Experimental site

The experimental site was located in the south-central Okanagan Valley (49°33′59″N, 119°38′12″W) at the Summerland Research and Development Centre of Agriculture and Agri-Food Canada, Summerland, BC, Canada. The area is characterized by cool winters (mean December–February temperature: −0.5 °C), warm summers (mean June–August temperature: 20.0 °C), and low annual precipitation (346 mm year⁻¹; Environment and Climate Change Canada 2020) (Table 1). The experiment was located on south–south-west-facing slope in a Skaha loam (Brown Chernozemic soil) (Wittneben 1986; Soil Classification Working Group 1998). More details about the experimental site were described by Hannam et al. (2016b).

Table 1.

Monthly precipitation and mean temperatures for 2017–2020 and average 30-year normals at Summerland Research and Development Centre, Summerland, British Columbia, Canada (Environment and Climate Change Canada 2020).

The grape block was established in May 2011 with Merlot grape variety (clone 181 on SO4 rootstock) according to standard industry practices. Plots comprised 5 vines including a guard vine on either end and 3 experimental vines. Vines were planted 1.2 m apart in row and 3.0 m spacing between the rows, resulting in a density of 2778 vines ha⁻¹. General maintenances were performed according to Best Practices Guide by British Columbia Wine Grape Council (Best Practices Guide | BCWGC 2010). There was no N fertilizer application to vineyard's alleys prior to the experiment or during the experiment. The fertilizer N rate for the vine rows since the project establishment was 40 kg N ha⁻¹ year⁻¹. A dual irrigation system was used from 2017 to 2020: drip irrigation in vine rows and understory sprinklers for alleys. Drip emitters were placed at 0.3 m on both sides of each vine (two 4 L h⁻¹ emitters per vine) and were designed to replace 100% of water lost to evapotranspiration. Cover crops in alleys were mowed twice per each growing season when reaching 30 cm height. To control the weed growth under the vine, a 0.75 m strip on both sides of the vine row was sprayed with glyphosate 3 times per growing season.

Grapevine alleys with or without cover crop and/or B+Com were arranged in a randomized complete block design with 4 replications. Treatments included control, B+Com, cover crop, and cover crop with B+Com (cover crop/B+Com). Each alley treatment was comprised 2 alleys on both sides of a row. Therefore, every other row of vines was considered a guard row. The B+Com treatment consisted of a 1:1 ratio of biochar and compost that was blended 3 weeks before application and hand applied at a rate of 22 Mg ha⁻¹ on a dry weight basis every other year (May 2017 and May 2019) to the respective alleys before sowing of the cover crop. The B+Com was mixed with the top 5–7 cm soil using a rotor tiller. The application rate was based on recommendations from Schmidt et al. (2014) and Karhu et al. (2011). Certified organic biochar was purchased from Canadian AgriChar Company in Maple Ridge, British Columbia. The feedstock for the biochar was wood chips primarily from Pinus contorta var. latifolia, Picea glauca, and Pseudotsuga menziesii, and the pyrolysis temperature was 500 °C. The compost was a year old and composed of 15% grape pomace, 20% straw, 25% shredded bark and wood chips, and 40% cow manure obtained from BigHorn Composting Company, Penticton, British Columbia. No inorganic fertilizers were applied in the alleys. The total N applied through B+Com application during the experiment was 24.8 Mg N ha⁻¹. Biochar, compost, and B+Com characteristics are summarized in Table 2.

Table 2.

Mean biochar and compost chemical characterizations used in 2017 and 2019.

The cover crop mixture was a commercially available southern interior dryland forage mixture of 20% slender wheatgrass (Elymus trachycaulus (Link) Gould ex Shinners), 10% Proper orchardgrass (Dactylis glomerata L.), 10% TL dryland Porto orchardgrass, 20% Oracle creeping red fescue (Festuca rubra L.), 20% tetraploid common annual ryegrass (Lolium multiflorum L.), 10% Kirkcredted wheatgrass (Agropyron cristatum L.), and 10% creeping alfalfa (Medicago sativa L.) ( https://www.tlhort.com/images/files/docs//ta_223_southern_interior_dryland.pdf) sown at a rate of 22.4 kg ha⁻¹ plus bird’s-foot trefoil (Lotus corniculatus L.) sown at a rate of 6.7 kg ha⁻¹. These seeding rates were recommended by the seeding company. The cover crop was initially seeded in spring 2017 and reseeded in spring 2019. Southern Interior Dryland mix is designed for non-irrigated pasture or hay regions of the southern British Columbia interior. The weeds in control and B+Com treatments were controlled by glyphosate only in 2018, and no glyphosate was applied to cover crop in 2017 and 2019.

