Dissimilatory reduction is the process through which some microorganisms use the energy generated by the electron transport from an organic or inorganic source to nitrate or to a more reduced nitrogen oxide. Many microorganisms can use NO₃^- as their primary electron acceptor when low O₂ availability restricts their metabolism (Granli and Bøckman 1994):

5(CH₂O) + 4NO₃^- + 4H+ → 5CO₂ + 7H₂O + 2N₂ + energy

Furthermore, some groups of denitrifiers are able to use simultaneously both oxygen and nitrate or nitrite as electron acceptor. Therefore, denitrification by those organisms can occur under aerobic conditions. "Aerobic denitrification" can occur in the presence of significant amounts of oxygen (Robertson and Kuenen, 1991; Zumft and Kroneck, 1990). Thirteen arable Polish topsoils (0-30 cm), Calcaric Regosols (according to FAO/UNESCO), developed from different parent materials were used in the study. The soils showed a large variation of the soil texture, pH, organic C, and endogenous NO₃^- content. Soil samples originated from various regions and represent almost the whole territory of Poland. The 5-g portions of air-dried sieved (1-mm sieve) soils were placed in 38-cm³ glass vessels and enriched with KNO₃ at the rate of 100 mg NO₃^--N per kg dry soil. This N rate corresponded approximately to 300 NO₃^--N kg per ha (calculated on 20 cm layer basis). The soil/water ratio was about 1:1 (w/w); 0.5 ml of solution (containing 1g NO₃^- -N kg^-1) and distilled water (4.5 ml). The initial concentration of O₂ in the gas headspace at the start of the incubation was 10 % ± 0.5 (replaced by N₂). The soils were incubated at 20^oC for 34 days.

Table 1. N₂O and O₂ consumption and NO₃^-N reduced and denitrified for the investigated soils

Soil developed from	Texture/soil number	NO3-N reduced	NO3-N denitrified to N2O	Consumption
				N2O-N	O2
		[%]
sand	sand (S) 39	38.2	16.5	30.0	80.0
	loamy sand (lS) 113	20.2	11,3	18.4	89.5
	loamy sand (lS) 224	22.1	10.7	30.2	92.3
	loamy sand (lS) 434	25.0	10.9	10.0	82.3
	loamy sand (lS) 772	31.1	13.4	20.0	89.1
silt	silty loam (iU) 951	100	40.1	100	99.0
	silty loam (iU) 984	100	59.5	100	98.8
loam	sandy loam (sL) 328	100	40.5	99.2	86.3
	sandy loam (sL) 342	59.4	13.3	36.6	94.6
	sandy loams (sL) 351	100	21.1	99.3	98.0
	sandy loams (sL) 543	87.9	33.5	98.4	94.4
	sandy loams (sL) 922	100	36.6	99.9	98.6
	sandy loam (sL) 947	100	39.3	100	98.9

The period of nitrous oxide production in the investigated soils was followed by its consumption. Higher nitrate reduction and N₂O and O₂ consumption were observed in finely (e.g. loamy and silty) than in coarsely textured soils. Denitrifiers in the investigated soils were able to use, simultaneously as well as oxygen, nitrate and nitrous oxide as electron acceptors, with some preference. All soils developed from sand used simultaneously three electron acceptors without any preference. Two of the soils developed from loam (No 351 and 543) used nitrous oxide faster than nitrate. The rest of the soils developed from loam and all of those developed from silt preferred nitrate to nitrous oxide. The use of nitrate and nitrous oxide as electron acceptors during all the time of incubation was accompanied by oxygen consumption.

1. Granli, T. and Bøckman, O. (1994) Nitrous oxide from agriculture. Norw. Agric. Sci. Suppl. 12. p.128.
2. Robertson, L.A. and Kuenen, J.G. (1991) Physiology of nitrifying and denitrifying bacteria. In Rogers, J.E. and Whitman, W.B. (eds) Microbial production and consumption of greenhouse gases: methane, nitrogen oxides, and halomethanes. American Society for Microbiology, Washington D.C. 189-235.
3. Zumft, W.G. and Kroneck, P.M.H. (1990) Metabolism of nitrous oxide. Revsbech, N.P. and Sorensen, J. (eds) Denitrification in soil and sediment. Plenum Press, New York pp.37-55.