Introduction

In the last decade, there has been a rise in the adoption of sustainable soil management practices that reduce soil erosion and bolster soil organic matter to counter the impacts of climate change on agricultural soils (Lal, 2004a; Powlson et al., 2011a; Lal, 2012a; Lazcano et al., 2020a). Traditionally, vineyard rows were kept bare with the use of herbicides and tillage. However, there is disagreement on the utility of this practice due to the detrimental effects of tillage on air quality and soil physical, chemical, and biological properties (Patiño-Zúñiga et al., 2009a; Ferreira et al., 2020; Gatti et al., 2022a). Thus, the adoption of cover crops and reduced tillage is considered a sustainable alternative to the traditional management of vineyard floors (Alsina et al., 2013b). Furthermore, environmental regulations and public perception serve as an additional incentive to adopt climate-smart practices (Guerra and Steenwerth, 2012).

The benefits of cover crops on the properties of soils are well documented. They can increase soil organic matter (SOM), total nitrogen, microbial biomass, β-glucosidase, and alkaline phosphomonoesterase, improve water infiltration and aggregate stability, reduce soil erosion and greenhouse gas emissions (GHG), and increase vineyard biodiversity (Ingels et al., 2005; Steenwerth and Belina, 2008b; Gattullo et al., 2020; Abad et al., 2021, Zumkeller et al., 2023). Nevertheless, the adoption of cover crops in vineyards is limited by the concern of excessive competition between the cover crop and grapevine for water and nutrients (Smith et al., 2008; Steenwerth and Belina, 2008a; Celette et al., 2009a; Steenwerth et al., 2013; Pérez-Bermúdez et al., 2016). Thus, the presence of a cover crop is generally reported to affect grapevine water status detrimentally (Naor et al., 1997; Monteiro and Lopes, 2007; Hatch et al., 2011a; Pou et al., 2011; Steenwerth et al., 2016; Tomaz et al., 2021). Despite wide acceptance of this particular effect, results depended on the specific conditions of the study, as some studies have shown that cover crops may improve early-season water status (Ingels et al., 2005; Delpuech and Metay, 2018; Reeve et al., 2016); yet others have concluded that cover cropped vineyards do not display better water status compared to those with bare soil (Celette et al., 2005; Costello, 2010; Daane et al., 2018, Torres et al., 2017). Ultimately, previous works agree that changes in grapevine physiological response to cover crop adoption are largely driven by the climatic conditions and irrigation regime at a given site (Delpuech and Metay, 2018; Tomaz et al., 2021).

There is an agreement in the literature that cover crops may reduce vegetative growth, with more pronounced yield losses in warmer regions. However, some studies had found no effect (Monteiro and Lopes, 2007; Lopes et al., 2008a; Smith et al., 2008; Costello, 2010; Jordan et al., 2016; Steenwerth et al., 2016) or yield increases when vineyards were planted with annual species such as oats or legumes (Fourie and Freitag, 2010; Ovalle et al., 2010; Steenwerth et al., 2013, 2016; Messiga et al., 2016; Fourie et al., 2017). These effects were dependent on the experimental conditions (Morlat and Jacquet, 2003; Ingels et al., 2005; Tesic et al., 2007; Lopes et al., 2008a; Smith et al., 2008; Costello, 2010; Pou et al., 2011; Steenwerth et al., 2013; Giese et al., 2015a; Gattullo et al., 2020), the cover crop species (reviewed by Abad et al., 2021) and the cover surface (Reeve et al., 2016). Consequently, any changes to berry composition as a result of cover crop adoption were closely associated with changes in yield, such as smaller berry size and purportedly greater content of berry flavonoids (Lopes et al., 2008b; Lopes et al., 2011; Lee and Steenwerth, 2013; Tomaz et al., 2021).