Cover crop biomass and C and N content

Cover crop samples were collected before each mowing using a 0.25 m² quadrate per alley section of each plot (2 quadrates per plot). Cover crop comprised >80% of the total vegetative ground cover in the alleys determined by quadrate percentage coverage and were mowed, as part of the typical management practice in this vineyard, in July and after harvest. The mowed clippings were not removed from plots, except for clippings in the quadrates. The aboveground biomass in each quadrant was cut at 2.5 cm above the soil surface, stored on ice in coolers for transfer, and fresh weighed within 3 h. The biomass samples were immediately dried at 60 °C until they reached a constant weight. Oven-dried samples were weighed, ground with a Thomas Wiley mill (Thomas Scientific) followed by ball milling. Tissue C and N concentrations were measured using a LECO 628 (LECO Corporation, St. Joseph, MI). Cover crop C and N contents were calculated by multiplying dry biomass, and tissue C and N concentrations.

Soil and biochar compost

To establish a baseline for soil properties, composite soil samples consisting of eight sub-samples were collected from each replication at the 0–15 and 15–30 cm soil depths using soil probe with 2.5 cm diameter in May of 2017 before B+Com application or cover crop sowing. For each plot, soil samples were collected from alleys on both sides of the grape vine row. Soil samples were immediately air-dried and passed through a 2 mm sieve. The pH and electrical conductivity (EC) were measured in 1:2 ratio soil:0.01 N CaCl₂ with pH and EC electrodes. Mehlich-III extractable macro- and micro-nutrients were measured by an ICP-OES (Spectro Analytical Instruments, Kleve, Germany). Organic C (after carbonate removal with HCl 1.0 N) and total N were determined using a LECO 628 CHN analyzer (LECO Corporation, St. Joseph, MI). Carbon sequestration and soil N gain values were calculated for the period of 39 months from the first application of the B+Com on 10 May 2017 to the last soil sampling on 14 August 2020. Particle size distribution was determined by laser analyzer with H₂O₂ pretreatment (Konert and Vandenberghe 1997). Soil texture was loam and soil developed on calcareous parent materials. Soil properties are reported in Table 3.

Table 3.

Mean soil characteristics in alley location at the 0–15 and 15–30 cm depths before start of the experiment in October 2016.

Composite soil samples, each consisting of eight sub-samples, were collected from the 0–15 and 15–30 cm depths at alley (intra-row) location for determination of soil organic C and total N concentrations in June 2017, May 2018, May 2019, and Aug 2020. Soil organic C and total N concentrations were determined as previously described. At the same time as soil sampling, another set of soil samples was taken to determine bulk density at 0–15 and 15–30 cm using 5 cm inner diameter cores. The soil was oven-dried at 105 °C for 24 h to determine bulk density.

Four composite samples of each biochar, compost, and their mixture (1:1 ratio) were air-dried and finely ground using a ball mill, then re-dried at 60 °C for 2 h and their chemical properties were determined. Biochar and compost pH and EC were measured in a 1:2 ratio of amendment to water by pH and EC electrodes. Amendments’ organic C and total N were determined by the same procedure as for soil.

Yield, fruit quality, growth, and leaf petiole N concentration

Grape clusters were harvested in late October from 2018 to 2020. The 2017 yield was lost due to severe animal damage. The grape yield was determined by harvesting clusters on the 3 innermost vines of each plot with a set of harvesting pruners. Harvested clusters were counted and weighed. The average cluster weight was calculated by dividing the total weight of clusters per plot by the number of clusters. After harvest, a representative sub-sample of 12 fruit clusters was weighed and berries were plucked from the upper, middle, and lower third of each cluster for a total of 216 berries. The 216 berries were weighed to determine average berry weight. Berries were crushed by hand and the juice was immediately filtered through several layers of cheesecloth to remove skins and seeds in preparation for analysis. The filtrate was measured for pH and titratable acidity, expressed as grams per litre tartaric acid determined by automated titration with 0.1 mol/L NaOH to an endpoint pH of 8.1. Soluble solid concentration (Brix) was determined by refractometry using Cole-Parmer digital refractometer. The concentration of yeast assimilable N was determined on 15 mL juice samples using the formal titration method (Gump et al. 2001). Vines were pruned in the dormant season, between late February and late April, and pruned materials were weighed to assess canopy growth and vine balance.

