Commonly reported benefits of under-vine cover crops include reduced vine vegetative growth, herbicide elimination, decreased nutrient runoff, enhanced species diversity, and decreased management costs. Here we consider potential for an additional benefit: stabilization of grapevine responses to variable soil moisture availability in the Northeast. In wet years, grapevines exhibit excessive vegetative growth, which can be mitigated by under-vine cover crops’ competition for resources. In dry years, water stress can threaten grape productivity. This inter-annual variation is anticipated to exacerbate with climate change. While the benefits of under-vine cover crops in wet years are well documented, benefits under dry conditions are more speculative. Cover crops have been shown to shift grapevine root distribution deeper. This may improve stability of production in grapevines when conditions are dry. Our proposal will examine Noiret (Vitis vinifera hybrid) vine response to a grass cover crop (Festuca rubra) and moisture availability in Pennsylvania. Above- and belowground responses will be measured in vegetative growth, depth of resource uptake, root characteristics, and rhizospheric microbiota abundance. With targeted examinations of water uptake by roots and whole-vine response, we will provide inference on whether reductions in productivity with drought are mitigated in grapevines with cover crops. In addition, we will address previously undescribed mechanisms of resource competition, including any impacts on microbiota. We anticipate this project to lead to more informed decisions on how to maximize the multitude of benefits associated with under-vine cover crops in the variable weather conditions of the Northeast.
- In comparison to grapevines growing in bare soil:
- (a) Under normal precipitation conditions, do under-vine cover crops limit grapevine resource uptake, vegetative growth and fruit production?
- (b) Under variable soil moisture, do grapevines with under-vine cover crops show deeper roots and more stabilized production?
- (c) Do grapevine roots that compete with roots of under-vine cover crops have shorter lifespans and lower root production in shallow soil?
- Will rhizospheric microbial communities by grapevine roots, including mycorrhizal fungi, be enhanced with under-vine cover crops?
- Provide information that will assist growers understanding of how under-vines cover crops can regulate vine growth and productivity under variable water conditions.
The achievement of all of these objectives are still in progress, although steps and deadlines are being met as outlined in the proposal with adjustments due to unforeseen obstacles (fully described in part 3). The inability to construct a rain-out shelter and rely on irrigation manipulation has caused a slight shift in objective 1b from the original proposal. The wording has thus changed to “variable soil moisture” from “restricted rainfall”.
Experimental Design and Installation
Since the award of the SARE grant my project has steadily moved along, but has encountered a few obstacles that required augmentations of the original experimental design. Late summer and early fall 2016, much of the project time was spent on troubleshooting a rain-out shelter design to impose a drought. We ran into several unanticipated obstacles on how to account for factors such as tractor movement and belowground water flow. Although we were able to find a construction solution, the costs were far beyond the scope of this grant. As a result, in order to examine influence of water limitations, we decided to rely on variations in irrigation if an extended dry period occurred during the summer of 2017. This also meant needing to move the project from the commercial vineyard of our farmer-cooperator, which did not have irrigation installed, to a vineyard located at the Penn State Agricultural Experiment Station (Rock Springs, PA) that does have irrigation.
The study is taking place in an eight-row Noiret (interspecific Vitis vinifera hybrid) section of the vineyard. The vineyard was planted in spring 2015 (2.4 m x 2.7 m intra- and inter-row spacing) in order to study the vigor-control potential of two rootstocks. Both rootstocks were chosen to be used in this project in order to allow for a more descriptive examination of cover crop influences. The experimental design for this study is a 2 x 2 split-plot factorial with four blocks for a total of 16 experimental units. An experimental unit consists of two randomly assigned 9-vine subreplicates (32 subreplicates total). Rootstock is the main plot and under-vine groundcover is the split- plot (Figure 1). Half of the Noiret vines planted (n = 144) were grafted on the low vigor rootstock Riparia (Vitis riparia), while the other half (n = 144) on the medium vigor rootstock 101-14 Mgt
(Vitis riparia x Vitis rupestris). As a randomized sub-plot treatment, the under-vine area is kept weed-free through herbicide applications or maintained with creeping red fescue (Festuca rubra). Creeping red fescue, a popular cover crop choice and aggressive competitor, was hand planted in September 2016 and bare patches were reseeded in fall 2017.
In order to observe and access roots of a known type, order, and age, root observation boxes 0.6 x 0.6 m and 1m deep (Comas et al. 2000) were installed in May 2017 in the under-vine area between two vines. Once the ground thawed, a small bobcat excavator was used to dig pits between center vines in each subreplicate approximately the size of each box. The walls of these pits were used to collect soil bulk density at 40 cm and 70 cm depths and root distribution data as baseline information for each experimental unit. Each root box has two vine-facing sides (32 boxes, 64 vine-facing sides). Each side is comprised of viewing windows at three depth increments down the length of the box: 0-0.3, 0.3-0.7, and 0.7-1.0 m (Figure 2). Each viewing window is covered in replaceable clear acetate and buffered from light, temperature, and moisture fluctuations with a styrofoam insulated lid. Also at this time, one meter deep soil moisture probe access tubes (Delta-T Devices; Cambridge, UK) were installed as approximately 10 cm from the boxes as possible to best reflect the soil environment in the box windows.
