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Mechanisms leading to high mean resi-

Background and aims Plant litter quality and water availability both control decomposition. The interaction of both parameters was never studied. We used a grassland site, where litter of contrasting quality, i.e. green litter (fresh leaves; high quality) and brown litter (dead leaves, which underwent senescence but which are still attached to the plant; low quality), is returned to soil. Green and brown litter were exposed in the field under regular weather and drought conditions. The objective of this study was to evaluate the effect of drought on the decomposition of both litter types. Methods We incubated green and brown litter of three different grassland species (Lolium perenne, Festuca arundinacea and Dactylis glomerata) alone or as litter mixture (1/3 of each of the three grassland species) in litterbags for 28 weeks. Drought conditions were simulated by rainfall exclusion. After incubation, litter residues were analysed for C and nitrogen (N) content and stable isotope composition. Additionally, we determined the response of the lignin and carbohydrate signatures to the contrasting conditions. Results C decomposition kinetics of green and brown litter under drought conditions could be explained by two pools of contrasting turnover times. Drought decreased leaf litter C and N decomposition by more than 50% compared to regular weather conditions, mainly by strongly decreasing the decomposition rate constants. The lowest C decomposition occurred for mixtures of litter from all three grassland species. Brown litter showed on average 15% higher reduction in carbon decomposition than green litter following drought. Lignin content remained similar for green and brown litter after drought and regular weather conditions, while sugar content remained similar in green litter and decreased by 18% for brown litter under drought conditions. Conclusions Our results showed different response of decomposition of litter with contrasting quality to drought. Low quality brown litter is likely to be more affected than high quality green litter. Thus, litter quality must be taken into account, when assessing the effect of drought on decomposition.

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