A survey on actual agricultural practices and their effects on the mineralnitrogen concentration of the soil solution

In intensive integrated crop-livestock farming systems, the surplus of N at the farm scale may be large and reflects on the N balance at the field sca le. A study was conducted to assess the N fertilizer efficiency in four pri vate farms in intensively cropped areas of NW Italy, and to monitor the eff ects of agricultural practices on the mineral N concentration of the soil s olution, sampled every 2 weeks for 2 years and considered as an indicator o f potential leaching. Two cultivation systems were compared in each farm, o ne involving continuous maize rotation, the other assuring a continuous soi l cover (permanent meadow or winter cereal-maize double cropping system). T he fertilization level in the arable crops was high (369-509 kg N ha(-1) ye ar(-1)) compared to the crop removals, and resulted in a low efficiency, as indicated by the four examined efficiency indexes (calculated N surplus, N removal-fertilizer ratio, N apparent recovery, N use efficiency). The soil -water-nitrate concentration showed large temporal variations in the range of 1-150 mg l(-1) for five out of the eight cropping situations, while conc entrations smaller than 10 mg l(-1) were always recorded in the meadows and in one of the four soils (Aeric epiaquept). The fertilizer management that characterized each cropping system affected the soil-mineral-nitrate conte nt in shallow arable soils. The longer soil cover duration in double-croppi ng systems did not result in a reduction of soil N compared to maize as a s ingle crop, not even in winter (the bare-soil intercropping period in maize -based systems). However, the temporal oscillations of the concentration we re buffered by the crop cover duration and by the presence of a shallow wat er table (1 m deep) in the soil profile. The average nitrate content of the soil could be predicted by the N uptake of the crop, the N removal-fertili zer ratio, the soil pH and sand content, however no simple explanatory rela tionship was found with the experimental factors. Hence, in farm conditions , in the absence of sufficient data for a deterministic model approach, the target of reducing the risk of leaching should be achieved by maximizing t he fertilizer efficiency. (C) 2000 Elsevier Science B.V. All rights reserve d.