Cover cropping and reduced tillage management are two practices that directly alter the growing environment of grapevines. Thus, the selection of appropriate vineyard floor management practices is critical to maximise benefits to the soil while minimising the impact on grapevine function and productivity. This selection involves decisions in space (cover crop in vineyard rows vs under-vine), type (grasses vs broadleaves; monoculture or species mixture), and time including perennial vs annual species selection and timing of termination (Bowles et al., 2017b; Gatti et al., 2022b). Factors such as cultivar, vineyard age, macroclimate, soil physiochemical characteristics, and the overall goals for the use of the selected cover crop and tillage system must also be considered. These elements have been shown to contribute to the effect of the practices on grapevine functioning and production (Ingels et al., 2005; Sweet and Schreiner, 2010; Steenwerth et al., 2013; Abad et al., 2021). The adoption of reduced or no-till management preserves SOM, reduces soil erosion, improves soil structure, and is considered integral to reducing GHG emissions from the agriculture sector (Álvaro-Fuentes et al., 2008b; Gaudin et al., 2010; Dobrei et al., 2015; Wolff et al., 2018a). A greater amount of SOM exerts positive effects not only on the structure of the soil but also on the crops, even reducing many negative effects (i.e., mutagenic) of chemical compounds adopted in agriculture (Ferrara et al., 2000; Ferrara et al., 2004; Ferrara et al., 2006). Thus, the influence of tillage on soil properties, while not entirely understood, is more studied than the impact on crops themselves, and few reports have investigated the influence of tillage on grapevine physiology under the presence of a cover crop. Thus, according to previous studies, there is little evidence of tillage having a direct influence on grapevine stomatal conductance and net leaf photosynthesis (Pou et al., 2011), and although no-till practices are often promoted for their positive influence on soil infiltration and conservation of soil water, few studies have found that this effect translated to ameliorated plant water status in grapevine (Pool et al., 1990; Patiño-Zúñiga et al., 2009b; Myburgh, 2013; Steenwerth et al., 2016; Van Huyssteen and Weber, 2017). The grapevine vegetative growth has been reported to be greater under conventional tillage, while yield reductions are typically associated with no-till management, despite research that indicated no effect on production (Myburgh, 2013; Steenwerth et al., 2013; Wolff et al., 2018a). Similarly, previous studies reported a limited impact of tillage on berry composition (Van Huyssteen and Weber, 1980; Lee and Steenwerth, 2013; Reeve et al., 2016; Chrysargyris et al., 2018; Buesa et al., 2021).

Therefore, the objective of this work was to investigate the interactive effects of tillage and cover crops on grapevine water status, leaf gas exchange, components of yield, berry composition and resulting water footprint in two contrasting production regions in California, USA.

Materials and methods

1. Site description and experimental design

Field experiments were conducted at two sites (Figure 1A) for two consecutive growing seasons (2019-2020 and 2020-2021). The first site was located at a Winkler Index V vineyard in Fresno, CA (36.671°N, –119.925°W) in a Ruby Cabernet/Freedom (27 % V. vinifera hybrid) vineyard. Grapevines were planted in 2012 with a spacing of 3.0 × 1.2 m (row × vine) with a row orientation of E-W. The grapevines were cane-pruned and trained to quadrilateral cordons at 1.38 m with catch wires at 1.54 m and at 1.68 m above the vineyard floor. The soil texture of the site was classified as Hanford fine sandy loam. Hanford fine sandy loam is a coarse-loamy, mixed, super active, non-acid, thermic Typic Xerothents. (websoilsurvey.sc.egov.usda.gov). The A1 horizon is 0 to 30 cm deep with a moist, weak fine granular structure; slightly hard, very friable, nonsticky and nonplastic; many fine roots in the upper few inches; many fine interstitial pores; slightly acid; with gradual smooth boundary between 15 and 35 cm thick. The C1 horizon is 30 cm to 96 cm moist; massive; slightly hard, very friable, nonsticky and nonplastic; common fine interstitial pores; neutral; diffuse boundary 25 to 60 cm thick. The vines were drip-irrigated with two emitters per plant, delivering 4.0 L/h each at 100 % crop evapotranspiration replacement. Irrigation was scheduled daily to meet the vineyard demand according to methods reported by Torres et al. (2021) and 2900 m³/ha and 3073 m³/ha of irrigation water were applied in 2020 and 2021, respectively. The second site was located at a Winkler Index III vineyard in Oakville, CA (38.428 °N, –122.409 °W) in a Merlot (clone 181)/3309 C (V. riparia × V. rupestris) vineyard. Grapevines were planted in 2018 at a spacing of 3.0 by 2.0 m (row × vine) with a row orientation of E-W. The grapevines were spur-pruned and trained to quadrilateral cordons 1.38 m above the vineyard floor with catch wires at 1.68 m. The soil of the site was classified as Baley clay loam. The Bale soil series consists of very deep, somewhat poorly drained soils formed in stratified, gravelly and sandy alluvium from mixed sources. In these soil series, the Ap horizon is distributed from 0 to 0.15 m deep, the B horizon is distributed from 0.15 to 0.61 m, the Ab horizon is distributed from 0.61 to 1.12 m, and the C horizon is distributed from 1.12 to 1.47 m. (websoilsurvey.sc.egov.usda.gov). The vines were drip-irrigated with two emitters per plant, delivering 2 L/h each at 80 % crop evapotranspiration replacement. Irrigation at this location was scheduled weekly (Torres et al., 2021). In 2020 and 2021, 978.5 m³/ha and 824 m³/ha of irrigation were applied, respectively, at this location.

