Evaluation of long-term water management strategies for saving water and reducing nitrogen and phosphorus losses from agricultural fields: controlled drainage and surface flow constructed wetland case studies in Venice Lagoon drainage basin

In the context of increasing water scarcity and surface water pollution caused by agricultural activities, new water management practices that tackle these issues and that could be used in different environments should be identified. The Venice Lagoon drainage basin (in north-eastern Italy) is a sensitive area to surface water pollution. The prevalence of flat lands and the presence of shallow phreatic groundwaters, however, create suitable conditions for the implementation of two water management practices that can reduce N and P loads coming from agricultural fields: controlled drainage (CD) and surface flow constructed wetlands (SFCWs). Long-term monitoring of the performances of these practices is required to provide sound results that are not contingent on annual weather variability. This work evaluates the performances of a CD and SFCW system in a long-term experiment. CD was monitored during the periods 1995-2002 and 2006-2013 for water balance and crop yield, and from 2007 to 2013 for N and P losses. The SFCW was monitored from 2007 to 2013 for N and P removal loads. CD permitted to reduce water outflows of 69%, and provided an overall increase in maize grain yield of 26.3% and in silage maize yield of 4.0%. NO3-N and PO4-P losses to surface waters were reduced by 92% and 65%, respectively. The SFCW showed annual apparent removal rates of 83% and 79% respectively for NO3-N and total N, and of 48% and 67% respectively for PO4-P and total P. Both CD and SFCW proved effective in reducing N and P loads, and CD helped increasing crop yield through water saving. For these reasons, the application of these two water management practices is advisable in this environment.