Lee Marissa R, Flory S Luke, Phillips Richard P, Wright Justin P
Department of Biology, Duke University, Durham, North Carolina 27708 USA.
Department of Biological Sciences, George Washington University, Washington, D.C 20052 USA.
Ecosphere. 2018 Oct;9(10):1-13. doi: 10.1002/ecs2.2454.
Invasive plant species can alter critical ecosystem processes including nitrogen transformations, but it is often difficult to anticipate where in an invaded landscape, these effects will occur. Our predictive ability lags because we lack a framework for understanding the multiple pathways through which environmental conditions mediate invader impacts. Here, we present a framework using structural equation modeling to evaluate the impact of an invasive grass, (.), on nitrogen cycling based on a series of invaded sites that varied in invader biomass and non-. understory biomass, tree basal area, light availability, and soil conditions. Unlike previous studies, we did not find an overall pattern of elevated nitrate concentrations or higher nitrification rates in .-invaded areas. We found that reference plot conditions mediated differences in mineralization between paired invaded and reference plots at each site through indirect (via . biomass), direct, and interactive pathways; however, the strongest pathways were independent of . biomass. For example, sites with low reference soil nitrate and high non-. understory biomass tended to have faster mineralization at 5-15 cm in invaded plots. These findings suggest that more attention to reference conditions is needed to understand the impact of invasive species on soil nitrogen cycling and other ecosystem processes and that the greatest impacts will not necessarily be where the invader is most abundant.
入侵植物物种会改变包括氮转化在内的关键生态系统过程，但通常很难预测在被入侵的景观中，这些影响会发生在哪里。我们的预测能力滞后，因为我们缺乏一个框架来理解环境条件介导入侵者影响的多种途径。在这里，我们提出一个使用结构方程模型的框架，基于一系列在入侵者生物量和非林下生物量、树木基部面积、光照可用性和土壤条件方面存在差异的入侵地点，来评估一种入侵草（.）对氮循环的影响。与以往的研究不同，我们没有发现在被.入侵的地区硝酸盐浓度升高或硝化速率更高的总体模式。我们发现，对照样地条件通过间接（通过.生物量）、直接和交互途径介导了每个地点成对的入侵样地和对照样地之间矿化的差异；然而，最强的途径与.生物量无关。例如，对照土壤硝酸盐含量低且非林下生物量高的地点，入侵样地在5 - 15厘米深度处的矿化往往更快。这些发现表明，需要更多地关注对照条件，以了解入侵物种对土壤氮循环和其他生态系统过程的影响，而且最大的影响不一定发生在入侵者最丰富的地方。