Petiole samples were collected at veraison. Samples were taken from the sixth or seventh leaf from the cane tip; if there was a cluster at the sixth or seventh leaf, it was taken directly across from the cluster. Ten petioles were collected from each of the three data vines from each plot and placed in bags on ice until transported. Petiole samples were then weighed and dried in an oven at 60 °C for 48 h. Samples were finely ground using a ball mill, and stored until analyses for N using a LECO 628 (LECO Corporation, St. Joseph, MI).

Statistical analysis

Data were analyzed with JMP software (SAS Institute, Inc., V.15.0.0). The normality of data distribution was tested with Shapiro–Wilk test. When normality could not be assumed, data were transformed. Data transformation was performed on yield (square root), total cluster number (square root), and soluble solids (log). Data were analyzed using a two-way Analysis of variance (ANOVA) by considering B+Com and cover crop as fixed factors, and year and replications as random factors. As the effect of soil depth was always significant on soil C and N content at each sampling date with no interactions with other factors, soil depth was not included in the analysis as a factor. The exception was cover crop parameters in 2018, where a one-way ANOVA was used as weeds in non-cover cropped alleys controlled by glyphosate application. When a treatment's effect on a parameter was significant, differences between treatment means were evaluated using the Tukey's HSD test at a significant level of P < 0.05. Only significant differences at P < 0.05 reported decrease or increase in the “Results” section.

Cover crop biomass and C and N content

Cover crop biomass and C and N contribution to soil were measured from 2017 to 2019 in vineyards alleys (Table 4). In 2017, vegetative ground cover generated 5.67 Mg ha⁻¹ dry biomass and contributed 2.18 Mg C ha⁻¹ and 0.15 Mg N ha⁻¹, but was not affected by B+Com or cover crop treatments. The vegetative ground cover C concentrations in 2017 were about 1% lower in B+Com treatment, but were not affected by cover crop treatment or interactions. The vegetative ground cover N concentrations were about 18% lower in cover crop treatment compared with control, but were affected by B+Com or interactions in 2017. Averaged across control and B+Com treatments, cover crop generated 5.63 Mg ha⁻¹ year⁻¹ dry biomass and contributed 2.41 Mg C ha⁻¹ year⁻¹ and 0.140 Mg N ha⁻¹ year⁻¹ in 2018. In 2019, groundcover vegetation dry biomass and biomass N content were significantly affected by B+Com or cover crop treatments. Dry biomass was 12.4% lower in amended relative to non-amended treatment and was 4.69 times greater in cover crop compared with non-cover cropped treatments in 2019. Biomass N content was reduced by 28% in amended compared with non-amended treatment, and was greater by 3.58 times in cover crop compared with non-cover cropped treatments in 2019. Biomass C and N concentrations and biomass C content were not affected by any of the treatments or their interactions in 2019.

Table 4.

Mean values and ANOVA for the effect of biochar–compost (B+Com) and cover crop treatments on total dry biomass of weeds or cover crop+weeds in alleys, and their C and N concentrations and contents from 2017 to 2019.

Soil C and N content

Across all sampling dates, B+Com alone or in combination with cover crop significantly increased the soil C content only at the 0–15 cm soil depth compared with the control by 11% and 17%, respectively (Table 5). Cover crop treatment did not affect soil C content at the 0–15 or 15–30 cm soil depths in all sampling dates. The B+Com did not affect soil N content, while cover crop decreased soil N content in both soil depths by 8%–17% and cover crop/B+Com increased the soil N content only at the 0–15 cm soil depth by 4% across all sampling dates (Table 5). Overall, over the period of 39 months, organic C increased in vineyard's alleys by 2.3 times from 0.62 Mg ha⁻¹ in control to 1.42 Mg ha⁻¹ in cover crop/B+Com in the top 30 cm soil. The annual C sequestration rates were increased from 0.156 Mg ha⁻¹ year ⁻¹ in control to 0.356 Mg ha⁻¹  year ⁻¹ in cover crop/B+Com treatment. These values translate to the C retention values of 1.5%–2.9% in B+Com and 2.3%–8.8% in cover crop/B+Com assuming 25 Mg C ha⁻¹ input by B+Com and 31.3 Mg C ha⁻¹ input by cover crop/B+Com. Over the period of 39 months, soil total N in 0–30 cm soil depth was not affected by any of the treatments or their interactions. The average annual soil total N gain rate across the treatments was 0.019 Mg ha⁻¹.

