Diversity and feeding strategies of soil microfauna along elevation gradients in Himalayan cold deserts

High-elevation cold deserts in Tibet and Himalaya are one of the most extreme environments. One consequence is that the diversity of macrofauna in this environment is often limited, and soil microorganisms have a more influential role in governing key surface and subsurface bioprocesses. High-elevation soil microfauna represent important components of cold ecosystems and dominant consumers of microbial communities. Still little is known about their diversity and distribution on the edge of their reproductive and metabolic abilities. In this study, we disentangle the impact of elevation and soil chemistry on diversity and distribution of rotifers, nematodes and tardigrades and their most frequent feeding strategies (microbial filter-feeders, bacterivores, fungivores, root-fungal feeders, omnivores) along two contrasting altitudinal gradients in Indian NW Himalaya (Zanskar transect from 3805 to 4714 m a.s.l.) and southwestern Tibet (Tso Moriri transect from 4477 to 6176 m a.s.l.), using a combination of multivariate analysis, variation partitioning and generalized additive models. Zanskar transect had higher precipitation, soil moisture, organic matter and available nutrients than dry Tso Moriri transect. In total, 40 species of nematodes, 19 rotifers and 1 tardigrade were discovered. Species richness and total abundance of rotifers and nematodes showed mid-elevation peaks in both investigated transects. The optimum for rotifers was found at higher elevation than for nematodes. Diversity and distribution of soil microfauna was best explained by soil nitrogen, phosphorus and organic matter. More fertile soils hosted more diverse and abundant faunal communities. In Tso Moriri, bacterivores represented 60% of all nematodes, fungivores 35%, root-fungal feeders 1% and omnivores 3%. For Zanskar the respective proportions were 21%, 13%, 56% and 9%. Elevational optima of different feeding strategies occurred in Zanskar in one elevation zone (4400–4500 m), while in Tso Moriri each feeding strategy had their unique optima with fungivores at 5300 m (steppes), bacterivores at 5500 m (alpine grassland), filter-feeders at 5600 m and predators and omnivores above 5700 m (subnival zone). Our results shed light on the diversity of microfauna in the high-elevation cold deserts and disentangle the role of different ecological filters in structuring microfaunal communities in the rapidly-warming Himalayas.

西藏与喜马拉雅地区的高海拔寒冷荒漠是最为极端的环境之一。这类环境的一大特征是大型动物群（macrofauna）的多样性往往受限，而土壤微生物在调控关键地表与地下生物过程中发挥着更为关键的作用。高海拔土壤微型动物（soil microfauna）是寒冷生态系统的重要组成部分，也是微生物群落的优势消费者。目前学界对其在繁殖与代谢能力极限边缘的多样性与分布格局仍知之甚少。本研究依托印度西北喜马拉雅（赞斯卡样带，海拔范围3805~4714米a.s.l.）与西藏西南部（措莫瑞里样带，海拔范围4477~6176米a.s.l.）两条截然不同的海拔梯度，结合多元分析、变异分区分析与广义可加模型（generalized additive models），解析了海拔与土壤化学属性对轮虫（rotifers）、线虫（nematodes）、缓步动物（tardigrades）及其最常见取食策略——包括微生物滤食者、食细菌者、食真菌者、根食真菌者、杂食者——的多样性与分布的影响。相较于干旱的措莫瑞里样带，赞斯卡样带的降水量、土壤含水量、有机质与有效养分含量均更高。本次调查共发现线虫40种、轮虫19种与缓步动物1种。两条样带中，轮虫与线虫的物种丰富度及总个体数均呈现中海拔峰值特征。轮虫的适宜海拔区间高于线虫。土壤微型动物的多样性与分布格局主要由土壤氮、磷与有机质含量所解释。土壤肥力越高，其孕育的动物群落多样性与丰度也越高。在措莫瑞里样带，食细菌线虫占所有线虫类群的60%，食真菌线虫占35%，根食真菌线虫占1%，杂食线虫占3%；而赞斯卡样带的对应占比分别为21%、13%、56%与9%。赞斯卡样带中，不同取食策略的微型动物的海拔适宜区集中在4400~4500米这一单一海拔区间；而在措莫瑞里样带，每种取食策略均拥有独特的适宜海拔：食真菌线虫的适宜区位于5300米（草原地带），食细菌线虫位于5500米（高山草甸），滤食者位于5600米，捕食者与杂食者则分布于5700米以上的亚雪带（subnival zone）。本研究结果阐明了高海拔寒冷荒漠中微型动物的多样性特征，并解析了在快速变暖的喜马拉雅地区，不同生态过滤因子在塑造土壤微型动物群落结构中的作用。