At both experimental sites, the experiments were arranged as a split-plot 3 × 2 factorial arrangement of treatments (three cover crops and two tillage managements) with four (Oakville) and three replications (Fresno). Each treatment replicate consisted of 15 grapevines. Three grapevines in the middle of each replicate were used for measurements, and the distal plants on either end served as buffer plants. Treatments included tillage as the main plot [conventional tillage (CT) and no-till (NT)], and the sub-plot was randomly applied within the main plots as i) Perennial grass (Poa bulbosa hybrid cv. Oakville Blue) (Figure 1B); ii) Annual grass (Barley, Hordeum vulgare) (Figure 1C); iii) Resident vegetation (natural weed population) (Figure 1D). Resident vegetation was mainly composed of grasses from the Poaceae family, Plantago sp., Trifolium sp., Convolvulus sp., and Barbarea vulgaris. The cover crop seed was drilled in a 1.5 m wide strip with a 1.5 m wide Frontier GS1160 (John Deere &Co. Deerfield, IL USA) seed drill according to the seed manufacturer's recommended rate prior to receiving fall/winter rains in 2019 (19 November 2019 in Napa County, 26 November 2019 in Fresno County) and 2020 (16 October 2020 in Napa County and 23 October 2020 in Fresno County) at a rate of 605 kg/ha and 84 kg/ha for the perennial grass (PG) and annual grass (AG) treatments, respectively. Resident vegetation (RV) was allowed to grow within the 1.5 m strip and mowed once with a 1.5 m wide Rears IFA60 (Rears Mfg. Co. Coburg, OR USA) flail mower (6 April 2020 and 11 April 2021 in Napa County; 12 March 2020 and at the vineyard manager's discretion. Conventional tillage was performed with a Schmeiser TSN60 tandem disk (Schmeiser Co. Selma, CA USA) within the 1.5 m planted strip according to the vineyard manager´s discretions once per year (22 April 2020 and 2 May 2021 in Napa County; 18 April 2020 and 21 April 2021) and removed cover crop was left on the field. All other cultural practices, including weed control on the berm once during March of each year with glyphosate (Zumkeller et al., 2023), were conducted according to University of California Cooperative Extension guidelines (Christensen, 2000).

Figure 1. Location of experimental sites in the state of California, U.S.A. (A), Perennial grass (Poa bulbosa hybrid) (B), Annual grass (Hordeum vulgare) (C), Native vegetation (D) in the vineyard.

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Results

2. Leaf gas exchanges and plant water status over the season

In Fresno, no treatment effects were observed on season-long integrals of g_s (Table 2). Likewise, season-long integrals of A_net were also not affected by the treatments applied. However, in 2020 both gs and A_net were significantly lower than in 2021 in Fresno.

Table 2. Stomatal conductance and net carbon assimilation values of Ruby Cabernet grapevines subjected to different cover crops and tillage systems, as collected at various points in the 2019-20 and 2020-21 growing seasons a, b.

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Treatment	Stomatal conductance (mmol m−2 s−1)			Net carbon assimilation (μmol CO2 m–2 s–1)
2020	78 DAF	184 DAF	--	78 DAF	184 DAF	--
No Till - AG	259.5 ± 23.4	266.7 ± 54.5	--	10.2 ± 1.6	13.4 ± 0.7	--
No Till - RV	308.7 ± 34.7	230.8 ± 21.6	--	12.2 ± 1.9	13.5 ± 3.7	--
No Till - PG	380.1 ± 60.1	252.5 ± 7.4	--	15.7 ± 0.5	14.5 ± 1.9	--
Till - AG	281.2 ± 30.1	260.2 ± 24.4	--	16.2 ± 1.7	12.9 ± 1.9	--
Till - RV	262.7 ± 24.4	301.8 ± 59.9	--	14.5 ± 0.9	11.3 ± 3.9	--
Till - PG	403.0 ± 64.2	189.8 ± 3.5	--	14.8 ± 0.2	9.97 ± 2.9	--
Cover crop (CC)	ns	ns	--	ns	ns	--
Tillage (T)	ns	ns	--	ns	ns	--
CC x T	ns	ns	--	ns	ns	--
2021	26 DAF	91 DAF	128 DAF	26 DAF	91 DAF	128 DAF
No Till - AG	178.4 ± 41.92	415.0 ± 68.0	349.8 ± 56.5	12.2 ± 3.6	15.8 ± 2.9	18.5 ± 2.7
No Till - RV	139.9 ± 22.21	352.1 ± 55.8	349.6 ± 30.2	6.33 ± 1.9	16.1 ± 0.9	20.8 ± 2.7
No Till - PG	149.5 ± 18.10	293.5 ± 68.2	382.0 ± 60.8	8.67 ± 1.5	18.1 ± 3.1	19.8 ± 1.8
Till - AG	133.2 ± 16.63	418.4 ± 51.2	318.0 ± 59.9	9.27 ± 1.2	18.5 ± 1.4	20.4 ± 3.0
Till - RV	137.7 ± 30.52	360.5 ± 70.7	413.9 ± 42.5	5.57 ± 1.6	17.2 ± 1.9	19.1 ± 2.1
Till - PG	137.9 ± 7.87	421.2 ± 89.4	407.0 ± 73.9	9.07 ± 1.7	19.7 ± 2.7	19.8 ± 1.9
Cover crop (CC)	ns	ns	ns	ns	ns	ns
Tillage (T)	ns	ns	ns	ns	ns	ns
CC x T	ns	ns	ns	ns	ns	ns
Year	ns			**
Year x CC	ns			ns
Year x T	ns			ns
Year x CC x T	ns			ns

