The regulatory effects of salinity and inorganic nitrogen compounds on nitrification and denitrification were studied
in intertidal sandy sediments and rocky biofilms in the Douro River estuary, Portugal, over a 12-month period.
Nitrification and denitrification rates were measured in slurries of field samples and enrichment experiments using the
difluoromethane and the acetylene inhibition techniques, respectively. Salinity did not regulate denitrification in either
environment, suggesting that halotolerant bacteria dominated the denitrifier communities. However, nitrification rates
were stimulated when salinity increased from 0 to 15 practical salinity units. NO3
addition experiments revealed that
NO3
availability stimulates denitrification rates in sandy sediments, but not in rocky biofilms; however, in rocky
biofilms a positive and linear relationship was observed between denitrification rates and water column NO3
concentrations (r ¼ 0:92) during the monthly surveys. The N2O:N2 ratios increased rapidly when NO3
increased from
63 to 363 mM; however, results from monthly surveys showed that environmental parameters other than NO3
availability may be important in controlling the variation in N2O production via denitrification. Ammonium additions
to sandy sediments stimulated nitrification rates by 35% for the 20 mM NH4
+ addition, but NH4
+ appeared to inhibit
nitrification at high concentration addition (200 mM NH4
+). In contrast, rocky biofilmnitrification was stimulated by
65% when 200 mM NH4
+ was added.
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