Nahaa Alotaibi

Predicting the direction and magnitude of change in soil dynamics caused by invasive plant species has proven to be difficult because these changes are often reported to be species- and habitat-specific. This study was conducted to determine changes in three soil properties, eight soil ions, and seven soil microelements under established stands of four invasive plants, Prosopis juliflora, Ipomoea carnea, Leucaena leucocephala, and Opuntia ficus-indica. Soil properties, ions, and microelements were measured in sites invaded by these four species in southwest Saudi Arabia, and these values were compared to the results for the same 18 parameters from adjacent sites with native vegetation. Because this study was conducted in an arid ecosystem, we predict that these four invasive plants will significantly alter the soil properties, ions, and microelements in the areas they invaded. While the soils of sites with the four invasive plant species generally had higher values for soil properties and ions compared to sites with native vegetation, in most instances these differences were not statistically significant. However, the soils within sites invaded by I. carnea, L. leucocephala, and P. juliflora had statistically significant differences for some soil parameters. For sites invaded by O. puntia ficus-indica, no soil properties, ions, or microelements were significantly different compared to adjacent sites with native vegetation. Sites invaded by the four plant species generally exhibited differences in the 11 soil properties, but in no instance were these differences statistically significant. All three soil properties and one soil ion (Ca) were significantly different across the four stands of native vegetation. For the seven soil microelements, significantly different values were detected for Co and Ni, but only among stands of the four invasive plant species. These results indicate that the four invasive plant species altered soil properties, ions, and microelements, but for most of the parameters we assessed, not significantly. Our results do not support our initial prediction, but are in general agreement with previous published findings, which indicate that the effects of invasive plants on soil dynamics vary idiosyncratically among invasive species and among invaded habitats.

Metagenomic approach was used to detect microbial gene abundance and relative abundance in the rhizosphere of Moringa oleifera and surrounding bulk soil and to detect the response of soil microbes to watering. Expectedly, the number and abundance of non-redundant genes were extremely higher in bacteria followed by archaea, eukaryota and viruses. Results demonstrated unexpected high abundance of some microbes (ex., endophyte genus Nocardioides) in the rhizosphere that are supposed to exist mainly in other rhizocompartments. We suggest this differential distribution of microbes is due to the specific pattern of host-microbe interaction. Other endosymbiont microbes, ex., fungi Mucoromycota and Ascomycota, were highly abundant in the bulk soil possibly because they are phytopathogens where plant exudates might inhibit their growth or force these fungi to approach reverse chemotaxis. Our data indicated high abundance of other symbiont microbes in the rhizosphere of M. oleifera at phylum (ex., Actinobacteria) and genus (ex., Streptomyces) levels. Watering experiment indicated that phylum Actinobacteria and the descending genus Streptomyces are among the highest. Rhizobiome of M. oleifera seems to harbor a wealth of new species of the genus Streptomyces that are required to be deciphered for function in order to be eventually utilized in pharmaceutical and agricultural applications.