Parthenium hysterophorus L. (Asteraceae) invasion had limited impact on major soil nutrients and enzyme activity: Is the null effect real or reflects data insensitivity?

Parthenium hysterophorus L. (Asteraceae) is an annual weed of worldwide significance in grasslands and riparian corridors. In view of the invasibility and known allelochemical properties exhibited by its presence in a given habitat, the weed is hypothesized to change soil properties significantly. Soil samples were collected in Parthenium weed invaded and non-invaded vegetation patches over two years and across twelve sites in central and south east Queensland of three land-use types, and analyzed for differences in physicochemical and biotic contents. Soil response to the weed invasion varied significantly between years, regions, sites, and land-use types (riparian corridor > grazing grassland cropping land), but invasion impact was minimal. Overall, there was a tendency for increased soil major nutrients and microbial traits with Parthenium weed invasion. However, the differences, using conventional parametric statistics and bias-corrected Hedges' effect size procedures were non-significant - due to a lack of consistency in the direction or magnitude of the impact. In five sites, the 95% confidence intervals of mean impact of the effect size estimates spanned both the null and the rejection regions, indicating data insensitivity. Only in one site was the null hypothesis upheld, while for the remaining six sites, there was evidence to reject the null hypothesis of no invasion impact, but direction varied significantly. Impact of Parthenium weed invasion on soil processes is context and trait-of interest-dependent, and the magnitude and/or direction of the weed invasion is affected by complex interactions among environmental factors that might change across invaded habitats and survey periods, making broad generalizations un-informative for management.