Evaluation of fall residual nitrogen influenced by soil chemical characteristics and crop history in Flanders (Belgium)

In the framework of the Directive concerning the protection of waters against pollution caused by nitrate from agricultural resources (91/676/EEC), losses of nitrate nitrogen to groundwater and surface water have to be limited. In the moderate climate regions of western Europe, drainage losses mainly occur during wintertime. The residual nitrate nitro-en in the soil profile after the harvest in autumn is considered as the main determinant of nitrate losses. In Flanders, a preliminary limit value of 90 kg NO3-N ha(-1) up to a soil depth of 0.90 in between 1 October and 15 November is proposed. During this period in 2001, the Soil Service of Belgium sampled > 18,000 fields. From these results, some interesting conclusions can be drawn. As expected, differences in residual nitrate after harvest are highly influenced by the crop, soil type, and carbon content of the soil. The percentage of fields with a residual nitrate nitrogen content below the target value of 90 kg N ha(-1) differed from 39% to 86%, because of the influence of the crop grown on the parcel. On an average, a high nitrate value was found after early harvested crops (cereals, flax [Linum L.]) and after crops with high protein content due to plant residues remaining on the field after harvest (peas [Pisum sativum L.] and beans [Phaseolus vulgaris L.]). Soil type was also important because, only 42% of the fields with clay soils were below 90 kg NO3-N ha(-1), and in sandy soils this figure was 75%. This is probably partly caused by leaching in the period after harvest until sampling. A very important factor was the carbon content of the soil. Because of a higher mineralization rate in the period after harvest, only 38% of the fields characterized by a high C content were found with a nitrate content below 90 kg NO3-N ha(-1) in contrast to 78% of the fields with a very low C content. Finally, because of the influence of the climatological conditions on nitrogen mineralization and nitrate leaching, the changing weather conditions during the 6-wk-long sampling period influenced the amount of residual nitrogen in the soil.