Dynamic of denitrification activity in the soil of a wooded riparian strip assessed for nitrogen removal

Human activities drastically altered the nitrogen cycle on a global scale, mainly causing a strong increase of combined nitrogen release to freshwater ecosystems. Located at the interface between terrestrial and aquatic environments, riparian buffer zones have been proven very effective in nitrogen removal from surface and subsurface water flows (Hunter and Faulker, 2001; Spruill, 2004; Carline and Walsh, 2007), essentially due to the metabolic activity of soil bacterial communities (particularly denitrification process) and to the uptake and assimilation by plants and microbes (Pinay et al., 1993; Haycock et al., 1997). However, there is no agreement (Haycock et al., 1997) on the relative significance of these two processes in different landscapes, though denitrification is considered to be the base process because it permanently removes nitrogen from the soil bringing it back in the atmosphere in a gaseous form. This research is focused on the comprehension of the main mechanisms which regulate denitrification in the soil of a wooded riparian strip. In order to reach this objective a relationship between the denitrification rate measures (effects) and the composition, biomass and distribution of bacterial community (causes) in different soil layers, was undertaken. To this objective, an appropriate experimental site which allows the total control of the irrigation water volume flowing through a forested area was designed and assessed in 1999 along the riparian strip of the Zero river, within the Venice lagoon watershed, on an area previously exploited as an arable land. The buffer zone, organized with an irrigation ditch carrying water from the Zero river, produces a difference in elevation between the irrigation and the drainage ditches, resulting in a sub-surface water flow running through the entire buffer strips. New selected plants were placed in the ground and organized in four parallel rows for each plot. In this way two processes, vegetation/microbial uptake and denitrification, can work together to provide an effective buffer zone to reduce excess of nitrogen from the aquatic ecosystem. Microbial denitrification, continuously monitored in the different zones and depths of the pilot site during a period of time of three years, was directly measured in situ, while the denitrification potential was evaluated on specifically treated soil samples by lab-scale experiments. The results obtained indicated that i) the denitrification process is very active already during the first year from the conversion of the arable crop to the forested area, but it reaches the higher rates after 3 year; ii) the upper soil samples show lower denitrification rates due to the aerated nature of the soil environment; iii) the nitrate rich water pumped from the Zero river flows through the medium layer (40- 55 cm) where maximum values of denitrification were recorded; iv) higher denitrification rates occur in summer and autumn but the process is active during all the year; v) the denitrification potential study indicated that the most limiting factors could be the availability of organic carbon.