Abstract
AbstractA peat deposit (Zennare basin, Venice coastland, Italy) was monitored in previous field studies to investigate the hydrological response of organic soil to meteorological dynamics. Field tests and modelling predictions highlighted the risk of the complete loss of this peat layer during the next 50 years, due to oxidation enhanced by the increased frequency of warmer periods. Unfortunately, despite the considerable impacts that are expected to affect peat bogs (in this area and worldwide), only a few experimental studies have been carried out to assess the hydrologic response of peat to severe water scarcity. Because of that, an undisturbed 0.7 m3 peat monolith was collected, transferred to the laboratory and instrumented. The total weight (representative of the water content dynamics of the peat monolith as a whole), and two vertical profiles of matric potentials and water content were monitored in controlled water‐scarce conditions. After an extended air‐drying period, the monolith was used as an undisturbed peat lysimeter and a complete cycle of wetting and drainage was performed. Supplementary measurements of matric potential ψ and water content θ were collected by testing peat subsamples on a suction table apparatus. A set of water retention curves was determined in a range of matric potentials broader (ψ down to −7 m) than the current natural conditions in the field (minimum ψ = −1 m). While water content at saturation showed values similar to those in the original natural conditions (θ ≅ 0.8), a remarkable loss of water holding capacity (even for low potentials) has been highlighted, especially in deep layers that are now permanently below the water table. The retention curves changed shape and values, with a more pronounced hysteresis visible in an increasing distance between wetting and drying data. Hydraulic non‐equilibrium between the water content and water potential could be a possible cause and it is worth modelling in future studies. The parameters of the van Genuchten retention curves were obtained for the wetting and the drying phases.
Highlights
Peat soils are commonly characterized by high water-holding capacities and low hydraulic conductivities
The volumetric water content (VWC) values detected by the time-domain reflectometry (TDR) probes and the matric potential (MP) records are depicted as functions of time and depth
The monolith was transferred to the laboratory and instrumented to monitor matric potential, volumetric water content and total weight under drying/wetting cycles and extreme drought conditions
Summary
Peat soils are commonly characterized by high water-holding capacities and low hydraulic conductivities. Peat forms when plant material lies in anaerobic conditions (e.g. high water table) and does not fully degrade This leads to a progressive reduction in water table depth, which lowers the decomposition rates of organic carbon itself, in a positive feedback loop, and creates conditions that allow peatlands to expand. This bidirectional interaction between hydrology and biogeochemistry is well known in organic soils These interventions alter peatland hydrology, the accumulation processes and carbon storage
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