Saturday, 15 July 2006

The Preference for Electron Acceptors during Nitrate Reduction.

Teresa Włodarczyk1, Witold Stępniewski2, Małgorzata Brzezińska1, and Urszula Kotowska1. (1) Institute of Agrophysics PAS, Doswiadczalna 4, Lublin, Poland, (2) Technical Univ of Lublin, Nadbystrzycka 40b, Lublin, Poland

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 20oC 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.

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