Low soil nitrogen and moisture limit the expansion of exotic invasive grass, Megathyrsus maximus (Guinea grass)

Pushpa Soti and Vanessa Thomas, Biology, University of Texas Rio Grande Valley, Edinburg, TX, Pen Holland, Biology, University of York, York, United Kingdom

Background/Question/Methods
Habitat soil characteristics play an important role in determining the distribution of invasive plant species. Identifying the determinant habitat characteristics is essential in the management of the invaded habitats, and to estimate the potential range expansion for the species. The goal of this study was to predict the range expansion potential of an exotic invasive forage grass, Megathyrsus maximus (Guinea grass). In southwest Texas, M. maximus has invaded field edges, roadsides, and the Tamaulipan thornscrub. We collected rhizosphere soil samples of M. maximus and coexisting species from 150 different locations along a rainfall gradient (14 to 34 mmy-1) in March 2020. We analyzed the soil samples for carbon, nitrogen, organic matter, pH, salinity, and moisture, and sourced average annual minimum and maximum temperatures for each location. We conducted a correlation analysis between all pairs of environmental variables and used t-tests to explore differences between each environmental variable by plant species. We estimated the probability of M. maximus presence as a function of soil moisture, organic matter, pH, salinity, rainfall, temperature (min and max), C:N ratio, and total N, using logistic regression.
Results/Conclusions
Presence of M. maximus was associated with higher soil moisture, higher organic matter, pH, and nitrogen, and lower salinity and C:N ratio. Soil nitrogen and moisture were key factors for predicting the presence of M. maximus. Soil C:N ratio ranged from 19:1 to 36:1 for the central 50% of M. maximus samples (interquartile range) but was significantly higher (25:1 to 3222:1) for the other coexisting species, indicating that the other coexisting plants are better adapted to soils with low nitrogen availability. Paired interactions from the predictive model suggest that wetter soil and high nitrogen concentrations give M. maximus a strong competitive advantage over other species: more nitrogen reduces the effect of otherwise adverse environmental conditions and allows M. maximus to capitalize on moisture.
Currently, M. maximus is primarily found along roads and farm edges. Our results suggest that spread into irrigated and fertilized summer crops in the region, such as corn, cotton, and sugarcane, poses a serious and credible threat. We also expect that a regional climate shift towards a wetter rainfall regime would facilitate a range expansion by M. maximus further into the Tamaulipan thornscrub, countering efforts to restore native plant communities.