VARIABILITY OF N MINERALIZATION AND NITRIFICATION IN A SIMPLE, SIMULATED MICROBIAL FOREST SOIL COMMUNITY

It is generally accepted that N transformations in natural terrestrial ecosystems are regulated by: (1) climatological and geomorphological conditions; (2) soil physics and chemistry; and (3) quality and quanti ty of soil organic matter. Consequently, spatial variability of N tran sformations within experimental plots have been related to fine-scale heterogeneity of these regulating factors, e.g. spatial differences in micro-climate. However, it has also been argued that spatial differen ces in the composition of the microbial community, which can be the re sult of stochastic events, may be an important source of spatial heter ogeneity of decomposition processes. The aim of our study was to detec t to what extent net N mineralization and nitrification can vary in so il samples as a result of differences in the composition of a simple m icrobial community of acid forest soils. This community consisted of t wo species of chitin decomposers, a mycelium-forming fungus and a rod- like bacterium, and two species of nitrifying bacteria. Characterizati on of the chitin decomposers in liquid cultures demonstrated two impor tant differences between the fungus and the bacterium: (1) chitin-N mi neralization by the bacterium was much slower than that by the fungus, and (2) the bacterium showed an antibiotic-type of inhibition against the nitrifying bacteria whereas the fungus did not. The effect of dif ferences in the composition of the chitinolytic community on N mineral ization and nitrification was studied using environmentally controlled incubations of the microorganisms in Petri-dishes containing purified sand with solid chitin or its soluble monomer, N-acetylglucosamine, a s substrate. As seen in liquid cultures, the differences in chitin-N m ineralization between series of sand incubations of either the fungus or the bacterium were considerable. Surprisingly, chitin-N mineralizat ion in sand that had been inoculated with both chitinolytic decomposer s. was relatively slow. This was most likely due to mycolytic activity of the bacterium against the fungus. Variation of N mineralization wi thin series of identical inoculation, consisting of 10 replicates, was generally low (CV <15%). In contrast, nitrification was extremely var iable within many sand series (CV >200%), especially those in which th e chitinolytic bacterium was present. This high variability of nitrifi cation was most likely due to instability of the antibiotic compound i n sand. In conclusion, the results show that spatial variability in th e composition of a simple microbial community and interactions therein can be an important source of small-scale heterogeneity of N transfor mations. The relevance of these results for the field situation is dis cussed.