Carbohydrate composition of plant materials determines N mineralisation

To improve the synchrony between N mineralisation and crop N demand it is important to identify properties of the plant materials that can be used to predict the N mineralisation. We hypothesised that the timing of decomposition of plant carbohydrates relative to that of plant proteins could be used to determine the course of net N mineralisation. We started by studying the decomposition of several plant carbohydrates, both as single substrates and in combination, to find out how they affected the net N mineralisation from proteins. As the next step, we combined nitrogen-rich Medicago sativa leaves with Phleum pratense that contained large amounts of sugars and fructans. We also combined M. sativa leaves with Trifolium pratense that contained large amounts of pectic substances and hemicellulose. Finally, we combined M. sativa leaves with Lolium perenne that contained large amounts of lignocellulose but low amounts of sugars and fructans. With the addition of P. pratense, the course of N release changed to an initial N immobilisation followed by a net N mineralisation. Addition of L. perenne only slightly affected the N mineralisation rate of M. sativa leaves initially, but reduced amounts of N mineralised thereafter. The effect of T. pratense was intermediate between that of P. pratense and L. perenne. Our results show that by combining plant materials with different carbohydrate and protein compositions it is possible to affect the N mineralisation. However, to utilise subtle differences between plant materials, detailed analysis of non-cellulose polysaccharides may be necessary.