Carbon cycling in a tidal freshwater marsh ecosystem:a carbon gas flux study

Abstract

A process-based carbon gas flux model was developed to calcuIate total macrophyte and microalgal production, and community and belowground respiration, for a Peltandra virginica dominated tidaI freshwater marsh in Virginia. The model was based on measured field fluxes of CO2 and CH,, scaled to monthly and annual rates using empirically derived photosynthesis versus irradiance, and respiration versus temperature relationships. Because the gas exchange technique measures whole system gas fluxes and therefore includes turnover and seasonal translocation, estimates of total macrophyte production will be more accurate than those calculated from biomass harvests. One limitation of the gas flux method is that gaseous carbon fluxes out of the sediment may underestimate true belowground respiration if sediment-produced gases are transported through plant tissues to the atmosphere. Therefore we measured gross nitrogen mineralization (converted to carbon units using sediment C/N ratios and estimates of bacterial growth efficiency) as a proxy for belowground carbon respiration. We estimated a total net macrophyte production of 536 to 715 g C m-' yr-', with an additional 59 g C m-' yr-' fixed by sediment microalgae. Belowground respiration calculated from nitrogen mineralization was estimated to range from 516 to 723 g C m-2 yr-' versus 75 g C m-' yr-l measured directly with sediment chambers. Methane flux (72 g C m-' yr-') accounted for 11 to 13 % of total belowground respiration. Gas flux results were combined with biomass harvest and literature data to create a conceptual mass balance model of macrophyte-influenced carbon cycling. Spring and autumn translocation and re-translocation are critical in controlling observed seasonal patterns of above and belowground biomass accumulation. Annually, a total of 270 to 477 g C m-2 of macrophyte tissue is available for deposition on the marsh surface as detritus or export from the marsh as particulate or dissolved carbon.

Salinity Influence on Methane Emissions from Tidal Marshes

“…Polyhaline marshes had consistently low CO 2 eq methane emissions, indicating that it may be feasible to assign carbon credits in these systems without accounting for methane emissions. The total carbon sequestered by an ecosystem can be estimated through intensive monitoring of net carbon dioxide exchange (e.g., Neubauer et al 2000, Cornell et al 2007, or by measuring changes in the size of the major carbon pools. The latter approach is most common because it is less data intensive, particularly in marshes where there is relatively little carbon accumulation in long-lived (100 year) woody biomass.…”

Electron donors and acceptors influence anaerobic soil organic matter mineralization in tidal marshes

“…The pool of labile organic carbon in soils is derived largely from plant inputs, therefore differences in plant community composition and plant ecophysiological traits, such as the C:N ratio of plant tissues, have important effects on the quality of soil carbon pools. The soils from the freshwater site were collected from a part of the marsh in which the dominant plants are broadleaf, emergent vegetation, including P. virginica and P. cordata, both of which have low shoot C:N ratios (16.3 and 11.5, respectively) (Neubauer et al, 2000), while the vegetation at the brackish site, included S. alterniflora, S. patens, and D. spicata, which have higher C:N ratios than at the freshwater site (28.6, 53.1, and 45, respectively) (Frasco and Good, 1982;Hunter et al, 2008). Carbon inputs from freshwater plants, including P. virginica and P. cordata, decompose more quickly than brackish or saline species, including S. alterniflora, because they have a higher N content (i.e., a lower C:N ratio) and a lower cellulose and lignin content (Odum and Heywood, 1978).…”

Atmospheric CO2 evasion, dissolved inorganic carbon production, and net heterotrophy in the York River estuary

“…In the upper York where CO 2 is supersaturated, phytoplankton are light-limited (Sin et al 1999), and bacterial production is greatest (Schultz 1999). The combination of these two factors in conjunction with high rates of marsh respiration in the upper York (Neubauer et al 2000) favor net heterotrophy, the accumulation of pCO 2 , and high rates of CO 2 evasion. Interestingly, other patterns in system metabolism are consistent with pCO 2 trends in the York River estuary.…”