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 dynami cs in an unfertilized soil (NIL plot) and a soil annually fertilized with o rganic (FYM plot) or inorganic fertilizer (NPK plot). The soils amended wit h or without rye, i.e. 500 mg C and 30 mg N kg(-1) dry soil (D.S.), were in cubated for 180 d at 25 degrees C and microbial biomass C and N, CO2 produc tion and inorganic N (NH4+, NO2-, NO3-) were monitored. The production of C O2 was greater in the NIL plot than in the NPK plot but three times lower t han in the FYM plot. The soil microbial biomass C and N decreased in the FY M 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 bur the application of rye had no significant effect on it. In the unamended soil, an efficienc y for C of 40%, i.e. the amount of C incorporated into the microbial biomas s 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 FY M 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. We conclude that the dynamics of C and N as governed by the microbial bioma ss C and N were different for the FYM plot compared with the NPK and NIL pl ots. 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 measure d values, or/and a difference in the dynamics of N. (C) 2000 Elsevier Scien ce Ltd. All rights reserved.