MINERALIZATION OF NITROGEN FROM AN INCORPORATED CATCH CROP AT LOW-TEMPERATURES - EXPERIMENT AND SIMULATION

The release of nitrogen from incorporated catch crop material in winte r is strongly influenced by soil temperatures. A laboratory experiment was carried out to investigate this influence in the range of 1-15 de grees C. Samples of sandy soil or a mixture of sandy soil with rye sho ots were incubated at 1-5-10-15 degrees C, and samples of sandy soil w ith rye roots were incubated at 5-10-15 degrees C. Concentrations of N -min (NH4+-N and NO3--N) were measured after 0-1-2-4-7-10 weeks for th e sandy soil and the sandy soil:rye shoot mixture, and after 0-2-7-10 weeks for the sandy soil:rye root mixture. At 1 degrees C, 20% of tota l organic N in the crop material had been mineralised after ten weeks, indicating that mineralisation at low temperatures is not negligible. Maximum mineralisation occurred at 15 degrees C; after ten weeks, it was 39% of total applied organic nitrogen from shoot and 35% from root material. The time course of mineralisation was calculated using an e xponential decay function. It was found that the influence of temperat ure in the range 1-15 degrees C could be described by the Arrhenius eq uation, stating a linear increase of In(k) with T-1, k being the relat ive mineralisation rate in day(-1) and T the temperature (degrees C). A simulation model was developed in which decomposition, mineralisatio n and nitrification were modelled as one step processes, following fir st order kinetics. The relative decomposition rate was influenced by s oil temperature and soil moisture content, and the mineralisation of N was calculated from the decomposition of C, the C to N ratio of the c atch crop material and the C to N ratio of the microbial biomass. The model was validated first with the results of the experiment. The mode l was further validated with the results of an independent field exper iment, with temperatures fluctuating between 3 and 20 degrees C. The s imulated time course of mineralisation differed significantly from the experimental values, due to an underestimation of the mineralisation during the first weeks of incubation.