Biogeochemistry of Nitrous Oxide Production in the Red Mangrove (rhizophora Mangle) Forest Sediments

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

ABSTRACT

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 Ñ"Эmole.m-2.h-1 (overall mean = 0.495 Ñ"Эmole.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.

INTRODUCTION

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 Ñ"Эm 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 worldÐŽ¦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 therefore this system has been extremely undersampled . The goals of the study were to quantify the emission of nitrous oxide and to explore the relation between dissolved nitrous oxide concentration with pertinent physical and chemical parameters in the mangrove forest sediment. We present evidence that mangroves are a significant sources of nitrous oxide to the atmosphere and that nitrification is the main source of nitrous oxide production in the mangrove sediments studied.

MATERIALS AND METHOD

Experiments were carried out in a mangrove fringing forest dominated by the red mangrove (Rhizophora mangle) on the northeast side of Magueyes island, the University of Puerto Rico, Department of Marine Sciences field station. Magueyes island is located at about 17o 58ÐŽ¦ N 67o 03ÐŽ¦ W at the southwestern corner of Puerto Rico. The mangroves at our study site are classified as a fringe forest (Lugo and Snedaker, 1974) and, for the most part, are located above mean sea level so that roots in this mangrove forest are not continuously submerged but rather undergo only occasional

submergence during the highest tides. The Magueyes Island mangrove fringe has a muddy substrate of organic peat derived largely from the roots of the mangroves themselves and leaf litter. Detailed analyses of the ecological structure and metabolism of this forest have been presented by Golley et al. (1962).

Three stations along a transect normal to the coastline, separated by approximately 20 meters each, were established for diel variations measurements of nitrous oxide fluxes. These stations were the landside station (A), the middle station (B), and the seaside station (C), near the island channel. Only the middle station was sampled for the detailed study of physical and chemical parameters.

Intact sediment cores were taken using a perforated piston-type corer (30 cm long and 3.6 cm wide) for sampling of the upper 14 cm of sediment. The corer was designed to reduce sediment compaction during sampling. From each sediment core, samples of 8 cm-3 were obtained at vertical