While it has long been held that invasive plants alter ecosystem processes, the magnitude and direction of these effects have rarely been quantified in situ. We measured the effects of an invasive C₄ grass (Microstegium vimineum) on soil organic matter (SOM) decomposition in a deciduous forest in south-central Indiana, USA. The unique ¹³C signature of the C₄ grass relative to the C₃ trees allowed us to partition soil CO₂ fluxes and estimate M. vimineum effects on decomposition. The magnitude and direction of priming effects hinged on the soil characteristics, which related to the mycorrhizal association of dominant trees. In forest plots dominated by ectomycorrhizal trees, with low nitrogen availability and most SOM in particulate (i.e., unprotected) forms, M. vimineum increased SOM decomposition by 58%. In contrast, in plots dominated by arbuscular mycorrhizal trees, characterized by high nitrogen availability and most SOM in mineral-associated (i.e., protected) forms, M. vimineum decreased decomposition by 14%. Collectively, our results demonstrate that invasive species can play a large role in altering ecosystem processes and suggest that the magnitude and direction of such effects depend on the dominant trees and edaphic characteristics of the stand.

长期以来，人们一直认为入侵植物会改变生态系统过程，但这些影响的程度和方向却很少在原地量化。我们在美国印第安纳州中南部的落叶林中测量了侵害性C ₄草（微草）对土壤有机质（SOM）分解的影响。C ₄草相对于C ₃树的独特¹³ C特征使我们能够分配土壤CO ₂通量并估计M. vimineum对分解的影响。引发作用的大小和方向取决于土壤特性，这与优势树的菌根结合有关。在以外生菌根树为主的森林地块中，氮的利用率较低，且大多数SOM以颗粒状（即未保护的）形式存在，而M. vimineum使SOM分解增加了58％。相比之下，在以丛枝菌根树为主的地块中，氮的利用率高，且大多数矿物质相关（即受保护）形式的SOM都属于M. vimineum。分解降低了14％。总体而言，我们的结果表明，入侵物种可以在改变生态系统过程中发挥重要作用，并表明这种影响的程度和方向取决于林分的优势树和深层特征。