Gross mineralisation of nitrogen during the decomposition of leaf protein I (ribulose 1,5-diphosphate carboxylase) in the presence or absence of sucrose

Ribulose 1,5-diphosphate carboxylase (Rubisco) comprises 35–40% of the protein in plant leaves. It is an ideal analogue for the nitrogenous part of leaf material. We have used 15N isotope dilution to study the mineralisation of nitrogen and carbon during Rubisco decomposition in a sandy soil in the presence or absence of sucrose. The results showed that gross N mineralisation was only slightly affected by the addition of sucrose but that gross N immobilisation increased markedly. They confirmed earlier work showing that the decomposition was effected by bacteria and that direct assimilation of peptides or amino acids occurred without prior mineralisation. One interpretation of these findings is that there are two functional groups of bacteria in this soil: one which uses protein for energy and C and N and another which uses carbohydrate as an energy and C source and NH4+ mineralised by the first group as an N source. They may provide a link between bacterial functionality in terms of nutritional requirements and the two main N transformations in soil-mineralisation and immobilisation. The implications of these conclusions in terms of current models of the soil N cycle are explored and a modified soil N cycle is proposed.