Nutrient constraints on plant community production and organic matter accumulation of subtropical floating marshes

In the cycle of delta growth and decay, peat-forming wetlands span a time and space continuum. Later in the delta cycle, freshwater floating marshes become increasingly removed from external sediment subsidy and internal nutrient cycling controls plant productivity and organic matter accumulation. An interesting question is whether increased external nutrient loading can affect the basic processes that lead to peat formation—plant production and organic matter decomposition. I conducted several field studies to understand whether belowground organic matter accumulation and decomposition were affected with increased nutrient exposure; in addition, the plant community was monitored over three years. A laboratory soil respiration experiment was run to determine potential limiting nutrients to microbial activity. After two years, there was a reduction of total belowground organic matter accumulation with increased nutrients; this partially resulted from reduced belowground root deposition coupled with accelerated root decomposition. This reduction in belowground accumulation, however, may be related also to the disappearance of a common plant of the community following fertilization, and not necessarily from a phenotypic shift in the allocation pattern by the dominant plant species. Fertilization accounted for an approximate doubling in aboveground biomass, accompanied by decreased stem density, and decreased species richness. Compared to other species, the grass, maidencane (Panicum hemitomon) responded most positively to increased nutrient availability at both sites, while at one site goldenrod (Solidago sempervirens) eventually captured a significant proportion of biomass. Nutrient resorption efficiency of nitrogen (N) and phosphorus (P) were both decreased with increased fertility. The degree of N- or P-limitation observed across coastal populations of P. hemitomon reflect the composite influences of site geology, water source, fire frequency, and the more conservative cycling of P compared to N. Plant community nutrient limitation may not be a reliable predictor of the type of nutrient limitation to soil microbes. Floating marshes have developed and are sustained under nutrient restricted conditions compared to other coastal marshes. Although floating marshes may function as nutrient sinks, a change in community and plant allocation pattern can be predicted with increased nutrient enrichment of these wetland types

Component greenhouse gas fluxes and radiative balance from two deltaic marshes in Louisiana: Pairing chamber techniques and eddy covariance

Coastal marshes take up atmospheric CO2 while emitting CO2, CH4, and N2O. This ability to sequester carbon (C) is much greater for wetlands on a per area basis than from most ecosystems, facilitating scientific, political, and economic interest in their value as greenhouse gas sinks. However, the greenhouse gas balance of Gulf of Mexico wetlands is particularly understudied. We describe the net ecosystem exchange (NEEc) of CO2 and CH4 using eddy covariance (EC) in comparison with fluxes of CO2, CH4, and N2O using chambers from brackish and freshwater marshes in Louisiana, USA. From EC, we found that 182 g Cm-2 yr-1 was lost through NEEc from the brackish marsh. Of this, 11 g Cm-2 yr-1 resulted from net CH4 emissions and the remaining 171 g Cm-2 yr-1 resulted from net CO2 emissions. In contrast, -290 g Cm2 yr-1 was taken up through NEEc by the freshwater marsh, with 47 g Cm-2 yr-1 emitted as CH4 and -337 g Cm-2 yr-1 taken up as CO2. From chambers, we discovered that neither site had large fluxes of N2O. Sustained-flux greenhouse gas accounting metrics indicated that both marshes had a positive (warming) radiative balance, with the brackish marsh having a substantially greater warming effect than the freshwater marsh. That net respiratory emissions of CO2 and CH4 as estimated through chamber techniques were 2–4 times different from emissions estimated through EC requires additional understanding of the artifacts created by different spatial and temporal sampling footprints between techniques