duced to gaseous products (King and Nedwell
1987). Laverman et al. (2007) have shown that
denitrification in estuarine sediments is nitratelimited
and the resident denitrifying community
rapidly adjusts its level of activity to increased
nitrate availability. Denitrification accounts for
27% to 57% of the nitrate consumption in estuarine
and coastal sediments (Nishio et al. 1982).
The Divar mangrove ecosystem is prone to high
nutrient input from mining rejects, land runoff and
domestic sewage discharge. The increase in denitrification
activity especially in the first few centimeters
is indicative of the high nitrate removal
capacity of these sediments. Corredor and Morell
(1994) have confirmed that mangrove sediment–
microbial communities are capable of depurating
up to 10–15 times the nitrate added. Recently,
Krishnan and Loka Bharathi (2009) have shown
that nitrification rates in the Divar sediments vary
between 2.7 to 18.2 nmol g−1 h−1. A strong coupling
between redox processes of the N cycle
could exist in these sediments wherein nitrate
supplied continuously through the nitrification
process could be fueling denitrification especially
in the upper few centimeters.
Organic carbon addition stimulated denitrification
activity mostly at depths ≥4 cm
(Fig. 6). However, maximum activity of only 35.24
(±9.93) μmol N2O-N m−2 h−1 was recorded at 4–
6 cm depth. A 0.5% amendment of labile organic
carbon (glucose) was found to effectively stimulate
denitrification activity at all depths suggesting
that the process is optimal at this concentration.
Statistical analyses did not show significant increase
in denitrification activity on organic carbon
addition as compared to amendments with
nitrate (one-way ANOVA; n = 15; p < 0.001) at
all depths indicating that organic carbon was not
a limiting factor for denitrification in mangrove
sediments. Denitrification in oxygen minimum
zones is known to be fueled almost entirely by
organic matter supplied by particles sinking vertically
from the euphotic zone (Anderson et al.
2007; Ward et al. 2008). In these environs, organic
carbon is the main limiting factor controlling
denitrification. In contrast, estuarine systems
have considerable organic C loading and labile
organic matter is readily available for metabolic
activity. In such circumstances, denitrification is