Failure to simulate C and N mineralization in soil using biomass C-to-N ratios as measured by the fumigation extraction method?

A C and N mineralization model (DETRAN) was used to simulate C and N dynamics in an unfertilized soil (NIL plot) and a soil annually fertilized with organic (FYM plot) or inorganic fertilizer (NPK plot). The soils amended with or without rye, i.e. 500 mg C and 30 mg N kg−1 dry soil (D.S.), were incubated for 180 d at 25°C and microbial biomass C and N, CO2 production and inorganic N (NH4+, NO2−, NO3−) were monitored. The production of CO2 was greater in the NIL plot than in the NPK plot but three times lower than in the FYM plot. The soil microbial biomass C and N decreased in the FYM soil but not in the NIL and NPK plots. The N mineralization was 7.5 times greater in the FYM plot than in the NPK and NIL plots but the application of rye had no significant effect on it. In the unamended soil, an efficiency for C of 40%, i.e. the amount of C incorporated into the microbial biomass while the rest or 60% evolved as CO2, had to be used to link the C and N dynamics in the NPK and NIL plots but the efficiency was only 26% in the FYM plot. The efficiency for C added with the rye was 45% in the NIL plot and 60% in the NPK plot but it was difficult to link C and N mineralization in the FYM plot. clude that the dynamics of C and N as governed by the microbial biomass C and N were different for the FYM plot compared with the NPK and NIL plots. The difference, a result of long-term organic fertiliser application, could be due to a difference in efficiency for C, a difference in the C-to-N ratio of the microbial biomass; a difference not reflected in the measured values, or/and a difference in the dynamics of N.