^a ANOVA to compare data (p-value indicated); Letters within columns indicate significant mean separation according to Tukey's HSD test at p-value < 0.05, where "*": p-value < 0.05; "**": p-value < 0.001, "***": p-value < 0.0001.

^b Abbreviations: AG: Annual grass; RV: resident vegetation; PG: perennial grass; DAF: days after flowering; na: not applicable; ns: not significant; --: not applicable.

In Oakville, g_s was significantly lower among grapevines grown with AG compared to RV and PG in 2020, 96 DAF and 110 DAF (Table 3). However, a similar effect was not evident throughout the rest of the experiment. Additionally, no differences were measured in A_{net ,} although 2020 values were lower than in 2021. A similar response was observed at the Fresno vineyard.

Table 3. Stomatal conductance and net carbon assimilation values of Merlot grapevines subjected to different cover crops and tillage systems, as collected at various points in the 2019-20 and 2020-21 growing seasons a, b.

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Treatment	Stomatal Conductance (mmol m−2 s−1)				Net Carbon Assimilation (μmol CO2 m–2 s–1)
2020	61 DAF	96 DAF	110 DAF	145 DAF	61 DAF	96 DAF	110 DAF	145 DAF
No Till - AG	136.6 ± 18.9	141.5 ± 6.3 b	147.7 ± 18.2 b	220.4 ± 22.2	10.0 ± 1.35	13.1 ± 0.96	19.2 ± 7.99	16.5 ± 1.6
No Till - RV	161.3 ± 27.6	195.5 ± 26.5 a	213.7 ± 14.0 a	217.0 ± 29.1	7.7 ± 0.72	17.3 ± 0.94	14.4 ± 1.04	16.6 ± 1.7
No Till - PG	174.0 ± 28.5	182.5 ± 17.1 a	217.0 ± 29.1 a	205.4 ± 17.3	11.7 ± 2.01	14.3 ± 1.85	17.1 ± 1.67	17.0 ± 0.9
Till - AG	109.2 ± 13.0	124.3 ± 19.3 c	167.8 ± 11.7 b	159.0 ± 18.9	7.7 ± 0.24	11.4 ± 1.31	13.2 ±  0.95	14.8 ± 1.0
Till - RV	143.0 ± 25.1	194.2 ± 32.5 a	229.2 ± 10.8 a	184.4 ± 10.8	6.3 ± 2.58	16.0 ± 2.32	16.0 ± 1.75	17.2 ± 0.3
Till - PG	172.2 ± 26.3	167.2 ± 30.0 b	201.4 ± 24.5 a	237.0 ± 20.8	10.4 ± 2.44	12.9 ± 2.07	14.0 ± 2.14	19.0 ± 0.9
Cover crop (CC)	ns	*	*	ns	ns	ns	ns	ns
Tillage (T)	ns	ns	ns	ns	ns	ns	ns	ns
CC x T	ns	ns	ns	ns	ns	ns	ns	ns
2021	56 DAF	74 DAF	98 DAF	109 DAF	56 DAF	74 DAF	98 DAF	109 DAF
No Till - AG	198.3 ± 26.4	191.7 ± 18.1	179.0 ± 17.9	150.3 ± 15.9	9.0 ± 2.06	13.1 ± 2.01	13.6 ± 1.51	12.4 ± 1.2
No Till - RV	180.9 ± 23.2	156.6 ± 25.1	155.0 ± 24.4	173.6 ± 18.7	9.4 ± 1.42	11.9 ± 1.41	11.5 ± 1.15	13.2 ± 1.3
No Till - PG	209.0 ± 28.9	146.5 ± 13.3	165.7 ± 16.9	177.1 ± 25.6	11.5 ± 2.08	13.2 ± 0.47	13.2 ± 1.63	12.2 ± 1.8
Till - AG	193.5 ± 19.2	160.5 ± 32.8	194.2 ± 21.5	163.2 ± 27.1	9.5 ± 1.31	14.8 ± 1.70	11.9 ± 1.51	12.1 ± 2.0
Till - RV	231.0 ± 24.6	168.7 ± 32.1	177.4 ± 26.7	193.3 ± 23.3	10.8 ± 1.87	13.4 ± 1.29	10.9 ± 1.85	13.3 ± 1.6
Till - PG	199.3 ± 26.2	171.4 ± 38.6	145.8 ± 18.3	181.2 ± 20.3	10.5 ± 1.73	12.5 ± 2.72	10.3 ± 1.59	13.9 ± 1.1
Cover crop (CC)	ns	ns	ns	ns	ns	ns	ns	ns
Tillage (T)	ns	ns	ns	ns	ns	ns	ns	ns
CC x T	ns	ns	ns	ns	ns	ns	ns	ns
Year	ns				***
Year x CC	ns				ns
Year x T	ns				ns
Year x CC x T	ns				ns