Table 5.

Mean values and ANOVA for the effect of biochar–compost (B+Com) and cover crop treatments on soil organic C and total N content in alleys at the 0–15 and 15–30 cm and combined 0–30 cm depths.

Yield, fruit quality, pruning weights, and petiole N concentration

From the measured yield and fruit quality parameters in 2018, only the number of clusters and soluble solids were affected by treatments (Table 6). B+Com treatment resulted in the greater number of clusters relative to control and cover crop/B+Com in 2018. The number of clusters in cover crop treatment was not different from B+Com, control, or cover crop/B+Com. In 2018, greater soluble solids were measured in cover crop/B+Com and control compared with B+Com, but on par with cover crop. In 2019, the effects of treatments on measured parameters were not significant except for the effect of cover crop on cluster size, where larger average cluster weights were measured in cover-cropped treatment relative to non-cover cropped. Total yield was significantly greater in cover crop/B+Com than B+Com or CC (32% and 34% differences, respectively), but on par with control treatment in 2020. Cluster size was significantly greater in cover crop/B+Com than B+Com, but on par with cover crop and control treatments in 2020. Soluble solids were significantly greater in B+Com than control and cover crop/B+Com cover crop/B+Com, but on par with cover crop treatment in 2020. Pruning weights in 2020 were reduced by 29.4% in cover crop treatment relative to non-cover cropped treatments. Leaf petiole N concentrations in 2018 and 2020 were significantly affected by cover crop treatment (Table 6), where cover crop treatment reduced petiole N concentrations by 44.9% and 29.8%, respectively, compared with non-cover cropped treatment (0.49% vs. 0.71%; 0.57% vs. 0.74%). In 2019, leaf petiole N concentrations were not affected by the treatments.

Table 6.

Mean values and ANVOA for the effect of alley biochar–compost (B+Com) and cover crop treatments on grape yield and fruit quality.

Cover crop biomass and C and N content

Our hypothesis that the addition of B+Com will increase cover crop biomass and consequently cover crop-driven C and N input to soil was rejected, because B+Com application did not change total dry biomass in the first 2 years and reduced it in the third year. The cover crop-driven C was not affected by B+Com application, while cover crop-driven N was affected and reduced only in the third year. Only few studies tested the effect of a biochar on cover crop biomass and cover crop-driven C and N content in vineyards of temperate regions. Our results were supported by Hüppi et al. (2016), who reported no significant effect of 20 Mg ha⁻¹ wood chip biochar application on the yield of green rye (Secale cereale L.) cover crop or its biomass C and N content in a 2-year study at Reckenholz Zurich in sandy loam and silt loam soils. In another study, Verhoeven and Six (2014) observed a 155% increase in cover crop (a mixture of sweet pea (Lathyrus odoratus L.), vetch (Vicia sativa L.), faba bean (Vicia faba L.), and barley (Hordeum vulgare L.)) biomass with an application of 10 Mg ha⁻¹ pine chip biochar in the first year, but no differences in biomass in the second year, and no differences in the biomass C and N content in both years in a commercial vineyard located in Sacramento County, USA, with sandy clay loam soil. The inconsistency in the reported biochar application effects on cover crop biomass or C and N contents between years can be attributed to existence of an un-pyrolyzed labile C fraction in the biochar that encourages the N mineralization after a short N immobilization phase in the year of application (Nelissen et al. 2015). Their results were different from ours likely due to differences in biochar feedstock, cover crop species, and soil type among other factors.

