The immobilization of nitrogen by straw decomposing in soil

Immobilization of nitrogen (N) in decomposing straw varies between soils, and 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 ¹⁵N, for periods up to 1 year. Straw was recovered from the soils periodically and analysed for microbial biomass and different forms of N using chemical methods and CPMAS ¹⁵N 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 glucosamine N and a concomitant decrease in the carbohydrate content of the straw. Using direct and plate counts for bacterial and ergosterol content for fungal estimation, we found that fungal biomass was much greater than that of bacteria. This correlated with the forms of N in the straw as determined by CPMAS ¹⁵N 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.