
• decay (microorganisms metabolize & break down organic nitrogen compounds in excretions and dead organisms into ammonia - can be taken up directly by plants - usually through their roots),
• nitrification (or 'ammonium oxidation' - most of the ammonia either in the soil or produced by decay is converted into nitrates - ammonium N is taken up by special nitrifying bacteria, and converted to nitrate N),
• denitrification (specific anaerobic bacteria reduce nitrates to nitrogen gas, replenishing the atmosphere)

Microorganisms play major roles in all four of these processes.

Nitrogen in Agriculture:

In modern agriculture, factors such as nitrogen deficiency are now seen as major constraints to production and yield 1. As a result, crop production systems have become overly-dependent upon nitrogen fertilisation, and the temptation to over-fertilise with Nitrogen is therefore high 2. However, only 40-50% of fertiliser nitrogen is taken up by plants. The large remainder converts to volatile ammonia gas (volatilisation losses from urea in the field can range up to 20 per cent of the nitrogen applied 3), nitric oxide or to nitrate - which can pollute the environment. In addition, excess plant-absorbed nitrogen can be lost through ammonium or nitrate efflux from roots or ammonia volatilisation from leaves 4.

Little ammonium nitrogen (eg ammonium sulfate) is leached - except on very poor sandy soils. The longer the nitrogen stays in the ammonium form in the soil, the less susceptible it is to leaching. However, urea readily leaches while it remains as urea; but in its nitrate form, nitrogen is extremely soluble and so is readily leached from soils into ground water reservoirs 3.

Nitrogen nutrition has significant effects on root and shoot relations 5,6,7. Excess nitrogen nutrition causes excessive cell elongation, accelerated shoot and vegetative growth, reduces availability of assimilates to the root, reduces the Root/Shoot ratio 8,9, and can have a negative impact on general plant viability (frost and pest damage). Excessive nitrogen fertilisation leads to excess vigor, poor fruit or grain set, poor produce quality, and inadequate hardening off.

Imbalances in uptake and assimilation for N compounds do occur - with nitrate regularly accumulating in leaves and research has demonstrated a role for water in nitrate homeostasis 10. Chauboussou 11,12 (1980, 1985) demonstrated that plant susceptibility to pest or disease attack was related to an imbalanced use of fertiliser (excess nitrogen and deficiency in trace-elements in the soil). Excessive use of nitrate fertilisers inhibit the formation of normal plant proteins & stimulates an over-abundance of unused amino acids that attracts insects 12.

Beneficial Microbes and Nitrogen :

Research in plant nutrition has mainly concentrated on the role of rhizobia for fixed nitrogen uptake 13 and mycorrhizas on their importance for P, K and water. However, more recently significant uptake and transfer of N from both soil ammonium and amino acid sources has been demonstrated for mycorrhizas 14,15,16.

Ammonium is the preferred nitrogen source of bacteria and fungi. Biologically, the rate of "nitrification" carried out by microorganisms depends on several factors, including soil pH and soil moisture. The process is slow on low pH soils and rapid on alkaline soils. Plants preferably acquire nitrogen from the soil and from microorganisms in the form of ammonium 17.

Microbial biomass strongly decreases and the ratio of fungi : bacteria becomes unbalanced when excessive amounts of nitrate fertilisers are applied to the soil 18. When beneficial microbes are depleted from the soil, they are no longer present to convert inorganic minerals into bioavailable organic minerals needed by plants.

Herbicides can have a detrimental effect on the balance of soil microbiology & also on nutrient uptake 19. The phytotoxic residues of herbicides can be detected in the soil up to 84 days after application 20. SU (Sulfonyl Urea) herbicides inhibit an enzyme (acetolacetate synthase - involved in the synthesis of amino acids - valine, leucine and isoleucine) in certain plants and bacteria, but not in fungi. This can result in an in imbalance in the fungal : bacterial ratio (by increasing fungal dominance), and can lead to a Nitrogen reduction in plants.

Conclusion:

Therefore, the type and amount of nitrogen applied to plants must be optimal for root and shoot relations. Adequate Nitrogen fertiliser (and not excessive amounts!) promotes good root development, extending roots ability to extract water and nutrients in soil, increases leaf photosynthesis 21 and encourages beneficial microorganisms.

Western Mineral Fertilisers recommends and uses only biologically-friendly forms of nitrogen in our Mineral fertilisers. WMF's Plants-Microbe-Mineral-Soil management system assists with more efficient uptake, retrieval and internal use of nitrogen (and other nutrients). This can help to decrease overall nitrogen fertiliser input, and nitrogen load to the environment.

