Extreme temperature events have increased in intensity, duration and frequency in the last century, with potential consequences on organisms and ecosystems. In many streams, leaf litter of terrestrial origin is a key resource for microorganisms and some detritivores, and its decomposition has a main role on ecosystem functioning and is often used as an indicator of ecological integrity. As litter is often exposed to atmospheric conditions before entering the stream, extreme warming and freezing events may alter its physicochemical structure and affect decomposition and associated detritivores. We tested this prediction in a microcosm experiment by exposing litter of three tree species (in single-species treatments and the 3-species mixture) to different temperature pre-treatments: heating (40 °C), freezing (−20 °C) and both (heating followed by freezing). We then examined changes in litter traits due to leaching (72 h), litter decomposition in the absence and presence of detritivores, and detritivore growth (28 d), with focus on mass and nutrient (nitrogen and phosphorus) changes. Nutrient leaching was promoted mostly by the heating pre-treatment, which apparently produced lower-quality litter. However, microbial activity mostly resulted in litter mass and nutrient gain, which were reinforced by the heating pre-treatment, while freezing had the opposite effect. When detritivores were present, decomposition showed high variation among litter types but, again, the heating and freezing pre-treatments tended to reduce and enhance nutrient loss, respectively. The greatest and more consistent effects occurred for detritivore growth, which was reduced by temperature pre-treatments, particularly in the highest-quality litter type. In general, the sequential application of heating and freezing pre-treatments showed no synergistic effect, and the litter mixture showed similar responses to single-species treatments. Our results demonstrate that short-term extreme temperatures can modify litter quality in riparian soils and have subsequent effects on its decomposition within the stream and associated fauna, potentially altering stream food webs, ecosystem functioning and biogeochemical cycles.

在上个世纪，极端温度事件的强度，持续时间和频率增加，对生物和生态系统有潜在的影响。在许多河流中，陆生凋落物是微生物和某些有害生物的重要资源，其分解对生态系统功能起主要作用，通常被用作生态完整性的指标。由于垃圾在进入河流之前经常暴露于大气条件下，极端的变暖和冰冻事件可能会改变其理化结构并影响分解和相关的有害物质。我们通过微观实验对这一预测进行了测试，方法是将三种树的凋落物（单种处理和3种混合物）暴露于不同的温度预处理下：加热（40°C），冻结（−20°C）和两者（加热然后冻结）。然后，我们研究了由于浸出（72小时），在没有和有杀虫剂存在下的凋落物分解，以及除草剂生长（28天）而引起的凋落物性状的变化，重点是质量和养分（氮和磷）的变化。加热预处理主要促进了养分的淋洗，这显然产生了质量较低的垫料。然而，微生物活性主要导致垫料质量和养分增加，通过加热预处理可以加强垫料质量和养分吸收，而冷冻则相反。当存在有害菌时，分解表现出不同类型的凋落物，但再次，加热和冷冻预处理分别倾向于减少和增加养分流失。最大的，更一致的影响发生在有害生物的生长上，通过温度预处理（尤其是最高质量的垫料类型）可以减少这种情况。通常，加热和冷冻预处理的顺序应用没有显示出协同作用，而凋落物混合物对单种处理也显示出相似的响应。我们的结果表明，短期极端温度会改变河岸土壤中的凋落物质量，并对其在河流和相关动物群内的分解产生后续影响，从而可能改变河流食物网，生态系统功能和生物地球化学循环。