Implications of Sustainable Vineyard Floor Management Practices on California Vineyard Agroecoystems: The Effect of Cover Crops and Tillage on Whole Grapevine Physiology and Net Ecosystem Carbon Balance
Vineyard floor management is comprised of several cultural practices used to accomplish various goals including weed control, water, nutrient, and fertilizer availability. The selection of floor management practices should thus balance the goals of the given vineyard, while taking into consideration factors such as vineyard age, climate, and soil physio-chemical properties (i.e., texture, pH, CEC, etc.). One such floor management practice is the adoption of winter cover crops. The use of this technique has increased across vineyards of Mediterranean climates over the last decade (Karlen et al., 2019). This is especially true among California vineyards, where adoption of climate smart agriculture practices is encouraged and, in some instances, incentivized (Lewis and Rudnick, 2019). Furthermore, reducing soil tillage has been identified as an integral strategy to meeting the State of California’s greenhouse gas emissions (GHG) reduction goals (Steenwerth et al., 2016). Cover crops may include a single or mixture of species, are seeded in the fall prior to winter rains, and allowed to grow in vineyard alleyways during the dormancy season. This complementarity in space and time is a key advantage to using cover crops to reduce soil erosion, improve water infiltration, and increase above and belowground biodiversity (Bowles et al., 2017a). Despite the well-documented benefits of cover crops on soil, concern exists regarding competition between the grapevine and cover crop for water and nutrients which may cause reductions in vegetative growth and yield (Ingels et al., 2005; Celette et al., 2009a). However, reports of the influence of cover crops and tillage on grapevine growth, yield, and juice characteristics in irrigated vineyards are inconsistent (Monteiro and Lopes, 2007a; Costello, 2010; Reeve et al., 2016a; Coniberti et al., 2018; Delpuech and Metay, 2018). Tillage, while an effective weed control method, exacerbates soil erosion and can result in soil compaction over time (Álvaro-Fuentes et al., 2008a; Lal, 2012b). Reduced tillage is defined as any tillage system that utilizes fewer cultivation passes compared to what is considered conventional at a given site. While reduced and no-till management have been found to increase soil organic matter (SOM) and improve soil aggregation in soils of most semi-arid regions, the influence of various tillage management systems on grapevine performance is also uncertain (Myburgh, 2013a; Steenwerth et al., 2013a; Wolff et al., 2018). Both cover cropping and reduced tillage have been identified as soil carbon sequestration (SCS) strategies, despite skepticism of the longevity and stability of reported increases in carbon (C). While cover crops and reduced tillage are widely regarded as sustainable floor management practices, the utility of these practices as climate change mitigation tools must be investigated at both the grapevine and vineyard scale. Thus, this thesis research sought to quantify the effect of different combinations of cover crop and tillage systems on grapevine performance as well as C storage potential of the whole vineyard, including C contributions and losses of both the grapevine and vineyard floor. This work was conducted in two commercial vineyards in Fresno and Napa Counties, over two years (2019-2021). The first component of this work investigated the effects of cover crops and tillage on whole grapevine physiology and berry composition. At both sites, there were no treatment effects on leaf gas exchange which suggested negligible effect of cover crop and tillage on grapevine physiology in vineyards of these climates. In the mature Ruby Cabernet vineyard in Fresno, no changes to yield components or berry composition among cover crop and tillage were measured. In the young Merlot vineyard in Oakville, adoption of no-till management detrimentally affected grapevine water status, as grapevines under conventional tillage displayed higher water status over the two experimental seasons. Despite this effect, no changes to yield or water footprint were observed. Combined, these results provided evidence that both in mature and young vineyards cover cropping had negligible beneficial effects on grapevine physiology, mineral nutrition, or production; and tillage may be of benefit to young vineyards in semi-arid regions to improve plant water status. The second component of this work investigated the impact of cover crops and no-till management on net ecosystem carbon balance and the potential for vineyard systems to serve as carbon sinks. Results indicated that while vineyards of the investigated climates served as C sinks over the two experimental seasons, the influence of cover crops and tillage systems varied between the two sites. At the Fresno County vineyard, where soil texture is coarse and largely sandy, tillage and type of cover crop did not affect net ecosystem carbon balance (NECB). However, under the finer textured soil at the Napa County site, tillage reduced NECB via losses of soil organic carbon, and increased CO2 efflux (soil respiration) compared to no-till management. Grapevines under conventional tillage also contributed higher C inputs through annual growth of shoots and leaves. Furthermore, the cover crops that produced the greatest amount of biomass increased NECB. A lack of statistical interaction between the cover crop and tillage factors may suggest that organic C inputs from the cover crop is a key determinant of C storage potential in addition to soil texture. Combined, these results indicate that site characteristics such as vineyard age and soil texture may modulate the effect of cover crops and tillage systems observed on whole grapevine physiology and net ecosystem carbon balance. These findings reveal that the general considerations of cover cropping and reduced tillage as sustainable vineyard floor management practices may overlook important considerations to the grapevine itself as well as the vineyard as a whole.