Spatial and temporal variability of nitrate in irrigated salad crops

The objective of this study was to analyze the spatial and seasonal variations in NO3--N concentration in soil samples and solution samplers and the N leaching of an irrigated crop cultivated intensively in the Mediterranean zone. Although much information is available from controlled field experiments concerning N concentration and its spatial variability, quantitative estimates of nitrate fluxes under normal farming conditions and when the field is directly managed by farmers are rare. This is particularly true for gardening crops in the Mediterranean zone, where high evapotranspiration rates lead to intensive irrigation and may be responsible for N leaching. A field experiment was conducted in the Departement du Gard under agricultural conditions. Salads (Cichorium endivia, Lactuca sativa) were planted in three consecutive periods. The field was irrigated with sprinklers. Local measurements with a neutron probe were made at two sites (row, inter-row), and an experimental plot (95 m x 25 m) was surveyed at 36 points located on a 10 m x 10 m equilateral grid to analyze the spatial variability of water and NO3--N balances. To analyze the basic statistical properties of our sampling scheme, random fields of soil concentration were simulated with the turning-bands method. Sampling strategy simulations indicated that when a spatial structure exists, sampling according to a regular grid was more efficient than a purely random sampling strategy. Global trends indicated high spatial variability for nitrate leaching with differences between periods of different irrigation intensity (97 kg h(-1) NO3--N leaching during the spring and summer, and 199 kg ha(-1) NO3--N leaching during autumn and winter). Leaching caused temporal variations in the spatial distributions of NO3--N. The origin of the spatial variability of N leaching was explained by first, the variability in NO3--N concentration in the soil profile, and second, by spatial variability in irrigation. Furthermore, the spatial distribution of the NO3--N concentration was time dependent, and NO3--N spatial distributions became independent after approximately 2 or 3 months under our conditions. Our results show that better management of irrigation and fertilizer in spring and summer may reduce N leaching and, thus, improve ground water quality.