Litter quality impacts on grassland litter decomposition are differently dependent on soil fauna across time

The main factors controlling decomposition rate are climate, litter quality and soil organisms. We investigated how decomposition was affected by interactions between litter quality and the composition of the soil community. To do this, we designed an experiment using the litterbag technique and three grass species for which a gradient of four distinct litter qualities (defined as initial nitrogen content) had been generated. We manipulated the soil community composition using different mesh sizes to constrain access of specific soil fauna to the litter on the basis of body size. Litter of a single species and quality was placed into litterbags of each of four different mesh sizes (0.1, 2, 2.8 and 4.7 nun) and bags were retrieved from the field after 30 and 60 days. Whether litter quality was a significant determinant of litter decomposition rate was dependent on both the soil community composition and length of field exposure. After 30 days there was a significant positive relationship between litter quality and decomposition for the most complex community (coarsest mesh size). The strength of this relationship declined with decreasing mesh size and, for the most restricted community (smallest mesh size), no quality-decomposition relationship was apparent. In contrast, after 60 days, decomposition was most strongly related to litter quality in the smallest mesh size bags and the relationship between quality and decomposition in the two coarsest mesh bags was non-significant. The pattern of these interactive effects between litter quality, soil community composition and time was consistent across the three grass species. We hypothesize that the effect of litter quality on mass loss within a specific mesh size was dependent on time because, while soil organisms of all size-classes responded positively to increased litter quality, they did so at a rate dependent upon their mobility. (C) 2003 Elsevier B.V. All rights reserved.