A comprehensive experimental assessment of glyphosate ecological impacts in riparian forest restoration

Competition with invasive grasses is one of the most important drivers of tree planting failures, especially in tropical forests. A widely disseminated weeding approach has been glyphosate spraying, the most used herbicide globally in forestry and ecosystem restoration. However, glyphosate use in restoration is highly controversial and requires further studies to elucidate its effects on restoration processes and the environment. We evaluated the use of glyphosate in riparian forest restoration and its impacts on tree planting costs, weed control efficiency, planted seedling performance, herbaceous and woody species regeneration, soil bacteria, and environmental contamination, using mowing treatments as a reference and based on a controlled experiment established in the Brazilian Atlantic Forest. Glyphosate spraying reduced by one-half and one-third the accumulated aboveground biomass of, respectively, weeds in general and of the invasive grass Urochloa decumbens compared to mowing treatments, and it reduced the cost by half. The performance of planted tree seedlings was markedlyfavored by glyphosate spraying compared to mowing treatments, as expressed by improved seedling height (~twice higher), crown area (~59 higher), and basal area (~59 higher); the regeneration of both native woody and ruderal herbaceous plants were also enhanced. Neither glyphosate nor its metabolite Aminomethylphosphonic acid (AMPA) residues were detected in either water runoff or soil samples, but they were found at relatively high concentrations in the runoff sediments (from 1.32 to 24.75 mg/kg for glyphosate and from 1.75 to 76.13 mg/kg for AMPA). Soil bacteria communities differed before and after glyphosate spraying in comparison to mowing plots (without glyphosate). Glyphosate spraying was far more cost effective than mowing for controlling U. decumbens and greatly improved the performance of planted tree seedlings and natural regeneration, while not leaving residues in soil and water. However, the changes in the structure of bacterial communities and high concentration of glyphosate and AMPA residues in runoff sediments highlight the need for caution when using this herbicide in riparian buffers. We present alternatives for reducing glyphosate use and minimizing its risks in tree planting initiatives.

与入侵草本植物的竞争是导致热带森林植树造林失败的最核心驱动因素之一。当前广为推广的除草方案为草甘膦（glyphosate）喷施法，其是全球林业与生态修复领域应用最广泛的除草剂。然而，生态修复中草甘膦的使用存在极大争议，亟需开展更多研究以阐明其对修复过程与生态环境的影响。
本研究以巴西大西洋森林中建立的控制实验为基础，以刈割处理为对照，评估了草甘膦在河岸带森林修复中的应用效果，及其对植树造林成本、杂草防控效率、定植苗木生长表现、草本与木本物种自然更新、土壤细菌群落及环境污染的影响。结果显示，相较于刈割处理，草甘膦喷施可分别使一般性杂草和入侵草本植物俯仰臂形草（Urochloa decumbens）的累积地上生物量降低50%与33%，同时将造林成本降低一半。
与刈割处理相比，草甘膦喷施显著提升了定植苗木的生长表现：苗木株高提升约1倍，冠幅面积与基径面积均提升约59%；同时也促进了本土木本植物与杂草型草本植物的自然更新。在径流水样与土壤样品中均未检测到草甘膦及其代谢产物氨甲基膦酸（AMPA）的残留，但在径流沉积物中二者的检出浓度较高：草甘膦残留量为1.32~24.75 mg/kg，氨甲基膦酸残留量为1.75~76.13 mg/kg。相较于未施用草甘膦的刈割样地，喷施草甘膦样地的土壤细菌群落在处理前后存在显著差异。
总体而言，在防控俯仰臂形草方面，草甘膦喷施的成本效益远高于刈割处理，且可显著提升定植苗木生长表现与自然更新效果，同时不会在土壤与水体中留下残留。但细菌群落结构的改变，以及径流沉积物中较高的草甘膦与氨甲基膦酸残留浓度，均提示在河岸带缓冲带使用该除草剂时需谨慎行事。本研究同时提出了在植树造林项目中减少草甘膦使用、降低其相关风险的替代方案。