In order to determine the impact of nutrient enrichment on phosphorus (P) limited wetlands, we established experimental P additions in marshes throughout northern Belize. P significantly increased macrophyte primary production, which led to the rapid elimination of cyanobacterial mats. The replacement of cyanobacterial mats by macrophytes constrained autotrophic nitrogen (N) fixation, increased the quantity, and changed the quality of organic matter input to the sediments. We predicted that the activity of sediment heterotrophic N fixers will be impacted by these alterations in carbon input. We used the acetylene reduction technique to measure potential (glucose amended) nitrogenase activity (NA) in sediments from controls and treatment plots that have been P enriched for four years and dominated either by Eleocharis cellulosa, or Typha domingensis for two years. NA in P-enriched plots was 2-3 orders of magnitude higher than NA in controls. NA was positively correlated with the soil reactive P, both total organic and microbial carbon, live root biomass, and total phospholipid fatty acids (PLFA) as an indicator of active microbial biomass. It was negatively correlated with the concentration of ammonium-N. Path analysis revealed that the indirect effect of P on NA through the root biomass was more important than the direct effect of P. NA of the upper sediment layer was consistently higher in Eleocharis than in Typha dominated plots, despite the higher litter input by Typha. We feel that the higher levels of lignin and phenolics occurring in Typha litter, relative to Eleocharis, constrained NA in Typha plots.
This study presents rock strength variations at granite outcrops and in subsurface vertical profi les in the Jizerské hory Mountains, Czech Republic. Schmidt hammer rebound values in subsurface profi les change gradually from the bedrock surface downward. An exponential relation has been observed between the R-values and depth in rock outcrops to a depth of around 4·5 m. The exponential nature of the curve indicates that rock hardness increases more rapidly with depth in the uppermost 1 m section of the rock profi le. A detailed study of rebound values obtained from both intact and polished rock exposures reveal effects of surface grinding on results of the Schmidt hammer method. The range of data collected increases after grinding, allowing more precise discrimination of rock surfaces in respect of age and weathering. The Schmidt hammer method may be used effectively as a relative-age dating tool for rock surfaces that originated during the Late Pleistocene. It is concluded that this time limitation can be signifi cantly mitigated by surface grinding before measurement.
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