The effect of biochar on soil mineral N immobilization has been reported in several studies (Kolb et al. 2009; Novak et al. 2010), which can lead to negative effect on cover crop biomass and C or N content particularly in non-leguminous species. Bruun et al. (2012) showed that application of wheat straw biochar resulted in 43% increase in soil mineral N immobilization during 65 days of incubation at 23 °C. They corresponded this effect to existence of an un-pyrolyzed labile carbohydrate fraction in the biochar, which supported a larger microbial biomass C compared with control. Nelissen et al. (2015) also reported a considerable immobilization of nitrate and ammonium after application of a 20 Mg ha⁻¹ wood-based biochar in a field experiment in Belgium. The N immobilization after biochar application is temporary as the labile biochar C will be mineralized after a few months (Nelissen et al. 2015). Biochar can also accelerate soil micro-organism processes and further soil N immobilization through a change in soil pH, microbial protection in biochar pores, bacterial adhesion or sorption of compounds that would otherwise inhibit microbial growth (Lehmann et al. 2011). For N fixing species (i.e., legumes) soil available N is not a growth-limiting factor, while in grasses (the dominant species in our study) and cereals (Hüppi et al. 2016) soil available N is a critical factor for their biomass production (Verhoeven and Six 2014). The lack of cover crop biomass response to biochar application in 2017 may relate to carryover of mineral N from a previous season, and in 2018 to the diminished immobilization capacity of the biochar after a year of aging in the field. The reduction in cover crop biomass and N content in 2019 can be associated with a fresh application of biochar with a high N immobilization capacity plus limited carryover of the mineral N from the previous year (Nelissen et al. 2015).

Soil C and N content

Biochar average C and N concentrations were lower in C (78%–79%), but within the range reported for N (0.4%–0.9%) compared with data reported by Baronti et al. (2014) and García-Jaramillo et al. (2021). The hypothesis that addition of B+Com or planting cover crop in the vineyard's alleys will increase soil organic C in 0–30 cm soil depth was partially accepted for B+Com, but rejected for cover crop. The positive effect of B+Com on soil organic C was only in 0–15 cm soil depth. Few studies have evaluated the effect of biochar application on soil C and N in vineyard's alleys (Baronti et al. 2014; García-Jaramillo et al. 2021), and only 1 study evaluated the effect of B+Com on soil organic C (Niehues 2015). Our findings were supported by García-Jaramillo et al. (2021), who tested the effect of 18 and 35 Mg ha⁻¹ undervine application of a wood-based biochar on soil organic C and total N 6 months after application in 2 pinot Noir vineyards with loam to clay loam soil texture in Oregon, USA. They reported a 60% to >100% increase in soil total C in both application rates and in both locations compared with unamended control. The high magnitude of biochar effect on soil total C in García-Jaramillo et al. (2021) can be associated with 34% greater C concentration in their biochar compared with our study and their high rate of biochar application that was more than twice as high as our rate (11 Mg ha⁻¹). Baronti et al. (2014) did not report any data on the soil C. Niehues (2015) reported greater net C sequestration with application of B+Com compared with other organic fertilization practices in Napa Valley, USA, vineyard soils.

Several studies on other crops particularly on field crops (e.g., wheat, corn, and ryegrass) reported a significant increase in soil C as a result of biochar application (Weng et al. 2017; Shi et al. 2021). Shi et al. (2021) observed 16%–82% increase in soil organic C content by 10 years annual application of 4.5 and 9.0 Mg ha⁻¹ corn cobs biochar in winter wheat–corn rotation in a silt loam soil in Shandong Province, China. In their study, biochar C was mainly allocated to particulate organic C fraction (38%–166%). They attributed the greater soil organic C accumulation in biochar treatment compared with control to high C content of biochar, as well as its potential negative priming effect on the native soil C (Shi et al. 2021). Also, aromatic structure of C in the biochar resists the microbial degradation (Zhao et al. 2020). The negative priming effect of biochar may occur through 2 main mechanisms: enhanced formation of soil aggregates by improving organo-mineral interactions, therefore an increase in physical protection of soil organic matter (Maestrini et al. 2015; Weng et al. 2017), and adsorbing and stabilization of dissolved organic C or acid root exudates (Whitman et al. 2014). As the input of biochar C was through the soil surface in our study, the main accumulation of the C was found in 0–15 soil depth. Similarly, Weng et al. (2017) in a subtropical annual ryegrass field system found an increase of 13.3 Mg C ha⁻¹ in the total soil organic C stock (0–10 cm soil depth) 8.2 years after the incorporation of a 10 Mg ha⁻¹ eucalyptus biochar (C = 76%). The positive effect of biochar application on soil inorganic C was reported by Shi et al. (2021); however, soil inorganic C was not evaluated in our study.

