Climate and plant litter diversity are major determinants of carbon (C) and nitrogen (N) cycling rates during decomposition. Yet, how these processes will be modified with combined changes in climate and biodiversity is poorly understood. With a multisite field experiment, we studied the interactive effects of reinforced and prolonged summer drought (using rainout shelters) and tree species mixing on leaf litter decomposition in beech forests in the French Alps. Forests included monospecific stands of Fagus sylvatica, Abies alba and Quercus pubescens and two-species mixtures composed of beech and one of the other species. We hypothesized (1) slower C and N release during decomposition in response to experimentally prolonged summer drought, but (2) less so in mixed compared to monospecific tree stands, due to indirect canopy effects and direct litter mixing effects. Litter lost 35% of initial C and 15% of N on average across all sites and litter types over 30 months of decomposition. The experimentally prolonged summer drought led to weakly slower C loss but had no effect on N loss. Tree species mixing did not alter drought effects on decomposition but had non-additive effects on C and N loss, which were dominated by direct litter mixing rather than indirect plot-specific tree species composition effects. Our data suggest relatively small effects of reinforced and prolonged summer drought on decomposition, possibly because process rates are generally slow during summer and because microsite variability is larger than the effects of rainfall exclusion. The dominant contribution of litter mixing to the overall effect of plot-specific tree species mixtures on decomposition supports the importance of microsite conditions for C and N dynamics during decomposition, which should be accounted for more explicitly in climate and biodiversity change predictions.

气候和植物凋落物多样性是分解过程中碳（C）和氮（N）循环速率的主要决定因素。然而，人们对如何通过气候和生物多样性的综合变化来修改这些过程的了解却很少。通过多站点野外实验，我们研究了法国阿尔卑斯山的山毛榉森林中强化和长期的夏季干旱（使用防雨棚）和树木混合对叶片凋落物分解的相互作用。森林包括单生的林（Fagus sylvatica），白冷杉（Abies alba）和毛栎（Quercus pubescens）以及由山毛榉和其他物种之一组成的两种物种的混合物。我们假设（1）在分解过程中，由于实验延长的夏季干旱，碳和氮的释放较慢，但（2）与单种树种相比，混合林中的碳和氮释放较少，这是由于间接冠层效应和直接凋落物混合效应所致。在30个月的分解过程中，所有地点和垃圾类型中的凋落物平均损失35％的初始碳和15％的氮。实验上延长的夏季干旱导致C的损失较弱，但对N的损失没有影响。树木混合不会改变干旱对分解的影响，但对碳和氮的损失没有累加影响，这主要由直接的凋落物混合而非间接的地块特有的树木组成影响所决定。我们的数据表明，夏季干旱的持续时间延长对分解的影响相对较小，这可能是因为夏季的处理速度通常较慢，并且微地点的变异性大于降雨排除的影响。凋落物混合对特定树种混合物对分解的总体影响的主要贡献支持了分解过程中碳和氮动力学的微场所条件的重要性，在气候和生物多样性变化的预测中应更明确地说明这一点。