Effects of European buckthorn (Rhamnus cathartica) on soils and leaf litter communities in a Minnesota oak woodland
European buckthorn (Rhamnus cathartica) is an invasive shrub native to Eurasia that has become abundant in forests of the Eastern and Midwestern United States. Increased soil nutrient cycling and earthworm colonization, and decreased leaf litter cover under buckthorn have been suggested to negatively impact leaf litter arthropod and soil microbial communities. In contrast to this suggestion, we previously observed elevated abundance of leaf litter arthropods around individual buckthorn plants compared to plots without buckthorn in a pilot study conducted in mesic oak forest in Minnesota. The current study examined whether this pattern held at larger scales (~100 m2), and whether soils differed in chemical and physical properties and bacterial community composition under buckthorn. Leaf litter (n=24) and soil samples (n=6) were collected from six 10 x 10 m plots at Macalester College’s Ordway Field Station in Inver Grove Heights, MN. Soil arthropods were extracted, counted, and classified to order, and community composition was analyzed using MANOVA and cluster analysis. Bacterial community composition was surveyed via PCR amplicon length polymorphism of 16s-23S rDNA (rIGS) and nifD-K noncoding spacer loci, and terminal restriction fragment length polymorphism (T-RFLP) of 16S rDNA. Bacterial abundance was estimated for the culturable fraction of the bacterial community.
Buckthorn soils were significantly lower in bulk density, and higher in nitrogen, calcium, magnesium, potassium, and pH than control soils. Leaf litter arthropod communities did not differ in relative abundance among taxonomic orders, but total arthropods were more abundant in buckthorn-invaded plots than in control plots (P=0.04), identical to our previous findings at the single-plant scale. However, this result may be driven by differences in litter density between buckthorn and control plots.
Cluster analysis indicated that similarities in 16S-based bacterial communities were based on sampling location rather than presence of buckthorn, but buckthorn soils supported higher richness of nifD-K haplotypes than control soils (P=0.006). The latter result suggests the possibility of associative nitrogen fixation in buckthorn rhizospheres, which has also been suggested by previous workers. Preliminary data gathered by us in a pilot project at the single-plant level provide some support for this suggestion.