Abstract

Nitrogen mineralisation from green manure needs to be managed to meet crop N demand and minimise N losses. One way of achieving this can be to use differences in chemical composition between plant materials as a means to steer the processes either towards immobilisation or mineralisation during specific periods. The overall objective of this thesis was to improve the understanding of N mineralisation processes during decomposition of temperate-zone grasses and legumes, especially in relation to concentrations of specific carbohydrates and N compounds. Possible ways of using differences in composition as a means to steer the N mineralisation pattern were also investigated. Mineralisation of C and N during decomposition of pure plant components and materials were studied under controlled conditions. The most abundant plant components could be subdivided into three groups based on when they had their main influence on C and N mineralisation. This subdivision can be used when choosing plant material to achieve a desired effect on the pattern of N mineralisation. To enhance immobilisation of N during the initial days of decomposition plant materials rich in free sugars and fructans may be used. To prolong the initial immobilisation phase for a period of up to two weeks plant materials rich in starch, pectin, or with a hemicellulose characterised by a high arabinose-to-xylose ratio may be used. To reduce N mineralisation during later stages of decomposition plant materials dominated by slowly decomposable structural carbohydrates, such as xylan and cellulose, may be used. Under Swedish field conditions these periods will be approximately two to three times longer than this depending on the time of incorporation due to lower temperature and suboptimal moisture conditions. In order to obtain the necessary information about concentrations of pectin, arabinose and xylose detailed plant analysis, rather than the common proximate neutral/acid-detergent analysis, is required. More knowledge is also required concerning the significance of the effect of chemical composition on N mineralisation in a field situation. New tailor-made plant materials with a greater range of chemical properties, compared to the present plant materials, may be needed to achieve the intended effects in a field situation.