Cover crops with the goal of C sequestration have been increasingly used in the vineyards. Although we did not find any significant effect of cover crop treatment on soil C content, Poeplau and Don (2015) reported an annual soil organic C stock change rate of 0.32 ± 0.08 Mg ha⁻¹  year ⁻¹ in top 22 cm soil and during the observed period of up to 54 years when compiling the data from 139 cover crop plots at 37 different sites. However, they also noticed a reduction in soil organic C stock in 13 out of 139 plots after introducing cover crops. The lack of effect or reduction in soil organic C with cover cropping practice can be associated with (i) addition of rapidly decomposable plant material (low C/N ratio) that leads to enhanced microbial community activity, therefore fast turnover of newly added organic matter and positive priming effect on native soil organic C, and (ii) the high spatial variability of soil organic C concentrations in the soil that masks the small effects of cover crops on soil organic C (Poeplau and Don 2015). The former effect will be magnified when physical protection by soil particles is minimal, i.e., light-textured soils, which was the case in our study (Maestrini et al. 2015). The masking effect of spatial variability becomes more likely, when the time since establishment of cover crop treatment is not long (e.g., <7 years) (Poeplau and Don 2015). Goidts et al. (2009) found that only soil organic C concentrations changes >20% could be detected in the field. The C/N ratio of cover crop residues is also an important indicator of their turnover time in the soil (Schimel and Weintraub 2003). In our study, we only measured the soil organic C content at the top 30 cm soil depth, which might not fully capture the total effect of deep rooting cover crops on soil organic C stock. Also, we did not measure cover crop belowground C contribution; therefore, the input of C was likely underestimated. Gattullo et al. (2020) carried out a 3-year study with a fescue (Festuca arundinacea Schreb.) cover crop in a table grape vineyard in southern Italy on a silt loam soil and reported the average increase of 136% of soil organic C content in the cover crop treatment (0–20 cm soil depth) compared with control. The positive soil organic C response to cover crop in their study can be attributed to higher root/shoot ratio of grass, lower C/N ratio of residues, heavier soil texture, and lower initial organic C content of the soil (2.6 g kg⁻¹) compared with our study (Gattullo et al. 2020).

The hypothesis that addition of B+Com or sowing cover crop in the vineyard's alleys will increase soil total N in 0–30 cm soil depth was rejected, as B+Com did not affect and cover crop reduced soil total N in 0–30 cm soil depth. García-Jaramillo et al. (2021) reported an increase in soil total N only in 1 of the 2 studied vineyards with 35 Mg ha⁻¹ undervine application of a wood-based biochar in Oregon, USA. Biochar is not considered as a source of N in literature due to its low concentration of N (Baronti et al. 2014; García-Jaramillo et al. 2021). Cover crops, however, are reported as the main input of N to the soil. Tarricone et al. (2020) reported an average of 18% increase in soil total N using subterranean clover (Trifolium subterraneum L.) or vetch as undervine cover crop in a table grape vineyard in Italy. The reduction in soil N in 0–30 cm depth in our study can be related to low N input using a grass-based cover crop (Tarricone et al. 2020), light texture of the soil with low physical protection capacity (Maestrini et al. 2015), possibility of N leaching, and high spatial variability in soil total N concentration (Poeplau and Don 2015).

The hypothesis on the positive interaction of B+Com and cover crop on soil organic C and total N in 0–30 cm soil depth was accepted only for 0–15 cm depth. We observed a positive effect of B+Com on cover crop-derived C accumulation. The cover crop-derived C consists of aboveground and belowground residues and root exudates. Weng et al. (2017) found 20% greater belowground recovery of new root-derived C by field-aged biochar, which corresponded to facilitation of negative rhizosphere priming. Other studies reported 16%–48% reduction in soil organic C mineralization rate when incorporating plant residues in a soil that received biochar application (Whitman et al. 2014; Keith et al. 2015; Weng et al. 2015). Weng et al. (2017) measured higher recovery of the new root-derived C in a biochar-amended soil and suggested that biochar may have promoted the root exudation and consequently soil aggregation process. The greater protection of root-derived particulate organic matter C or exudates in biochar-amended soil associates with 2 mechanisms of physical occlusion or organo-mineral interactions (Joseph et al. 2020). Our results were also supported by Messiga et al. (2015), who reported 9%–11% greater soil organic C concentration and 8%–10% soil organic N in a combination of cover crop and an soil amendment treatment compared with amendment or cover crop alone in a 2-year study in a vineyard in Nova Scotia, Canada.

