Groundcover plantings in orchards and vineyards, as opposed to weed-free floor management, offer a range of environmental and soil health benefits, but can compete with fruit crops for soil resources. It is unclear if in addition to competing for soil resources, grass groundcovers directly and/or indirectly alter rhizosphere bacteria across the soil profile of the fruit crop. In a long term study in a Pennsylvania research vineyard, grapevines (Vitis sp.) were grafted on two rootstock genotypes with and without grass groundcover. We hypothesized that in comparison to an herbicide treatment, after three years of grass establishment: 1) vineyard soil bacterial diversity would increase, 2) rhizosphere communities of both grapevine rootstocks would shift towards grass-associated microbial communities at shallow depths (<30cm), and 3) grapevine rhizosphere communities of both rootstocks would be less dispersed at deep depths beyond grass roots (>60 cm). Soil and root sampling took place from 1-meter deep root boxes, a method that allows for efficient and careful sampling. To examine the bacteria communities, we extracted DNA from bulk and rhizosphere soils and performed NGS of the 16S rRNA gene. Sequences were classified taxonomically and subjected to common microbiome analyses and statistical tests of diversity, composition, and differential abundance.
Results/Conclusions
Bacteria community diversity and composition were primarily influenced by the sample type (soil, grapevine rhizosphere, or grass rhizosphere) and depth, and, to a lesser extent, grass presence and rootstock genotype. In shallow depths, grapevine rhizosphere samples tended to have higher diversity than grass rhizosphere samples or bulk soil. At medium and deep depths, bulk soil tended to have a higher bacterial diversity than either the grass or grapevines. The microbial community composition of vineyard soils was influenced by groundcover planting, but there was not a clear increase in bacterial diversity. Regardless of depth, the grass groundcover altered grape rhizosphere bacteria, including the abundance of specific genera. The two rootstock genotypes had distinct bacterial community composition, but their bacterial communities were not influenced by grass presence in contrasting ways. Preliminary results also suggest that even when grapevine roots are closely intertwined with a densely rooted grass, grapevine rhizosphere composition resembles grapevine rhizospheres from a non-competitive environment more than those of the competitive grass species.