Chemical Stability of Nitrate in Soils

Abstract

The chemical stability of nitrate and its reduction into nitrous oxide or nitrogen gases as end products was theoretically studied using Nernst's equation , under different sets of Eh (0,200 and 400mv) and pH (5, 7 and 9) comparable to those of soils.Calculations using Nernst's equation and published data on ion activities and redox potential declared that nitrate stability is very sensitive to little changes in soil reaction (pH) and/or redox (Eh) values. Nitrate transformation into N2O and N2 goes easier at low than at high pH and Eh values. A decrease of pH by one unit increases the activity of the produced N2O and N2 by 10 10 and 10 12 folds, respectively. Also, their activities increased by 10 11 and 10 16 folds due to decreasing Eh value by 100 millivolts. In addition, when increasing the nitrate activity by 10 folds, the activity of any of the two gases increases by 100 folds. Among the two gases, N2 production was more sensitive than N2O production towards the changes in pH and Eh values.Similar trends were found by other workers as to the effect of pH and Eh on nitrate reductions. Therefore, it is expected that small changes in soil pH and/or Eh can cause important chemical reduction of the applied nitrate fertilizers especially under extensive cropping systems to form N2O gas which shares in the environmental problems through its reaction with the ozone layer.

SOIL REDOX AND pH EFFECTS ON NITRATE REDUCTION UNDER FLOODED CONDITIONS

“…It seems that nitrate reduction works better under low pH value when combined with high Eh than with low Eh values. Similar conclusion was drawn by Lindsay et al (1981) and El-Sebaay (2000) who confirmed that acid oxidized conditions favour chemical reduction of nitrate under oxidized conditions. However, even at 400 mV and pH 9 there was still some nitrate reduction.…”

“…400 mV) the rate of NO -3 disappearance was faster under acid than under alkaline conditions. This can be explained by the known reverse relationship between pH and Eh, beside the predominance of chemical NO -3 reduction under acidic and oxidized conditions over the biological one which is enhanced by alkaline and reduced conditions as was found by El-Sebaay and Khaled (1991), Hojberg et al (1996) and El-Sebaay (2000). On the other hand, under comparable reduced conditions (e.g.…”

“…On the other side, Atta and Van Cleemput (1981); Tiedje et al (1982) and Peoples et al (1995) stated that Eh values below 200 mV induced NO -3 reduction. Theoretical calculations made by El-Sebaay (2000) confirmed that a small decrease in Eh or pH can cause marked increase in the chemical reduction of nitrate.…”

“…However, it is still extremely difficult to elucidate to what extent NO -3 reduction is the result of biological or chemical reactions. In recent years, loss of nitrogen due to chemical process has gained wide research interest (El-Sebaay, 1991;El-Sebaay and Khaled 1991;Hojberg et al 1996;Abbasi and Adams, 1999;El-Sebaay, 2000). Loss due to denitrification has received a lot of attention although it represent a small portion of the total NO -3 loss.…”

“…Also, Stanford et al (1975) and Buresh and Patrick (1978) concluded that NH + 4 rather than NO -3 was the primary form of N-assimilation. Of the many environmental factors governing NO -3 reduction, the aeration status is very important (Lindsay, 1979;Firestone, 1982 andEl-Sebaay, 2000). Redox potential (Eh) in combination with soil reaction (pH) reflect this aeration status.…”