Yield, fruit quality, pruning weights, and petiole N concentration

The hypothesis that B+Com or cover crop or their combination in the vineyard's alleys will positively affect vine growth, grape yield, fruit quality, and vine N status was rejected except for the positive effect of cover crop/B+Com on yield in 2020. The effect of B+Com on increasing the number of clusters and reducing berry soluble solids in 2018 was aligned with a trend towards higher yield values in this treatment (Sivilotti et al. 2020; Bates et al. 2021). The mechanism of this effect is not clear, but could be due to a more balanced vine vigour due to immobilization of soil N in this inherently vigorous vineyard (Bruun et al. 2012; Zhang et al. 2012; Nelissen et al. 2015). In 2019, the average cluster size was increased by cover crop treatment, but the reason for this change is not clear. The lower yield in B+Com or cover crop treatments compared with cover crop/B+Com treatments in 2020 can be attributed to N immobilization by B+Com or competition for N between cover crop and vine (Bruun et al. 2012; Zhang et al. 2012; Nelissen et al. 2015). The effect of fresh application of B+Com on soil N immobilization was confirmed by small average cluster size and greater soluble solids compared with other treatments in 2020. The reverse relationship between yield and soluble solids was reported in several studies and was associated with the dilution of photosynthates (Sivilotti et al. 2020; Bates et al. 2021). The reduction in pruning weight in 2020, and petiole N concentration in 2018 and 2020 by cover crop treatment can be related to competition for resources between cover crop and vine. Petiole N usually exhibits a significant and positive correlation with available N in the depth of 0–30 cm (Bhat et al. 2017).

Grape yields and fruit quality parameters were within normal ranges typically observed for Merlot in the Okanagan Valley, BC, Canada (Neilsen et al. 2010; Hannam et al. 2016a), except for titratable acidity that was on the high end. The lack of B+Com treatment effect on yield and majority of fruit quality parameters in 2018 and 2019 was supported by García-Jaramillo et al. (2021), who reported no yield or berry chemistry significant responses to biochar application treatments in Oregon, USA. In contrast, Genesio et al. (2015) reported a significant increase in grape yield after the first year of biochar application at 22 Mg ha⁻¹. Their vineyards were under moderate to severe water deficits as discussed by Baronti et al. (2014). In the Baronti et al. (2014) study, soil water holding capacity and vine water status were improved by biochar application; therefore, the observed increased yield by Genesio et al. (2015) can be associated with improved vine water status. In our study, the vineyard was irrigated during the summer and therefore no soil water deficit was expected, and consequently no response of yield to B+Com application was observed. The lack of fruit quality parameter response to biochar application is also reported by Schmidt et al. (2014) and Genesio et al. (2015). It has been reported that vine yield and physiological response to biochar application is more likely to be seen under water deficit conditions; therefore, more studies on interactions of biochar and soil water status in Okanagan Valley region can be recommended.

The slight or no effect of cover crop treatments on grape yield and fruit quality is common and was reported in several studies (Giese et al. 2014; Sharifi et al. 2018; Tarricone et al. 2020). Gattullo et al. (2020) reported no effect of cover crops on yield or fruit quality except for 1 out of 3-year yield reduction that was associated with low precipitation in the growing season in that year. The low precipitation in the summer can trigger the competition for water between vine and cover crop, consequently leading to reduced vine vigour and yield. Reductions in grape yield as a result of cover crop practice were reported in a number of studies, which were mainly attributed to competition for the resources (Morlat and Jacquet 2003; Muscas et al. 2017). The competition for water and nutrients under cover crops and immobilization of nutrients by biochar can lead to reduction in the vegetative and reproductive growth of vigorous vines, therefore improving cluster exposure to light and air, reducing pest and diseases, and improving fruit composition. In our study, the reduction in pruning weight and petiole N, and consequently, enhanced soluble solids, was evident in some years. Messiga et al. (2016) reported an increase in the second year grape yield when a combination of soil amendment and cover crop was used in an infertile soil in southern Nova Scotia, Canada. They reported the greatest yield in oats and red clover (Trifolium pretense L.) mixture combined with muscle sediments (9.52 Mg ha⁻¹) followed by oats, pea, and vetch mixture combined with municipal solid food waste compost (9.49 Mg ha⁻¹). Their highest berry sugar concentrations among the cover crops were obtained under timothy grass (Phleum pratense L.) cover crop and attributed to elevated competition between vine and grass for nutrients compared with control.