Recovery of green manure N by succeeding sugar beet

Abstract

In the loam region of Belgium, a three-course rotation (sugarbeet, winter wheat and winter barley) prevails and the green manures are sown in late July immediately after the harvest of winter barley. N dressings of 80 or 120 kg/ha were applied to green manures of mustard (Sinapis alba), ryegrass (Lolium) and Phacelia grown in 65 m2 plots; 12 m2 plots received comparable dressings labelled with 15N (6 at.%). At the end of November they were turned under. They had taken up 110 to 180 kg N/ha, depending on the year (three years of experimentation). In the following spring, sugarbeet was sown. During the growing season, from 25 to 100 kg NO3-N were produced from green manure; mustard being characterized by the highest decomposition rate. Only 20 to 60 kg N/ha had been taken up by the beets: from 43 to 68% of green manure N was still present in the soil, in organic form; unaccounted N ranged from 10 to 32%, highest values were obtained for mustard. Only once in 3 years did green manures affect sugar yield (less sugar was produced after mustard than after bare soil in 1992). It is supposed that mustard has made part of the mineral N unavailable to the crop