EFFECT OF SOIL COMPACTION ON THE DYNAMICS OF SOIL AND FERTILIZER NITROGEN UNDER WINTER-WHEAT

In 1987 and 1988 the effect of severe soil compaction before sowing on the dynamics of soil and fertilizer nitrogen under winter wheat was studied in a field trial at the experimental station of Roggenstein. According to the course of soil nitrate values before winter soil compaction caused a loss of 34 kg N/ha in the first year whereas no such effect could be seen in the second year. During winter vertical nitrate movement was slower on the compacted plots because of the smaller volume of macropores. Without additional N-Fertilizer net N-mineralization during the vegetation period was 22 kg N/ha (32 %) lower in the compacted than in the well structured soil. Total N-yield at harvest was 95 kg N/ha on the control plots and 68 kg N/ha on the compacted plots. The unfavourable soil structure also led to a lower N-fertilizer efficiency, especially in the wet year of 1987, when about 20 % of the fertilizer-N was lost by denitrification whereas in 1988 all fertilizer nitrogen was found in the plant-soil system at harvest. Denitrification on the untreated soil was negligible in both years. The results showed that, besides a weakly developed root system, reduced N-mineralization and increased denitrification losses are responsible for nitrogen deficiency symptoms of plants growing in compacted soils.