Legacy effect of grazing intensity mediates the bottom‐up controls of resource addition on soil food webs.

Large‐scale studies have demonstrated that nitrogen (N) and water (W) availabilities greatly affect terrestrial ecosystems world‐wide, and this is especially true for the resource‐poor semi‐arid grasslands. Yet, experimental evidence is lacking for how N and W availabilities affect soil food webs across historical grazing intensity‐altered environments at a local scale.Here, we included N‐ and W‐addition treatments in an 8‐year grazing experiment (with four grazing intensities) to determine how the legacy effects of grazing intensity mediate the responses of key components of soil food webs (plants, micro‐organisms and nematodes) to resource addition in a semi‐arid grassland.After 4 years of N‐ and W‐addition treatments (with no grazing during that 4‐year period), we found that a legacy of grazing, even light grazing, had significant negative effects on the components of plant community and soil food webs. Both N and W addition increased above‐ and below‐ground plant biomass, especially under moderate and heavy grazing. N addition had negative effects on the biomass of bacteria under no grazing, while W addition increased the biomass of actinomycetes under light grazing. N addition decreased the abundance of omnivorous + carnivorous nematodes under light and heavy grazing, while W addition increased their abundance under heavy grazing. Overall, the effects of resource addition on soil food webs progressively decreased from the lowest trophic level (primary producers, i.e. plants), to intermediate trophic levels (micro‐organisms and root‐feeding nematodes), to higher trophic levels (microbial‐feeding nematodes and omnivorous + carnivorous nematodes).Synthesis and applications. Our results, which are the first data concerning the effects of resource addition on key components of soil food webs across a historical grazing‐induced environmental gradient, show that the strong bottom‐up controls of resource addition on soil food webs are mediated by the legacy of grazing intensity. These findings should be useful for predicting the responses of grassland ecosystems to future climate change and suggest that the recovery of degraded grasslands will require more than restoration measure of resource inputs alone. [ABSTRACT FROM AUTHOR]

中文摘要: 大量研究表明氮素和水分有效性是全球陆地生态系统的重要限制因子, 特别是资源贫乏的干旱半干旱草地尤其如此。然而, 有关历史放牧强度引起的环境梯度下, 氮素和水分有效性如何影响土壤食物网的实验论证相对匮乏。本研究在为期8年的放牧试验中选择4个放牧强度, 探讨了放牧遗留效应如何调控半干旱草地土壤食物网中关键生物组分(植物、微生物和线虫)对资源添加的响应。经过4年氮素和水分添加处理, 我们发现历史放牧强度, 即便是轻度, 对植物群落组成和土壤食物网依然有显著的负效应。资源添加均增加植物地上和地下生物量, 并且这种效应在中度和重度历史放牧样地中更加明显。在不放牧或轻度历史放牧样地中, 氮素添加降低细菌和丛枝菌根真菌生物量而水分添加增加丛枝菌根真菌和放线菌生物量; 在中度或重度历史放牧样地中, 资源添加都降低真菌和丛枝菌根真菌生物量。资源添加对土壤线虫数量几乎无显著影响, 仅在轻度或重度历史放牧样地中, 杂食捕食性线虫数量对资源添加有响应。总体上, 在不同历史放牧强度下的资源添加对土壤食物网的影响从低营养级生物(初级生产者植物), 到中间营养级生物(微生物和植食性线虫), 再到较高营养级生物(食微线虫和杂食捕食性线虫)依次递减。总结与应用: 本研究首次利用不同历史放牧强度营造的环境梯度, 研究了资源添加对土壤食物网关键生物组分的影响, 结果表明资源添加对土壤食物网上行效应的影响依旧受历史放牧强度的调控。这些发现除了有助于我们更加准确预测草地生态系统对未来气候变化的响应外, 还表明退化草地的恢复不仅需要资源投入, 还需要其它的辅助管理措施。 [ABSTRACT FROM AUTHOR]

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