Resumen de Monitoring water and NO3-N in irrigated maize fields in the Sorraia Watershed, Portugal

Luis S. Pereira, M.R. Cameira, R.M. Fernando

• The Sorraia Watershed has a long history of continuous irrigated maize. Imprecise water and fertiliser management has contributed to increase nitrate in the groundwater. Solving this problem requires the identification of problem sources and the definition of alternate management practices. This can be performed by an interactive use of selective experimentation and modelling. This paper presents the experimentation phase, where the field experiments were conducted under the irrigation and fertilisation management commonly found in the watershed. Two different soil representatives of the watershed were selected, presenting different water and solute transport properties. One is a silty loam alluvial soil, with a shallow water table, and the other is a sandy soil with a very low water retention capacity. The various terms of the water (consumption, drainage, soil storage) and nitrogen balance (plant uptake, mineralisation and leaching) were obtained from intensive monitoring in the soil profile up to 80 cm, corresponding to the crop root zone. The results showed that in the alluvial soil, up to 70 kg N ha-1 was produced by mineralisation. Current fertiliser management fail in that it does not consider the soil capability to supply mineral nitrogen from the organic nitrogen stored in the profile at planting. This leads to a considerable amount of NO3-N stored in the soil at harvesting, which is leached during the winter rainy season. In the sandy soil, the poor irrigation management (45% losses by deep percolation), leads to NO3-N leaching during the crop season and to inefficient nitrogen use by the crop.

  Author Keywords: Water and nitrogen balance; Experimental methods; Nitrate leaching