Soil mineral nitrogen dynamics under bare fellow and wheat in vertisols of semi-arid Mediterranean Morocco

The evoluion of NH4+-N and NO3--N was monitored during three growing seasons, 1992-1993, 1993-1994, 1994-1995 in the soil profile (0-60 or 0-90 cm) under bare fallow and wheat on a vertisol site of the Sais plateau, Morocco. The aim of this study was to relate the soil mineral N dynamics to crop N uptake and soil N transformation processes. The efficacy of the current N fertilisation rate (100 kg N ha(-1)) for wheat production in the region was evaluated. The high level of residual mineral N in the soil profile resulted from a low N plant uptake relative to the soil N supply and N fertilisation, and masked the effect of N fertilisation on dry matter accumulation. NH4+-N was present in considerable amounts, suggesting a low nitrification rate under the given pedo-climatic conditions. An artefact due to the sampling procedure was encountered shortly after the application of N fertiliser. Losses through leaching and denitrification occurred after heavy rainfall, but were limited. At least part of the exchangeable NH4+-N seemed to be barely taken up by the crop. NO3(-)-N was therefore considered to be a better indicator of plant-available N than total mineral N for this type of soil. The low N fertiliser use efficiencies demonstrated clearly that the current fertilisation rate (100 kg N ha(-1)) for wheat production in this region is unsustainable. The maximum N uptake ranged from 40 kg N ha(-1) to 180 kg N ha(-1). The estimation of the seasonal production potential is considered to be the main prerequisite for the determination of the best rates and timing of N fertiliser application in this region.