Mechanisms of Soil Erosion under Forest – The Role of Biological Soil Crusts

自从人类开始耕种土地以来，土壤侵蚀对陆地生态系统产生了负面影响。最突出的土壤流失出现在农业环境中。这可能是为什么森林下的土壤侵蚀很少被研究的原因之一。然而，在森林中可以观察到重要的土壤流失，如果保护土壤表面的覆盖物被破坏，例如在采伐木材后的滑道上。较高植被层中的这些缺口可以很快被生物土壤结皮(生物结壳)--蓝藻、绿藻、苔藓、真菌或地衣的组合--殖民。因此，生物结壳是防止森林下侵蚀的一个组成部分。然而，它们对温带气候特别是森林环境中的土壤侵蚀的影响在很大程度上被忽视了，保护土壤的机制和过程仍然不清楚。在这方面，我们将讨论在欧洲温带森林生态系统(德国Schönbuch自然公园)中，生物结壳对植被受滑行干扰后土壤侵蚀的影响。我们假设(H1)生物灰复盖是缓解植被干扰后土壤和养分流失的主要因素，(H2)它们的物种组成随着下层基质和周围森林植被的不同而不同，从而对土壤侵蚀过程产生不同的影响。此外，我们假设(H3)生物结壳的保护作用不仅取决于它们对土壤表面的物理保护能力，还取决于改变土壤有机质含量和土壤团聚体降低土壤可蚀性。为了验证这些假设，我们将通过三个实验建立一个土壤和生物结壳多样性的综合研究。它们将包括地面激光扫描仪的测量和滑道微型径流小区的降雨模拟(Ex I)，以调查植被干扰后的土壤和养分流失(H1)以及物种组成和不同下层基质(H2)的作用。随之而来的对野外生物群落的生物多样性分析以及实验室中最先进的DNA条形码将揭示生物土壤表面覆盖的模式。在第二次降雨模拟(Ex II)中，将在森林滑雪道上应用水悬浮中快速生长的苔藓物种和喷雾技术，以测试在受控条件(H1和H2)下生物结皮土壤的土壤流失。在第三个实验(Ex III)中，将测试生物结壳通过改变土壤物理特性(如团聚体稳定性)对土壤流失的影响(H3)。这项研究的结果将有助于更好地了解生物结壳在欧洲温带森林土壤侵蚀控制中的作用，包括植被干扰后生物结壳的发展和演替。此外，它们将允许评估生物壳(例如，选定的苔藓物种)在森林下土壤保持方面的应用。

Soil erosion negatively affects terrestrial ecosystems since humanity started to cultivate land. Most prominent soil losses appear in agricultural environments. This might be one reason why soil erosion under forest is much less in focus of research. Nevertheless, important soil losses can be observed in forests, if the protecting soil surface cover is disturbed e.g. on skid trails after timber harvesting. Those gaps in higher vegetation layers can be quickly colonized by biological soil crusts (biocrusts); assemblages of cyanobacteria, green algae, bryophytes, fungi or lichens. As such, biocrusts are an integral part of erosion prevention under forest. Nevertheless, their influence on soil erosion in temperate climates and especially in forest environments has been largely disregarded and mechanisms and processes to protect the soil are still not well understood. In this respect, we will address the influence of biocrusts on soil erosion after vegetation disturbance by skid trails in a temperate European forest ecosystem (Schönbuch Nature Park, Germany). We hypothesize that (H1) biocrust covers are a major factor in mitigating soil and nutrient losses after vegetation disturbances and (H2) they vary in species composition with different underlying substrates and surrounding forest vegetation and thus affect soil erosion processes differently. Furthermore, we hypothesize that (H3) the protecting effect of biocrusts relies not only on their ability to physically protect the soil surface, but also on reduced soil erodibility by altered soil organic matter content and soil aggregation.To test these hypotheses, we will set up an integrated soil and biocrust diversity study with three experiments. They will comprise measurements with a terrestrial laser scanner and rainfall simulations with micro-scale runoff plots in skid trails (Ex I) to investigate soil and nutrient losses after vegetation disturbances (H1) and the role of species composition and different underlying substrates (H2). An accompanying biodiversity analysis of biocrust communities in the field and by state-of-the-art DNA-barcoding in the laboratory will reveal patterns of biotic soil surface covers. In a second rainfall simulation (Ex II), fast-growing moss species in water suspension will be applied with spraying-up technique on a forest skid trail to test for soil loss from biocrusted soils under controlled conditions (H1 and H2). In a third experiment (Ex III) the effect of biocrusts on soil losses by altering soil physical characteristics such as aggregate stability will be tested (H3). The results of this study will help to attain a better understanding of the role of biocrusts for soil erosion control in temperate European forests including the development and succession of biocrusts after vegetation disturbances. Moreover, they will allow assessing the application of biocrusts (e.g. selected moss species) for soil conservation under forest.