Heterotrophic microbial activities and nutritional status of microbial communities in tropical marsh sediments of different salinities: the effects of phosphorus addition and plant species

Abstract

Oligotrophic, phosphorus (P) limited herbaceous wetlands of northern Belize are being impacted by P loading from fertilizer runoff. P enrichment causes a shift in autotroph communities from a microphyte (cyanobacterial mats, CBM) to macrophyte (Eleocharis spp., Typha domingensis) dominated system. To document potential effects of P, salinity, and macrophyte species on the heterotrophic microbial community nutritional status (represented especially by specific phospholipids fatty acids and specific respiration rate), biomass and activities, we took soil samples from established P enrichment plots in replicated marshes of two salinity levels. P addition increased microbial biomass carbon (C), nitrogen (N) and P, as well as soil nutrient transformation rates (nitrogenase activity, N mineralization and immobilization, methanogenesis). The effect of plant species (Eleocharis vs Typha sites) was generally lower than the effect of P addition (CBM vs Eleocharis sites) and was most evident at the low salinity sites, where Eleocharis dominated plots had enhanced nitrogenase activity and P microbial immobilization. Salinity reduced the overall rates of microbial processes; it also weakened the positive effect of both P addition and plant species on microbial activities. Lastly, the amount of N stored in microbial cells, likely in form of osmoprotective compounds, was enhanced by salinity.

Nutrient enrichment in tropical wetlands: shifts from autotrophic to heterotrophic nitrogen fixation

“…We did not measure these processes in the presented experiment, but data measured in 2005 and 2007 are available (Pivničková et al 2010). Those data show that both N mineralization and denitrification was enhanced to a great extent in the P-amended plots.…”

“…Note that autotrophic N fixation provides the N required for macrophytes to sustain their growth (macrophyte N demand). In addition, N mineralization measured in these control plots was negative (Pivničková et al 2010) indicating that N released during mineralization of organic matter was used by microbes to cover their N demand and as a result N was not available to higher plants, which is a common pattern for N limited ecosystems (Schimel and Bennett 2004). In this study, however, macrophyte tissue N in controls does not indicate any N limitation and is equal to or even higher than in P-amended plots and macrophyte N resorption efficiency is relatively low, suggesting that dominant macrophytes in controls are not limited by N.…”

“…The samples were taken from the same layer of the sediment and biomarkers were measured in the same way as referred here. For detail on sampling and NA activity see Pivničková et al (2010) Biogeochemistry (2010 N fixation (Welsh et al 1996a), yet they are a significant component of N fixation. We believe that if the marshes in our study site contained more sulfate, sulfur reducing bacteria would contribute more to N fixation, considering that the sulfate addition resulted in an increase in N fixation of sulfur reducing bacteria (Fig.…”

“…The exudation rate correlates with primary production and relates to the input of C to sediment via decomposition of dead plant material. The litter from the enhanced production due to P enrichment is also of better quality having lower C/N and C/P ratio (Rejmán-ková and Snyder 2008;Pivničková et al 2010). (4) The slurry technique used in our experiments may provide equivocal results in C addition experiments because roots may be damaged during slurry preparation.…”

Patterns and controls of anaerobic soil respiration and methanogenesis following extreme restoration of calcareous subtropical wetlands

“…However, an additional control on microbial decomposition can be a nutrient limitation in some ecosystems (Hogg et al, 1994). Increasing P concentrations have been linked to elevated CH 4 production (Pivničková et al, 2010) although this is not always the case (Bridgham and Richardson, 1992;Drake et al, 1996) suggesting the relationship between these parameters is complex and may depend on initial soil P concentrations and OM quality.…”