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 56,000mm 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 kg−1 and from 0.7 to 0.9 g kg−1 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 (1.3cm3 g−1) was significantly lower than in 1996 (1.49 cm3 g−1), while at depths greater than 20cm this increased to more than 1.6 cm3 g−1, 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), SFWproved to be a valuable sink for CO2, accumulating 1.4 t C ha−1 yr−1 which suggests a positive role in global warming attenuation.