Medium-term evolution of soil properties in a constructed surface flow wetland with fluctuating hydroperiod in North Eastern Italy

Knowledge of the evolution of soil properties in a constructed wetland is important for a correct design and performance assessment of the system over time. We evaluated the medium-term evolution of some chemical (pH, electrical conductivity [EC], organic carbon [OC], total nitrogen [TN], and total phosphorus [TP]) and physical (bulk density and penetration resistance) properties of the top soil layer in a constructed surface flow wetland (SFW) excavated in 1996 in North-Eastern Italy. The soil carbon sequestration performance of the SFW was assessed as well. Soil sampling was conducted in the years 1996, 2003, and 2007. During the period studied, the SFW was subjected to a fluctuating hydroperiod, receiving 56,000 mm of water, 7 times higher than the rainfall. With respect to the initial homogeneous conditions, two layers differentiated in the top profile, characterized mostly by the different OC content and physical characteristics. OC and TN content increased only in the top layer (0–5 cm), from 7 to 17 g kgÀ1 and from 0.7 to 0.9 g kgÀ1 respectively, leading to an average C/N ratio of 19. The other chemical properties showed no significant changes over time. In 2003, the bulk density in the 0–5 cm layer (1.3 cm3 gÀ1) was significantly lower than in 1996 (1.49 cm3 gÀ1), while at depths greater than 20 cm this increased to more than 1.6 cm3 gÀ1, evidencing the formation of a compact layer. Penetration resistance increased along the profile as well. With reference to other land uses (buffer strips, manured cropland), SFW proved to be a valuable sink for CO2, accumulating 1.4 t C haÀ1 yrÀ1 which suggests a positive role in global warming attenuation.

Different dynamics among the physical and chemical properties of wetland soil were observed in this medium-term experiment. In the top layer, the most dynamic properties were mainly the soil organic carbon and bulk density. As a consequence of their evolution, an initial differentiation of the original homogeneous soil profile was seen. Given the importance of these two parameters in the hydraulic and chemical performance of the wetland, this phenomenon should be considered in wetland design and modelling. The wetland proved also to be able to sequester C in the soil in the medium-term, and this suggests a potential positive role in the attenuation of global warming.