摘要 :
The rewetting of altered wetlands is becoming increasingly widespread. When flooding cultivated soils, the oxygen (O_2) availability is reduced, subsequently, ferric hydroxides can dissolve and associated inorganic phosphorus (P) ...
展开
The rewetting of altered wetlands is becoming increasingly widespread. When flooding cultivated soils, the oxygen (O_2) availability is reduced, subsequently, ferric hydroxides can dissolve and associated inorganic phosphorus (P) will be mobilized. This study shows the temporal and spatial dynamics of O_2 depletion following flooding using planar optodes and the subsequent release of Fe and P in two depth intervals in an experimental column set-up. The column was kept flooded for 48 days and thereafter partly drained and flooded again. Results document that large amounts of P (0.2 t P ha~(-1)) have accumulated in the present plough-layer (Ap) during the last 22 years, which represent roughly 15% of the present inorganic P stock in the Ap. As a result of flooding, fully anoxic conditions were observed within 3 days (within 10 h in Ap) and concentrations of dissolved Fe and P in the soil solution increased simultaneously after 7 days of flooding. Thus, P reaction kinetics was markedly delayed as compared to O_2 availability. P concentrations in soil water after flooding (up to 0.15 mg L~(-1)) accounted for only 0.034% of the inorganic P stock in the Ap which is a significantly smaller fraction of the potential P-release as compared to previous investigations. This is considered a result of only a minor fraction of the total inorganic P being directly associated with ferric hydroxides and thereby sensitive to short-term anoxic conditions as well as differences in the methodology used in this study (soil/water ratio). Finally, reactions releasing Fe and P were noted to be partly reversible upon draining.
收起
摘要 :
Oxygen (O_2) availability and diffusivity in wetlands are controlling factors for the production and consumption of both carbon dioxide (CO_2) and methane (CH_4) in the subsoil and thereby potential emission of these greenhouse ga...
展开
Oxygen (O_2) availability and diffusivity in wetlands are controlling factors for the production and consumption of both carbon dioxide (CO_2) and methane (CH_4) in the subsoil and thereby potential emission of these greenhouse gases to the atmosphere. To examine the linkage between high-resolution spatiotemporal trends in O_2 availability and CH_4/CO_2 dynamics in situ, we compare high-resolution subsurface O_2 concentrations, weekly measurements of subsurface CH_4/CO_2 concentrations and near continuous flux measurements of CO_2 and CH_4. Detailed 2-D distributions of O_2 concentrations and depth-profiles of CO_2 and CH_4 were measured in the laboratory during flooding of soil columns using a combination of planar O_2 optodes and membrane inlet mass spectrometry. Microsensors were used to assess apparent diffusivity under both field and laboratory conditions. Gas concentration profiles were analyzed with a diffusion-reaction model for quantifying production/ consumption profiles of O_2, CO_2, and CH4. In drained conditions, O_2 consumption exceeded CO_2 production, indicating CO_2 dissolution in the remaining water-filled pockets. CH_4 emissions were negligible when the oxic zone was >40 cm and CH4 was presumably consumed below the depth of detectable O_2. In flooded conditions, O_2 was transported by other mechanisms than simple diffusion in the aqueous phase. This work demonstrates the importance of changes in near-surface apparent diffusivity, microscale O_2 dynamics, as well as gas transport via aerenchymous plants tissue on soil gas dynamics and greenhouse gas emissions following marked changes in water level.
收起
