The impacts of altered biogeochemical cycles on ecological systems are likely to vary with trophic level. Predicting how these changes will affect ecological food webs is further complicated by human activities, which are simultaneously altering the availability of macronutrients like nitrogen (N) and phosphorus (P), and micronutrients such as sodium (Na). Here we contrast three hypotheses that predict how increasing nutrient availability will shape grassland food webs. We conducted a distributed factorial fertilization experiment (N and P crossed with NaCl) across four North American grasslands, quantifying the responses of aboveground plant biomass and volume, plant tissue and soil elemental concentrations, as well as the abundance of five arthropod functional groups. Fertilization with N and P increased plant biomass and foliar N and P concentrations in grasses but not forbs. Fertilization with Na had no effect on plant biomass but increased foliar Na concentrations. Consistent with the nutrient limitation hypothesis, we found strong evidence of nutrient limitation for insect herbivores across the four sites with sucking (phloem and xylem feeding) herbivores increasing in abundance with NP fertilization and chewing herbivores increasing in response to both Na and NP fertilization, and a trend for increased response of arthropods to lower plant nutrient availability. We found no evidence for an interaction of NaCl and NP on arthropod abundance as predicted by the serial colimitation hypothesis. Finally, consistent with the ecosystem size hypothesis, predator and parasitoid abundances increased with plant volume, but not fertilization. Our results suggest these functional group-specific responses to changes in plant nutrients and structure are key to predicting the future of grassland food webs in an era with increasing use of N and P fertilizers, and increasing terrestrial inputs of Na from road salt, saline irrigation water, and aerosols due to rising sea levels.

中文翻译：

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营养差异调节草原食物网：食草动物追踪食物质量，捕食者选择栖息地

改变的生物地球化学循环对生态系统的影响可能随营养级而变化。预测这些变化将如何影响生态食物网变得更加复杂，人类活动同时改变了氮 (N) 和磷 (P) 等常量营养素以及钠 (Na) 等微量营养素的可用性。在这里，我们对比了三个假设，这些假设预测增加的养分可用性将如何塑造草原食物网。我们在四个北美草原进行了分布式因子施肥实验（N 和 P 与 NaCl 杂交），量化了地上植物生物量和体积、植物组织和土壤元素浓度以及五个节肢动物功能组的丰度的响应。N 和 P 施肥增加了禾本科植物生物量和叶面 N 和 P 浓度，但不增加杂草。用钠施肥对植物生物量没有影响，但会增加叶面的钠浓度。与营养限制假设一致，我们发现了四个地点的昆虫食草动物营养受限的有力证据，吸食（韧皮部和木质部取食）食草动物随着 NP 施肥而增加，咀嚼食草动物随着 Na 和 NP 施肥而增加，并且节肢动物对降低植物养分可用性的反应增加的趋势。我们没有发现 NaCl 和 NP 对节肢动物丰度相互作用的证据，正如串行限制假设所预测的那样。最后，与生态系统规模假设一致，捕食者和寄生蜂的丰度随着植物体积的增加而增加，而不是受精。我们的研究结果表明，这些功能组对植物养分和结构变化的特异性反应是预测草原食物网未来的关键，因为氮和磷肥的使用越来越多，道路盐、盐水灌溉中钠的陆地输入也越来越多。海平面上升引起的水和气溶胶。