BIOGEOCHEMISTRY OF NITROUS OXIDE PRODUCTION IN THE RED MANGROVE (Rhizophora mangle) FOREST SEDIMENTS
Jorge Bauza, Julio M. Morell and Jorge E. Corredor
Department of Marine Sciences
University of Puerto Rico
Mayaguez, Puerto Rico 00680
RUNNING HEAD: Nitrous Oxide in Mangrove Sediments
keywords: nitrous oxide, nitrification, mangrove forest
The present study was undertaken to quantify the emission and distribution of nitrous oxide and to explore its relation with pertinent physical and chemical parameters of the red mangrove forest sediment. Rates of N2O evolution, which ranged from 0.052 to 1.37 Ýmole.m-2.h-1 (overall mean = 0.495 Ýmole.m-2.h-1), are comparable to those of other ecosystems that has been previously studied. A significant diel flux change in nitrous oxide emission was observed. Dissolved nitrous oxide concentration averaged 0.149 nmole.cm-3 (SD = 0.09, n = 54) with a range from 0.096 to 0.574 nmole.cm-3. Dissolved and exchangeable inorganic nitrogen was present mostly in the form of ammonium (from 199 to 272 nmole.cm-3) with lesser amounts of nitrate (overall mean = 29.0 nmole.cm-3). Redox potentials in the sediments generally decreased with depth, with a mean value of 377 mV at the sediment surfaces and lower mean value (159 mV) at 10 cm deep. We have explored the probable sources of nitrous oxide in the mangrove forest sediment using linear and multiple regression and correlation between the data obtained in this study and comparing this observations with previous studies of N2O metabolism. Our results, while not excluding the possibility of N2O production through denitrification, indicate that N2O is produced mainly by nitrification in sediments of this mangrove forest.
Nitrous oxide is a trace gas produced under natural conditions by the processes of nitrification and denitrification as part of the biogeochemical cycle of nitrogen (Firestone and Davidson, 1989). It was not until approximately 180 years after it discovery that was addressed the role of N2O in the destruction of stratospheric ozone (Crutzen, 1976) and in the radiative heat budget of the troposphere (Wang et al., 1976). Theoretical prediction has demonstrated that a doubling in the atmospheric abundance of N2O was therefore expected to yield a 20% decrease in total stratospheric ozone (Delwiche, 1981). At the same time, the characteristic absorption of the nitrous oxide molecule in the infrared range (7.8 and 17.0 Ým bands ) and its long atmospheric residence time of about 110-180 years make it act as a greenhouse gas about 300 time more powerful than CO2 (Wang et al., 1976). Troposheric N2O show an annual growth rate of about 0.25% to 0.31% per year (Prinn et al., 1990) and the estimated anthropogenic sources do not seem to be large enough to match this increase. However, recent compilation of different studies of N2O emission indicate that natural sources dominate the global budget. A growing interest has evolve to study these natural sources of nitrous oxide in view, not only of its great potential to influence directly the world's climate but on the uncertainties associated with the N2O global budget. The marine environments constitute a significant but poorly characterized sources of atmospheric N2O (Sorensen et al., 1981; Hansen et al., 1981; Nishio et al., 1983; Barge et al.,1996). Mangrove forest are an assemblage of tropical trees that dominate the intertidal zone in the world¡¦s tropical and subtropical coasts, paralleling the geographical distribution of coral reefs. Rhizophora mangle (red mangrove) is the dominant mangrove species of the neotropical intertidal regions and covers an area of 6,400 ha in Puerto Rico alone (Carrera and Lugo, 1978). Globally, this mangroves species may fringe as much as 60-75% of the tropical and subtropical coastline, or about 1.8 x 105 km2 (Spalding, 1997). However, their role in trace gas metabolism is poorly understood and...
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