The turnover of residue carbon in soil containing little available N can affect the management of crop residues. The effects of N deficiency on CO2 release from decomposing wheat straw were measured in an incubation experiment and interpreted by computer simulation. Straw with a C:N ratio of 91, incubated for 460 days in sand that was inoculated with a soil suspension, released CO2 much more slowly than when inorganic N was added to obtain a C:N ratio of 5. The evolution of CO2 continued longer without added N, approaching the amount released in the high N treatment with time. The simulation model NCSOIL was modified to simulate reduced CO2 release from decomposing residue when N limits microbial growth by (i) including the decomposers' biomass in the rate of residue decomposition in the form of a Monod-type equation, where the biomass reduced the rate when its concentration was small compared with a saturation constant, and (ii) including formation of a polysaccharide-like pool that received the decomposed C that could not be assimilated by the biomass because of insufficient N. The modified model simulated the reduced CO2 production in the absence of sufficient N, as a result of a smaller microbial biomass that reduced the rate of residue decomposition, and the formation of polysaccharides as long as N limited synthesis of microbial biomass.