Monitoring of N-uptake by green manures and of the influence of N-release on N-availability, production and quality of sugar beet

Abstract

To study N-cycling in a crop system with green manures, on two field trials, a monitoring programme was set up to evaluate the N-uptake and the N-release capacity of several types of green manure. The studied green manures vary not only with the efficiency with which they take up nitrate but also with which they release it again when ploughed- in. The N-release depends on the time of dying of the green manure (frost, chemical damage or mechanical cut), the time of ploughing-in, and, the type of green manure (easily decomposable or not). Important is whether this N is made available quickly enough to the following commercial sugar beet crop and if there is no reduction in yield and quality of the sugar beet. Monitoring of nitrate content in soil during the experiments, allows to classify green manures into several classes. One class are the winter-hardy green manures, like ryegrass and reply of winter barley. Notwithstanding their N-uptake capacity, there is seldom N available for the following sugar beet crop. Yield and sugar beet quality are not significantly influenced. A second class are green manures with a good developed aerial plant part like phacelia and mustard. A clear net mineralization of these green manures is determined but crop technical and climatic factors determine whether this mineralization results in a higher N-availability for the following crop. A third class are the legumes like vetch. In the two field trials the N-uptake by vetch is the highest and N is made available during a longer period in comparison with phacelia and mustard. The root and sugar yield were higher, but not significantly. No significant difference in technological quality of the beets is measured.