Global trends and uncertainties in terrestrial denitrification and N2O emissions.

Soil nitrogen (N) budgets are used in a global, distributed flow-path model with 0.5° × 0.5° resolution, representing denitrification and N2O emissions from soils, groundwater and riparian zones for the period 1900-2000 and scenarios for the period 2000-2050 based on the Millennium Ecosystem Assessment. Total agricultural and natural N inputs from N fertilizers, animal manure, biological N2 fixation and atmospheric N deposition increased from 155 to 345 Tg N yr(-1) (Tg = teragram; 1 Tg = 10(12) g) between 1900 and 2000. Depending on the scenario, inputs are estimated to further increase to 408-510 Tg N yr(-1) by 2050. In the period 1900-2000, the soil N budget surplus (inputs minus withdrawal by plants) increased from 118 to 202 Tg yr(-1), and this may remain stable or further increase to 275 Tg yr(-1) by 2050, depending on the scenario. N2 production from denitrification increased from 52 to 96 Tg yr(-1) between 1900 and 2000, and N2O-N emissions from 10 to 12 Tg N yr(-1). The scenarios foresee a further increase to 142 Tg N2-N and 16 Tg N2O-N yr(-1) by 2050. Our results indicate that riparian buffer zones are an important source of N2O contributing an estimated 0.9 Tg N2O-N yr(-1) in 2000. Soils are key sites for denitrification and are much more important than groundwater and riparian zones in controlling the N flow to rivers and the oceans.

Does microbial stoichiometry modulate eutrophication of aquatic ecosystems?

The stoichiometry of prokaryotes (Bacteria and Archaea) can control benthic phosphorus (P) fluxes relative to carbon (C) and nitrogen (N) during organic matter remineralization. This paper presents the first experimental data on benthic microbial stoichiometry. We used X-ray microanalysis to determine C : N : P ratios of individual prokaryotes from C-limited Baltic Sea sediments incubated under oxic or anoxic conditions. At approximately 400:1, C : P ratios of prokaryotes from both oxic and anoxic incubations were higher than the Redfield ratio for marine organic matter (106:1), whereas prokaryotic C : N ratios (6.4:1) were close to the Redfield ratio. We conclude that high microbial C : P ratios contribute to the enhanced remineralization of P from organic matter relative to C and N observed in many low oxygen marine settings.

Geochemistry of trace metals in a fresh water sediment: field results and diagenetic modeling.

Concentrations of Fe, Mn, Cd, Co, Ni, Pb, and Zn were determined in pore water and sediment of a coastal fresh water lake (Haringvliet Lake, The Netherlands). Elevated sediment trace metal concentrations reflect anthropogenic inputs from the Rhine and Meuse Rivers. Pore water and sediment analyses, together with thermodynamic calculations, indicate a shift in trace metal speciation from oxide-bound to sulfide-bound over the upper 20 cm of the sediment. Concentrations of reducible Fe and Mn decline with increasing depth, but do not reach zero values at 20 cm depth. The reducible phases are relatively more important for the binding of Co, Ni, and Zn than for Pb and Cd. Pore waters exhibit supersaturation with respect to Zn, Pb, Co, and Cd monosulfides, while significant fractions of Ni and Co are bound to pyrite. A multi-component, diagenetic model developed for organic matter degradation was expanded to include Zn and Ni dynamics. Pore water transport of trace metals is primarily diffusive, with a lesser contribution of bioirrigation. Reactions affecting trace metal mobility near the sediment-water interface, especially sulfide oxidation and sorption to newly formed oxides, strongly influence the modeled estimates of the diffusive effluxes to the overlying water. Model results imply less efficient sediment retention of Ni than Zn. Sensitivity analyses show that increased bioturbation and sulfate availability, which are expected upon restoration of estuarine conditions in the lake, should increase the sulfide bound fractions of Zn and Ni in the sediments.