Mapping the effects of Melinis minutiflora invasion on soil nitrogen dynamics in the Brazilian savanna: A dual-isotope approach

The invasion of exotic grasses in the neotropical savannas is closely linked to the conversion of the native landscape into agriculture and cultivated pastures. Molasses grass (Melinis minutiflora P.Beauv.) is one of the main invasive species in abandoned fields and native vegetation areas with the potential to alter both the structure and functioning of these ecosystems. We used the dual-isotope approach to evaluate the impact of molasses grass invasion on nitrogen dynamics in the soil of a savanna formation located in the Cerrado region of Central Brazil. We divided three plots (70×80 m) in 300 sampling units (7×8 m each) classified by predominant vegetation type: native grasses (NG), native cerrado sensu stricto (CSS), or molasses grass (MG). We interpolated the soil δ¹⁵N and δ¹³C (0–10 cm depth) in the three plots to continuous surfaces using semivariogram fit and ordinary kriging models. We also compared the aboveground biomass, litter decomposition rates, and soil N pools among vegetation types. MG and NG had higher litter decomposition rates than CSS. Soil pH was higher under MG compared to CSS and NG. The local soil δ¹⁵N isoscapes show the presence of MG in areas with higher soil δ¹⁵N. Soil δ¹³C under all vegetation types indicates a mixture between the C₃ and C₄ sources present in the soil organic matter, with the highest soil δ¹³C under MG. The dual-isotope approach showed the altered processes in the invaded areas with an intensification of the soil N dynamics in the long term compared to the areas dominated by the wood strata and by native grasses. The C and N isoscapes indicated that plant-soil interactions yielded different patterns and showedthe effect of the molasses grass invasion. Therefore, the spatial distribution must be accounted for when assessing the effects and outcome of species interactions and invasion pressure.