Ammonium accumulation in soil: the long-term effects of tillage, rotation and N rate in a Mediterranean Vertisol

Plant nutrition requires organic nitrogen to be mineralized before roots can absorb it. A 13-year field study was conducted on typical rain-fed Mediterranean Vertisol to determine the effects of tillage system, crop rotation and N fertilizer rate on the long-term NH4+-N content in the soil profile (0-90cm). The experiment was designed as a randomized complete block with a split-split plot arrangement and three replications. The main plots tested the effects from 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 examined the N fertilizer rate (0, 50, 100 and 150kgN/ha). Soil NH4+-N content was greatest in the rainiest years and greater under the no-tillage (NT) system than the conventional tillage (CT) system (57 and 48kg/ha, respectively). The deepest soil (30-60 and 60-90cm) contained a greater NH4+-N content (21.0 and 21.4kg/ha, respectively) than the shallowest soil (19.5kg/ha in 0-30cm). This observation may be related to Vertisol characteristics, especially crack formation that allows greater mineralization in the deepest layers by displacing organic matter.