Benefactor and allelopathic shrub species have different effects on soil microbial community along an environmental severity gradient. retama-thymus

Patches where shrubs have either positive or negative effects on their understory plant community are common in arid ecosystems. The intensity and balance of these effects change along environmental severity gradients but, despite the major role of soil microbes in plant interactions, little is known about the differences among soil microbial communities under these species and their possible influence on such contrasting shrub effects. We hypothesized that microbial communities associated to benefactor and allelopathic shrubs would differ among them and that differences would increase with environmental severity. To test these hypotheses we characterized soil microbial biomass, activity and community composition under a benefactor shrub species, Retama sphaerocarpa, an allelopathic shrub species, Thymus hyemalis, as well as in bare soil among plants (gaps) at three sites along an environmental severity gradient. Shrubs promoted an increase in soil bacterial diversity, being bacterial communities associated to benefactor shrubs, allelopathic shrubs and gaps very different in composition. Microbial enzymatic activity and biomass increased under shrubs and under more mesic conditions; nonetheless, they were highest under benefactor shrubs at the most arid site and under allelopathic shrubs at the less severe site. Compared to gaps, the presence of shrubs induced changes in microbial activity and community composition that were larger at the most severe site than at the less severe site. Along the gradient, benefactor shrubs enhanced the abundance of bacterial groups involved in organic matter decomposition and N fixation as well as plant pathogens, which could contribute to Retama’s outstanding positive effects on understory plant biomass and diversity. Plant patches mitigate the effects of extreme conditions on associated plant and soil microbial communities and promote soil biodiversity and ecosystem functioning in arid ecosystems, with shrubs actively selecting for specific microbial groups in their understory.

在干旱生态系统中，灌丛对林下植物群落（understory plant community）产生正向或负向调控效应的斑块广泛分布。这类效应的强度与平衡随环境胁迫梯度（environmental severity gradient）动态变化。尽管土壤微生物在植物互作中发挥核心作用，但目前针对这类灌丛下土壤微生物群落间的差异及其对这些差异化灌丛效应的潜在影响仍知之甚少。我们提出如下假设：与助益性灌丛（benefactor shrub）和化感灌丛（allelopathic shrub）相关的土壤微生物群落存在显著差异，且这种差异会随环境胁迫梯度加剧而增大。为验证上述假设，我们在沿环境胁迫梯度布设的三个样地中，分别对助益性灌丛物种球花荆豆（Retama sphaerocarpa）、化感灌丛物种冬百里香（Thymus hyemalis）林下，以及植株间裸地（林隙，gaps）的土壤微生物生物量、活性与群落组成进行了表征。研究发现，灌丛可提升土壤细菌多样性，且助益性灌丛、化感灌丛与林隙（gaps）的细菌群落组成差异显著。微生物酶活性与生物量在灌丛下及更湿润生境中更高；但在最干旱的样地中，其峰值出现在助益性灌丛下，而在胁迫程度最低的样地中则以化感灌丛下最高。与林隙（gaps）相比，灌丛的存在对微生物活性与群落组成的改变程度在胁迫程度最高的样地中显著大于胁迫程度最低的样地。沿环境胁迫梯度，助益性灌丛可提升参与有机质分解、固氮（N fixation）及植物病原菌的细菌类群的丰度，这或可解释球花荆豆对林下植物生物量与多样性的显著正向调控效应。干旱生态系统中，灌丛斑块可缓解极端环境对伴生植物与土壤微生物群落的影响，促进土壤生物多样性与生态系统功能，且灌丛会主动筛选林下特定的微生物类群。