The immobilization of nitrogen by straw decomposing in soil

Immobilization of nitrogen (N) in decomposing straw varies between soils, a nd the objective of this study was to identify the mechanisms responsible. Internode segments of wheat straw were incubated in Denmark and in Scotland in arable soils fertilized with NH4NO3, labelled with N-15, for periods up to 1 year. Straw was recovered from the sails periodically and analysed fo r microbial biomass and different forms of N using chemical methods and CPM AS N-15 NMR spectroscopy. The total N content of the straw increased, as long as the soil was not too wet, such that there was overall immobilization. This was accompanied by a rapid increase in the content of amino acid N and to a lesser extent of gl ucosamine N and a concomitant decrease in the carbohydrate content of the s traw. Using direct and plate counts for bacterial and ergosterol content fo r fungal estimation, we found that fungal biomass was much greater than tha t of bacteria. This correlated with the forms of N in the straw as determin ed by CPMAS N-15 NMR, which Showed spectra that were more typical of fungi than of bacteria. It seems that immobilization of N is primarily caused by fungi as they decompose the straw.