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 quantity of soil organic matter. Consequently, spatial variability of N transformations 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 differences in the composition of the microbial community, which can be the result of stochastic events, may be an important source of spatial heterogeneity of decomposition processes. The aim of our study was to detect to what extent net N mineralization and nitrification can vary in soil samples as a result of differences in the composition of a simple microbial community of acid forest soils. This community consisted of two species of chitin decomposers, a mycelium-forming fungus and a rod-like bacterium, and two species of nitrifying bacteria. Characterization of the chitin decomposers in liquid cultures demonstrated two important differences between the fungus and the bacterium: (1) chitin-N mineralization 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 differences in the composition of the chitinolytic community on N mineralization 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, as substrate. As seen in liquid cultures, the differences in chitin-N mineralization between series of sand incubations of either the fungus or the bacterium were considerable. Surprisingly, chitin-N mineralization in sand that had been inoculated with both chitinolytic decomposers. was relatively slow. This was most likely due to mycolytic activity of the bacterium against the fungus. Variation of N mineralization within series of identical inoculation, consisting of 10 replicates, was generally low (CV <15%). In contrast, nitrification was extremely variable within many sand series (CV >200%), especially those in which the chitinolytic bacterium was present. This high variability of nitrification was most likely due to instability of the antibiotic compound in sand. In conclusion, the results show that spatial variability in the composition of a simple microbial community and interactions therein can be an important source of small-scale heterogeneity of N transformations. The relevance of these results for the field situation is discussed.