In order to determine if cover crops induced deeper water use we developed a deuterium isotope tracer method for comparing vine water uptake at 40 cm soil depth. In early August 2017, one randomly selected vine in each subreplicate (8 vines per treatment combination, 32 vines total) was chosen for the study. At each vine, twenty 50 cm long x 0.8 cm inner diameter vinyl tubes were installed to a depth of 40 cm with 10 cm remaining above ground for tube-access. Tubes were installed centered on every other centimeter to create a 40 cm length transect at 50 cm from the vine base. Tubes were covered with parafilm “M” laboratory film (Beems; Needham, IA) until later use.
Throughout the study, rainfall, PAR, and air temperature were with a weather station located at the vineyard site (http://newa.cornell.edu/).
Measures: Plant Water Uptake and Status
Water Depletion: Soil moisture data was collected through frequency domain refractometer probe (PR2, Delta-T Devices; Cambridge, UK) in each access tube at depths of 10, 20, 40, 60, and 100 cm from July 172017 until October 10 2017. Soil moisture data is currently being analyzed for significant differences between treatments and depths.
Depth of Water Uptake: Before dawn on September 8, 5 mL of water enriched with 70% of the deuterium isotope (2H) was applied to each vinyl tube, for a total of 100 mL per vine. Transpired water was collected by sealing two sun-exposed shoot tips per vine (4-8 leaves) in plastic bags until 10-20 mL of water had been collected (approximately 2-4 hours). Measurements were collected for 7 days, from one day prior to deuterium application (September 7) until six days after (September 14). Samples were submitted to the Cornell Stable Isotopes Laboratory for percent deuterium enrichment and we await results. The enrichment of the transpired water following the application will be compared to the levels one day prior, this will provide insights on differences in depth of water uptake by the grapevines that may be particular to cover crop, rootstock, or their interaction.
Plant Water Status: Predawn stem water potential (September 10) and midday stem water potential (September 11) were measured on two mid-shoot leaves per subreplicate (16 per treatment) with a pressure chamber (Plant Water Status Console 3000, Soil Moisture Equipment Corp., Santa Barbara, CA, USA) to characterize the grapevine water status during the time of the isotope experiment. Concurrently, on September 10, leaf gas exchange (photosynthesis, transpiration, stomatal conductance) was measured on two mid-shoot leaves per subreplicate (16 per treatment) with a CIRAS-3 portable system (PP Systems, Amesbury, MA). Measurements were taken from 11 am to 2:30 pm. Leaf gas exchange and predawn leaf water potential were repeated as described on September 28 and 29 respectively.
Measures: Soil and Root Dynamics
On August 30 and September first, two 5.2 cm diameter soil cores to 1 m or depth of refusal were collected in each subreplicate and vertically divided in halves designated for microbial analyses or root distribution analyses. In the field, core-halves were separated into five depth increments (0-10, 10-20, 20-40, 40-60, 60-100 cm) and immediately placed on ice (160 samples total). Within eight hours the root-distribution set of core-halves (80 samples) were stored at 4ᵒC and the microbial diversity core-halves (80 samples) were dry sieved through a 2 mm sieve to separate roots and their adherent soil (rhizosphere soil) from the bulk soil. Bulk soil was then mixed and air dried for later submission to Penn State Ag analytical services for macronutrient analysis. Roots and adherent soil were stored at -20ᵒC for approximately one week before soil was removed from the roots with a sterile spatula. Rhizosphere soil and roots were then returned to -20ᵒC freezer for later analyses.
In order to determine the diversity of fungi and bacteria in the rhizosphere, DNA was extracted from the rhizosphere soil using Macherey-Nagel NucleoSpin kits by students of the Penn State PPEM 497 course “Studying and Shaping Microbiomes of the Environment” instructed by Dr. Terrence Bell. The Bell Microbiome Manipulation Lab in the Plant Pathology and Microbiology Department at Penn State amplified bacterial (16S rRNA gene) and fungal (internal transcribed spacer (ITS) region) portions of select universal primers recommended by Daum 2016 and amplicon libraries were prepared for sequencing on the Illumina MiSeq following Howard et al. (2017). Processed sequence data will be analyzed in early 2018. Analysis of arbuscular mycorrhizal fungi colonization of stored roots will be conducted in spring 2018 (% of root length as in Klodd et al. 2015).
