REDUCED CO2 RELEASE FROM DECOMPOSING WHEAT-STRAW UNDER N-LIMITING CONDITIONS - SIMULATION OF CARBON TURNOVER

The turnover of residue carbon in soil containing little available N c an affect the management of crop residues. The effects of N deficiency on CO2 release from decomposing wheat straw were measured in an incub ation experiment and interpreted by computer simulation. Straw with a C:N ratio of 91, incubated for 460 days in sand that was inoculated wi th a soil suspension, released CO2 much more slowly than when inorgani c N was added to obtain a C:N ratio of 5. The evolution of CO2 continu ed longer without added N, approaching the amount released in the high N treatment with time. The simulation model NCSOIL, was modified to s imulate reduced CO2 release from decomposing residue when N limits mic robial 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 wi th a saturation constant, and (ii) including formation of a polysaccha ride-like pool that received the decomposed C that could not be assimi lated by the biomass because of insufficient N. The modified model sim ulated the reduced CO2 production in the absence of sufficient N, as a result of a smaller microbial biomass that reduced the rate of residu e decomposition, and the formation of polysaccharides as long as N lim ited synthesis of microbial biomass.