Dynamic nature of the turnover of organic carbon, nitrogen and sulphur in the sediments of a Jamaican mangrove forest

Abstract

Rates of oxygen uptake, sulphate reduction, carbon dioxide production, ammonium turnover, nitrogen fixation and denitrification were measured in sediments in a transect across a mangrove swamp in Jamaica. Carbon and nitrogen mineralisation increased as the transect entered the mangrove, indicating a greater availability of organic matter, probably by root excretion. Time course measurements of sulphate reduction indicated a subsurface pool of labile carbon. Carbon dioxide production within the mangrove exceeded oxygen flux across the sediment surface, indicating a subsurface source of oxygen of 28 to 179 mm01 m-' d-' The turnover time for the sedimentary sulphide pools decreased as the transect entered the mangrove. The production of ammonium from organic nitrogen, measured by I5NH,+ turnover, also indicated active subsurface organic matter mineralisation. The source of this organic nitrogen is unknown. Measured rates of nitrogen fixation were too low to supply the required organic nitrogen. The estimated net availability of ammonium to the plant roots was 10 mm01 m" d-' This would give a productivity of 2000 g C m-2 yr-'.

Benthic decomposition rates and pathways in plantations of the mangrove Rhizophora apiculata in the Mekong delta, Vietnam

“…It is likely, however, that our measurements incorporate most of the benthic microbial activity in these forests, as sediments >40 cm depth consisted of very hard grey clay with minimal organic matter. Our findings are in agreement with similar l measurements made in high intertidal forests in southern Thailand, where sulfate reduction accounted for only 11 % of total carbon mineralization (Kristensen et al 1995), and in high intertidal forests in Jamaica (Nedwell et al 1994), where oxic respiration (78 mm01 C m-2 d-l) was greater than rates of sulfate reduction (23 mm01 C m-2 d-l 1.…”

“…Slow rates of denitnfication have been measured in other mangrove forests (Shaiful et al 1986, Shaiful 1987, Nedwell et al 1994, Knstensen et al 1995, RiveraMonroy & Twilley 1996. This phenomenon has been explained as a result of either immobilization of inorganic N or assimilation by mangroves, or a combination of both processes.…”

“…11) and rates of sulfate reduction were equivalent to rates measured in other high-intertidal mangrove forests (Kristensen et al 1992, 1995, Nedwell et al 1994, Alongi et al 1998, Alongi 2000. Rates of total mineralization, oxygen flux, and sulfate reduction varied greatly between sampling periods, although there were no obvious ecological or climatological changes over time.…”

“…The slow rates of sulfate reduction, particularly in surface sediments, may reflect oxidation of labelled sulfides produced during the core incubations, although this may have been limited by the short (4 to 6 h) incubation times. We did not evaluate time-course uptake of 35S04, although Nedwell et al (1994) observed linear uptake of 35S04 for up to 7-8 h in Jamaican mangrove sediments with higher rates of sulfate reduction activity.…”

“…These plant-microbial links are reflected in clear evidence that highly productive mangrove forests serve as sinks for many species of dissolved nutrients (Boto 1992, Kristensen et al 1995, Rivera-Monroy & Twilley 1996, Duarte et al 1998). Mangrove-microbenutrient interrelationships are complex (Boto 1992, Nedwell et al 1994) and need to be more fully understood in order to assist managers in maximizing mangrove yields. In this paper, we examine sediment biogeochemistry in 6, 8, and 35 yr old plantations of Rhiznphorir apiculata in the lower Mekong delta.…”