Nitrate accumulation in the soil profile: Long-term effects of tillage, rotation and N rate in a Mediterranean Vertisol

Excessive application of fertiliser in agriculture can have detrimental effects, one of which is diffuse contamination with nitrates. An 18-year field study was conducted on a typical rainfed Mediterranean Vertisol to determine the effects of the tillage system, crop rotation and N fertiliser rate on the long-term NO3−-N content in the soil profile (0–90 cm). The experiment was designed as a randomised complete block with a split-split plot arrangement and 3 replications. The main plots tested the effects of the tillage system (no-tillage and conventional tillage); the subplots tested crop rotation, with 2-year rotations (wheat–wheat, wheat–fallow, wheat–chickpea, wheat–faba bean and wheat–sunflower); and the sub-subplots tested the N fertiliser rate (0, 50, 100 and 150 kg N ha−1). The nitrate content increased with time. The tillage system showed an inconsistent effect on nitrates, although, overall, nitrate levels were higher under conventional tillage than with no-tillage. The wheat–faba bean rotation induced a larger accumulation of soil nitrates. Nitrates usually accumulated to a greater extent in the 30–60-cm depth of soil. As a rule, farmers should know the amount of residual N existing in the soil prior to crop fertilisation in order to avoid over-fertilisation.