Impact of fertilisation practices on nitrogen leaching under irrigation

Field experiments were carried out over a 2-year period on a loamy soil plo t under corn in Montpellier (south-east France). The effectiveness of impro ved irrigation practices in reducing the adverse impact of irrigation on th e environment was assessed. Different irrigation and fertiliser treatments were applied to identify the best irrigation and fertilisation strategy for each technique (furrow and sprinkler) to ensure both good yields and lower NO3- leaching. No significant differences in corn yield and NO3- leaching were found for the climatic scenario of 1999 between sprinkler and furrow i rrigation during the irrigation season. Following the rainy events occurrin g after plant maturity (and the irrigation season), differences in N leachi ng were observed between the treatments. The study shows that both the fert iliser method, consisting of applying a fertiliser just before ridging the furrows, and the two-dimensional (2D) infiltration process, greatly influen ce the N distribution in the soil. N distribution seems to have a beneficia l impact on both yield and N leaching under heavy irrigation rates during t he cropping season. But, under rainy events (particularly those occurring a fter harvesting), the N, stored in the upper part of the ridge and not prev iously taken up by plants, can be released into the deeper soil layers in a furrow-irrigated plot. In contrast, the ID infiltration process occurring during sprinkler irrigation events affects the entire soil surface in the s ame way. As a result the same irrigation rate would probably increase N lea ching under sprinkler irrigation to a greater extent than under furrow-irri gation during an irrigation period. In order to assess the robustness of th ese interpretations derived from soil N-profile analysis, a modelling appro ach was used to test the irrigation and fertilisation strategies under heav y irrigation rates such as those occurring at the downstream part of closed -end furrows. The RAIEOPT and STICS models were used to simulate water appl ication depths, crop yield and NO3- leaching on three measurement sites loc ated along the central furrow of each treatment. The use of a 2D water- and solute-transport model such as HYDRUS-2D enabled us to strengthen the conc lusions derived from the observations made on the N distribution under a cr oss-section of furrow. This model helped to illustrate the risk of over-est imation of N leaching when using a simplified 1D solute-transport model suc h as STICS.