Biological Processes in the Soil and Floodwater - The Biogeochemistry of Submerged Soils - page 139

5.1.2 NITRATE REDUCTION
Figure 4.1 shows that NO 3 − is the stable form of nitrogen over the usual range of
pe + pH in aerobic environments. The fact that most of the N 2 in the atmosphere
has not been converted to NO 3 − therefore indicates that the biological mediation
of this conversion in both directions is inefficient. Hence NO 3 − reductiontoN 2
occurs by indirect mechanisms involving intermediaries. Dissimilatory reduction
of NO 3 − (i.e. where the nitrogen oxide serves as an electron acceptor for the cell's
metabolism but the N reduced is not used by the microbes involved) potentially
occurs by two processes:
denitrification,
NO 3 − −−−→ NO 2 − −−−→ NO −−−→ N 2 O −−−→ N 2
andreductiontoNH 4 + ,
NO 3 − −−−→ NO 2 − −−−→ NH 4 +
Assimilatory NO 3 − reduction might also occur. But because concentrations of
NH 4 + and organic N are in general large in anaerobic environments, it is sup-
pressed and insignificant. The literature on NO 3 − reduction is reviewed by
Tiedje (1988).
In most submerged soils dissimilatory reduction to NH 4 + is much less impor-
tant than denitrification because reduction to NH 4 + is a strictly anaerobic process
and any NO 3 − entering the soil or formed in oxic zones is denitrified before
it reaches a sufficiently reduced environment (Buresh and Patrick, 1981). The
importance of dissimilatory reduction depends on the ratio of available carbon to
electron acceptors. Reduction to NH 4 + produces more electrons per unit NO 3 −
reduced (8 compared with 5), but less energy. It therefore dominates in contin-
uously anaerobic environments with a high ratio of available carbon to electron
acceptors, such as the rumen, whereas denitrification dominates in environments
with a low ratio of available carbon to electron acceptors, such as in most sub-
merged soils and in anaerobic microsites in otherwise aerobic soils. Buresh and
Patrick (1981) found 15% of NO 3 − reduction was to NH 4 + in unplanted sub-
merged sediment, and Buresh et al . (1989) found the equivalent figure for rice
soils was less than 5%.
Carbon acts as the electron donor for denitrification. The availability of carbon
often limits denitrification in anaerobic microsites in non-submerged soils. As a
result, the reaction does not go to completion and the intermediaries NO 2 − and
N 2 O accumulate. Completion of the reaction may also be hindered by low pH. But
under uniformly anaerobic conditions NO 3 − as electron acceptor is more likely
to be limiting than carbon as electron donor because NO 3 − is not regenerated.
Therefore the rate of denitrification is limited by the supply of NO 3 − rather
than carbon, and proceeds almost completely to N 2 .