Replacing herbicides with vegetation in vineyards alters root distributions, edaphic factors, and microbial diversity in a depth-dependent manner

Suzanne Fleishman, Terrence H. Bell, David M. Eissenstat and Michela Centinari, Intercollege Graduate Degree Program in Ecology, The Pennsylvania State University, University Park, PA, Suzanne Fleishman, Hayden W. Bock and Michela Centinari, Plant Science, The Pennsylvania State University, University Park, PA, Suzanne Fleishman, Hayden W. Bock and David M. Eissenstat, Ecosystem Science and Management, The Pennsylvania State University, University Park, PA, Terrence H. Bell, Department of Plant Pathology and Environmental Microbiology, The Pennsylvania State University, University Park, PA

Background/Question/Methods
Groundcover (GC) 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 crops for soil resources. Moreover, inherently more vigorous crops may mitigate competitive effects of GCs. During the third year of a study conducted in Pennsylvania, USA, we examined the soil environment under grapevines (Vitis spp.) grafted onto two rootstock genotypes that vary in vigor (medium versus low; MV verses LV) and planted with an under-vine fescue groundcover treatment or maintained with herbicide (Festuca rubra; GC versus noGC). Soils up to 1 meter were accessed through soil coring and root box techniques. Measurements included root length density, soil and rhizosphere microbial diversity, soil moisture, soil carbon (C), and soil macronutrient concentrations. We hypothesized that in comparison to the herbicide treatment, under GCs 1) grapevines would shift root distributions deeper into the soil profile, 2) soil macronutrients and moisture would decrease, while soil C and microbial diversity would increase, and 3) shifts in the soil environment would be depth and rootstock dependent; there would be more pronounced changes at shallow than deep depths and for MV than LV grapevines in the presence of competition.
Results/Conclusions
In shallowest GC soils (<20 cm), the fescue root system was very competitive and the absorptive root length of grapevines was 34% lower than in noGC soil, regardless of rootstock. In moderate soil depths (20-60 cm) GC grapevines on medium vigor rootstocks tended to maintain or marginally increase root length while LV rootstocks were uninfluenced by GC competition. In contrast, at the deepest soil depth (60-100 cm) the GC grapevines on LV rootstocks increased root length 184%. These GC-induced shifts in root density translated to increased soil C, alterations in microbial community diversity and composition, and decreases in most soil macronutrients. While many changes in the soil environment due to GC were present throughout the soil profile, shifts tended to be the most substantial at shallow depths where the groundcover roots were most competitive. We found little evidence that the differing root responses to GC between rootstocks led to changes in other soil properties, with the exception of soil moisture at 20 cm. Our results suggest that GCs, and to a lesser extent rootstock, can alter rooting and edaphic patterns in vineyard soils and may have the potential to improve soil health, shift soil-resource availability, and increase deep root growth.