J. W. Hansen’s research while affiliated with Aarhus University and other places

The effect of the seagrass Zostera capricorni on sediment microbial processes was studied in a tank experiment, where vegetated and unvegetated control sediments were incubated in 10 and 50% of incident light. Leaf and root-rhizome biomass, shoot density, and leaf productivity were significantly higher when plants were incubated in 50 % than in 10 % of incident Light. Nitrogen fixation, sulphate reduction, and urea turnover in the Z. capricorni vegetated sediment were higher in the 50% than in the 10% light treatment and higher in the vegetated than in the unvegetated sediment. The stimulation of microbial processes in the Z, capricorni vegetated sediment took place in the rhizosphere, where nitrogen fixation and sulphate reduction in particular were stimulated. The sediment studies were supplemented by measurements of nitrogen fixation, sulphate reduction, and urea turnover by microorganisms associated with the roots and rhizomes of Z. capricorni. The rates of nitrogen fixation and sulphate reduction associated with root-rhizomes were up to 40- and 7-fold higher, respectively, than the highest respective sediment rates, whereas the root-rhizome associated urea turnover was lower than sediment rates. Nitrogen fixation and sulphate reduction associated with root-rhizomes could account for up to 39 and 4%, respectively, of the depth-integrated sediment rates. Nitrogen fixed by microorganisms associated with root-rhizomes could supply up to 65 % of the nitrogen needed for plant growth. Further, it was estimated that 8 to 18% of the carbon fixed by Z. capricorni was released to the sediment by the roots and rhizomes. Urea turnover was suggested to be an important intermediate in the gross production of ammonium, and a low net production of ammonium indicated rapid internal nitrogen cycling within the sediment.

The leakage of urea, ammonium, dissolved organic nitrogen, and dissolved organic carbon from Zostera marina root-rhizomes during different porewater extractions was examined. In addition, the concentrations of these compounds were measured in root-rhizomes to estimate the potential leakage from root-rhizomes during sediment handling. The concentrations of ammonium, urea, and dissolved organic nitrogen in root-rhizomes were 22 to 51 times higher than the respective average porewater concentrations within the upper 5 cm of the sediment. Up to 70% of the compounds in question leaked from root-rhizomes during sediment handling, with a resultant, up to 8-fold, increase in the respective porewater concentrations. The porewater concentrations of ammonium, urea, and dissolved organic nitrogen and carbon were on average overestimated by a factor of 2 within the upper 5 cm of the sediment due to leakage from root-rhizomes. Thus, it is recommended that roots and rhizomes are removed from the sediment before porewater extraction by conventional methods.