Within 75 days of collection, the root distribution set of core-halves were wet sieved through a 2 mm sieve. Grapevine roots were separated from cover crop roots based on darker coloration and relatively large diameter. All roots were cleaned and classified as cover crop, fine grapevine (1st and second orders) and coarse grapevine (3rd order and greater). All roots within a classification were laid on transparent plastic sheets and scanned. Following scanning, roots were dried at 60C for 48 hours and weighed to estimate standing crop biomass and specific root length (g/cm). In January 2018, scanned root images will be analyzed for root length and diameter using WinRhizo image analysis software (Regent Instruments, Inc., Canada).
Measures: Aboveground Plant Production and Nutrient Status
Three days prior to harvest (October 1), 4 shoots per subreplicate (32 per treatment) were collected and deconstructed into leaf, stem, and cluster components. Fresh shoots were measured for length and width by ruler and total leaf area by LI-COR 3100 leaf area meter (LI-COR; Lincoln, Nebraska), dried at 70ᵒC for 3 days, and weighed. Dry matter will be ground and submitted to Agriculture Analytical Services for nutrient analysis for macronutrients.
Also on October 1, the percentage of cover crop cover was measured using a modified turfgrass scale (Giese et al. 2014). Within each experimental unit, a square frame (30cm x 30cm) split into a nine-square grid (10 cm x 10 cm) was positioned on two randomly selected under-vine areas. Within the frame, the area was visually evaluated for percent cover crop and weed coverage; weed type was noted. Above ground cover crop and weed biomass in each frame was collected, dried at 70ᵒC for 3 days, and weighed.
On October 4 and 5, grapes were harvested and cluster number and crop weight for each subreplicate were recorded. 12 randomly selected clusters per subreplicate were frozen and stored at -20ᵒC until juice chemistry analysis (soluble solids, titratable acidity, pH, and yeast assimilable nitrogen) to determine treatment effects on fruit maturity
A two-way analysis of variance (ANOVA) has been used so far to examine the effects of cover crop and rootstock treatments on aforementioned measures. Measures known to have interactions will be tested as potential covariates (ANCOVA). All analyses will use a mixed model in SAS software package (SAS Institute Inc., Cary, NC) with block and main plot factors as random effects.
Of the datasets collected during the 2017 growing season, our harvest data and seasonal precipitation and temperature have been examined. 2017 was a wet season with 136 rain events between April 1 and October 5 (49% of days, Figure 1). This meant that we were unable to manipulate water availability through irrigation (Objective 1b).
There was no effect of groundcover, rootstock, or their interaction on yield, cluster weights, or cluster number (Table 1; Objective 1a). This is not entirely surprising as it was a wet year, so anywater depletion by the cover crop may not have reduced water availability to the grapevine substantially enough to reduce grapevine yield. Vegetative growth is sensitive to water availability and one of the most sensitive measures to water stress; it is possible that there were vegetative growth reductions but not reductions in yield. Once we determine pruning weights, a measure of vegetative vigor, in early spring 2018 for each treatment we will be able to have the most complete picture of and potential influences of treatments on growth. Our results for harvest show promise as beneficial information to growers, as they could reap the ecological benefits of cover crops without sacrificing yield.
Education & Outreach Activities and Participation Summary
On Farm Tour, Oct 24: NCCC-212 Small Fruit and Viticulture Research Group and Coordinating Committee (12 Participants)
On Farm Tour, Aug 22: HORT 431 Small Fruit Culture Course Presentation (12 participants)
This project is the first in-depth investigation of cover crop competition with grapevines in Pennsylvania. Combined with results from other regional investigations, it provides further evidence that cover crops can provide benefits without necessarily sacrificing yield. Once the 2017 year’s data are analyzed and presented in future outreach and extension efforts, this project has the potential to assist in reducing risks of cover crop adoption for viticulturists in the region by presenting growers with specific information for hybrid vines and in a wet year. This will allow growers to make the most informed decisions on incorporating under vine cover crops as a sustainable vineyard practice.
Long-term, this project has provided the foundation for long term study and an in depth investigation. The study will be extended over the next three years with an AFRI Foundational grant to examine the microbiome on grapevine roots and as influenced by cover crops. Concurrently, yield and soil moisture measurements will continue in order to examine treatment influences on growth.
This project required the installation and management of large field equipment. Belowground processes are very difficult to study and grapevines have a deep root distribution, to our knowledge root boxes to this depth have not been installed in a replicated field experiment previously. This helped me understand the logistical difficulty with studying sustainable practices with respect specifically to soil function.
My experience with this project encouraged me to commit to a PhD program in Ecology to improve my background in ecology in order to apply it to research in agricultural ecosystems. I will expand upon concepts explored in this grant through a more in-depth investigation of how cover crops shift soil microbial communities (collaboration with Dr. Terrance Bell) and developing a model to help better predict how cover crop selection may impact grapevine growth(collaboration with Dr. Armen Kemanian). This PhD will facilitate my career plans in agroecological research, teaching, and outreach.