Data from: Strong indirect herbicide effects on mycorrhizal associations through plant community shifts and secondary invasions

Million of acres of US wildlands are sprayed with herbicides to control invasive species, but relatively little is known about non-target effects of herbicide use. We combined greenhouse, field and laboratory experiments involving the invasive forb spotted knapweed (Centaurea stoebe) and native bunchgrasses to assess direct and indirect effects of the forb-specific herbicide picloram on arbuscular mycorrhizal fungi (AMF), which are beneficial soil fungi that colonize most plants.
Picloram had no effect on bunchgrass viability and their associated AMF in the greenhouse, but killed spotted knapweed and reduced AM colonization of a subsequent host grown. Results were similar in the field where AMF abundance in bunchgrass-dominated plots was unaffected by herbicides one year after spraying based on 16:1ω5 phospholipid fatty acid (PLFA) and neutral lipid fatty acid (NLFA) concentrations. In spotted knapweed-dominated plots, however, picloram application shifted dominance from spotted knapweed, a good AMF host, to bulbous bluegrass (Poa bulbosa), a poor AMF host. This coincided with a 63% reduction in soil 16:1ω5 NLFA concentrations but no reduction of 16:1ω5 PLFA. Because 16:1ω5 NLFA quantifies AMF storage lipids and 16:1ω5 PLFA occurs in AMF membrane lipids, we speculate that the herbicide-mediated reduction in host quality reduced fungal carbon storage, but not necessarily fungal abundance after one year in the field. Overall, in greenhouse and field experiments, AMF were only affected when picloram altered host quantity and quality. This apparent lack of direct effect was supported by our in-vitro trial where picloram applied to AMF mycelia did not reduce fungal biomass and viability.
We show that the herbicide picloram can have profound, indirect effects on AMF within a year. Depending on herbicide-mediated shifts in host quality, rapid interventions may be necessary post herbicide applications to prevent loss of AMF abundance. Future research should assess consequences of these potential shifts for the restoration of native plants that differ in mycorrhizal dependency.

美国数百万英亩的野生土地曾被喷施除草剂以防控入侵物种，但目前学界对除草剂使用的非靶标效应仍所知有限。本研究结合针对入侵性非禾本科草本植物斑点矢车菊（Centaurea stoebe）与本土丛生禾草的温室、野外与室内实验，评估对草本专一的除草剂毒莠定（picloram）对丛枝菌根真菌（arbuscular mycorrhizal fungi, AMF）的直接与间接效应——丛枝菌根真菌是一类可定殖多数植物的有益土壤真菌。

温室实验中，毒莠定对丛生禾草的存活率及其共生的AMF无显著影响，但可杀死斑点矢车菊，并降低后续种植的宿主植物的菌根定殖率。基于16:1ω5磷脂脂肪酸（phospholipid fatty acid, PLFA）与中性脂脂肪酸（neutral lipid fatty acid, NLFA）的浓度检测结果显示，喷施除草剂一年后，以丛生禾草为优势种的样地中AMF丰度未受除草剂影响，野外实验得到了一致结果。但在以斑点矢车菊为优势种的样地中，喷施毒莠定将优势物种从AMF优质宿主斑点矢车菊转变为AMF劣质宿主球茎早熟禾（Poa bulbosa）。这一变化与土壤中16:1ω5 NLFA浓度降低63%的现象相吻合，但16:1ω5 PLFA浓度未出现下降。由于16:1ω5 NLFA可定量AMF的贮藏脂质，而16:1ω5 PLFA存在于AMF的细胞膜脂质中，因此我们推测：除草剂介导的宿主质量下降会降低真菌的碳贮存量，但在野外环境中经过一年后未必会影响真菌的丰度。整体而言，在温室与野外实验中，仅当毒莠定改变了宿主植物的数量与质量时，AMF才会受到影响。我们的体外实验结果进一步验证了这一非直接效应的结论：将毒莠定施加于AMF菌丝体时，并未降低真菌的生物量与存活率。

本研究表明，除草剂毒莠定可在一年内对AMF产生显著的间接效应。根据除草剂介导的宿主质量变化情况，可能需要在喷施除草剂后采取快速干预措施，以避免AMF丰度下降。未来研究应评估这些潜在变化对菌根依赖性各异的本土植物修复工作的影响。