References:
1. Hamblin, A. and G. Kyneur (1993): Trends in Wheat Yields and Soil Fertility in Australia. Bureau of Resource Sciences,Australian Government Printing Service, Canberra. 139 pp.
2. Rahn C., De Neve S., Båth B., Bianco V. V., Dachler M., Cordovil. M.d.S C., Fink M., Gysi C., Hofman G., Koivunen M., Panagiotopoulos L., Poulain D., Ramos C., Riley H., Setatou H., Sørensen J.N., Titulaer H. and Weier U., (2001): A comparison of fertiliser recommendation systems for cauliflowers in Europe, Acta Horticultura 563 : 39-45.
3. Mason, M. (1996): Nitrogen fertiliser sources for crops, Department of Agriculture, Western Australia Farmnote 27/96
4. Kolb, K. J. and Evans, R. D. (2002): Implications of leaf nitrogen recycling on the nitrogen isotope composition of deciduous plant tissues. New Phytol., 156, 57-64.
5. Feng, W.L. & Liu C.M. (1996): Regulation of soil water on the growth & distribution of root system of crops. Res. Ecol. Agro. 3: 5-9.
6. Lioert, F., Casanovas C., and Penuelas J. (1999): Seedling survival of Mediterranean shrub land species in relation to root: shoot ratio, seed size and water and nitrogen use. Functional Ecol. 13: 210-216.
7. Parsons, R. and Sunley, R.J. (2001): Nitrogen nutrition and the role of root-shoot nitrogen signalling particularly in symbiotic systems Journal of Experimental Botany, Vol. 52, No. 90001, pp. 435-443.
8. Layzell DB, Pate JS, Atkins CA, Canvin DT. (1981): Partitioning of carbon and nitrogen and the nutrition of root and shoot apex in nodulated legume. Plant Physiology 67, 30-36.
9. Passioura, J.B. (1983): Root and drought resistance. Agric. Water Manage. 7: 265-280.
10. Cardenas-Navarro R, Adamowicz S, Robin P. (1999): Nitrate accumulation in plants: a role for water. Journal of Experimental Botany 50, 334, 613-624.
11. Chauboussou F. (1980), Les plantes malades des pesticides, Ed. Debard, Paris.
12. Chauboussou F. (1985), Sante des cultures, une revolution agronomique, Ed. Flammarion, Paris.
13. Atkins CA, Sanford PJ, Storer PJ, Pate JS (1988): Inhibition of nodule functioning in cowpea by a xanthine oxidoreductase inhibitor, allopurinol. Plant Physiol 88: 1229-1234
14. Barea JM, Azcon R, and Azcon-Aguilar C. (1996): The use of 15N to assess the role of VA mycorrhiza in plant N nutrition and its application to evaluate the role of mycorrhiza in restoring Mediterranean ecosystems. In: Read DJ, Lewis DH, Fitter AH, Alexander IJ, eds. Mycorrhizas in ecosystems. Cambridge: Academic Press, 190-197.
15. Finlay RD (1996): Uptake and translocation of nutrients by ectomycorrhizal fungal mycelia. In: Read DJ, Lewis DH, Fitter AH, Alexander IJ, eds. Mycorrhizas in ecosystems. Cambridge: Academic Press, 91-97.
16. Leake JR & Read DJ. (1991): Proteinase activity in mycorrhizal fungi. 3.Effects of protein, protein hydrolysate, glucose & NH4+ on production of extracellular proteinase by hymenoscyphus-ericae (Read) Korf & Kernan. New Phytologist 117, 309-317.
17. von Wiren N., Gazzarrini S., Gojon A., Frommer W.B. (2000): The molecular physiology of ammonium uptake and retrieval. Curr Opin Plant Biol, 3:254-261.
18. Wallenstein,M.D.(2004): Effects of increased Nitrogen deposition on forest soil, Nitrogen cycling & Microbial community structure, PhD, Duke Uni.
19. Tworkoski, T.I. and Welker. W.V. (1996): Effect of twelve annual applications of diuron, simazine, and terbacil on a soil microbe community in West Virginia. Proc. Northeastern Weed Sci. Sac. 50:2- 6
20. Ismail B. S. and Kalithasan K. (1997): Effects of repeated application on persistence and downward movement of four herbicides in soil Australian Journal of Soil Research, 35(3) 503 - 514.
21. Li, Y.Z., Wang, F.X., and Liu L.H (1999): Use and management of soil water and nitrogen resource. I. Soil water and nitrogen conditions and root development. Plant Nutr. Fert. Sci. 5: 206-313.