RATES OF NET NITROGEN MINERALIZATION IN DISTURBED AND UNDISTURBED SOILS

Quantification of net nitrogen mineralization (NNM) in soils is indisp ensable in order to optimize N fertilization of crops. Two long-term l aboratory incubation methods were applied to determine rates of net ni trogen mineralization (r(NNM)) of soils from two sites of arable land (sandy loam soil, silty loam soil) at four temperature levels (2 degre es C, 8 degrees C, 14 degrees C, 21 degrees C). Since variability with in replicates was small, the modified 12-week incubation method of Sta nford and Smith (1972) using disturbed soils allowed to establish reli able Arrhenius functions with reasonable expenditure. The fit of the f unctions derived from the 5-month incubation of 23 undisturbed soil co lumns (4420 cm(3)) was worse. This was caused by greater variability a nd less differentiation between temperature levels. Results of both ex periments could be described best by zero-order kinetics. Mean mineral ization rates of disturbed samples were approximately twice as high th an those of undisturbed samples. The suitability of both methods for t he prediction of NNM at site conditions is discussed. Actual respirati on (AR) at incubation temperatures and substrate induced respiration ( SIR) were measured at the end of the incubation of undisturbed soil co lumns. The results presented reveal that soil microbial communities de velop in a different manner during long-term incubation at different t emperatures. This behavior offends the underlying assumption that soil microbes remain in steady-state during incubation and that rising rat es are physiological reactions to temperature enhancement. Therefore s oil microbial biomass (SMB) dynamics during the experiment has to be a ccounted for when rates of NNM and Arrhenius functions are established .