^a ANOVA to compare data (p-value indicated); Letters within columns indicate significant mean separation according to Tukey's HSD test at p-value < 0.05, where "*": p-value < 0.05; "**": p-value < 0.001, "***": p-value < 0.0001.

^b Abbreviations: AG: Annual grass; RV: resident vegetation; PG: perennial grass; DAF: days after flowering; na: not applicable; ns: not significant.

In Fresno, Ψs values ranged from –1.68 to –0.70 MPa in 2020 and –1.75 to –0.73 MPa in 2021. The cover crop and tillage treatments did not affect Ψ_s.integrals, and there was not an interaction of cover crop and tillage or an effect of the year (data not shown). Conversely, at Oakville, Ψ_{s integrals} ranged from –1.49 to –0.64 MPa in 2020 and –1.46 to –0.77 MPa in 2021. As presented in Figure 2, in 2020, Ψ_{S integrals} were affected by cover crops. Grapevines grown with AG displayed the most negative Ψ_{s integrals}, with PG the least negative. However, this effect was not observed in the second year of the study, nor was there an interaction effect between the cover crop and tillage system. Tillage was more effective in eliciting a Ψ_{S integrals} response more so than cover crop type. Grapevine water status was significantly lower in NT vines (–1.07 MPa) than in CT vines (–1.01 MPa) over both years at the Oakville vineyard.

Figure 2. Mid-day stem water potential integrals of Merlot grapevines subjected to different cover crop and tillage systems at the Oakville vineyard (Napa County) over the course of the 2019-20 and 2020-21 growing season a

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^a ANOVA to compare data (p-value of the respective factor indicated). The interaction between the till and cover crop factor was not significant and upper(tillage) and lower case (cover crop). Letters above treatment bars indicate significant mean separation according to Tukey’s honest significance difference (HSD) test for tillage and cover crop factors, respectively, at p-value < 0.05, where "*": p-value < 0.05; "**": p-value < 0.001, "***": p-value < 0.0001.

Discussion

Conclusions

Under the experimental conditions of this study, the use of inter-row cover crops had negligible beneficial effects on grapevine physiology, mineral nutrition or productivity, with no detrimental effects on vineyard water footprint, in both hot and warm climate vineyards. Additionally, differences between the cover crop species used in this study were not evident. The main effects of inter-row management were evidenced when comparing conventional tillage vs no tilling. Thus, conventional tilling accounted for increased pH and, consequently, decreased must acidity, and decreased water stress over the season, especially, when studying tillage on younger vineyards where it was beneficial to improve vine water and nutritional status in Napa Valley.

Acknowledgements

The authors would like to acknowledge the Wine Group, especially Mr Terry Violett, for his support and vision for the project as well as the access to the vineyard, equipment, and farming practices for their in-kind support for the project. The authors also acknowledge the farming staff at the Department of Viticulture and Enology Oakville Experimental Vineyard. A graduate stipend was provided to MZ and JEM from the Department of Enology and Viticulture. MZ has received research support Jastro-Shields Trust to complete her research.

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