1. Introduction
The effects of fully mechanized pruning systems on physiology and yield of grapevines and on grapes and wines quality have been studied since the early 1970′s [
1]. However, the interaction between the mechanical pruning and plant nutrition has not yet been evaluated, despite the fact that the interactions of irrigation with mechanical pruning [
2] as well as with pruning level have given significant results [
3]. It is known that mechanical pruning strongly increases the bud load per vine [
4,
5] and, due to that, the plant autoregulation capacity, which is influenced among other factors by soil fertility, becomes the major regulator of yield and, consequently, grape and wine quality.
Fully mechanized pruning systems induce a significant increase in bud load, which leads to compensation mechanisms that downregulate bud break [
6], shoot fruitfulness [
7] and, in a lesser extent, cluster weight [
6,
8]. However, the higher number of clusters per vine [
4,
8], usually increases yield [
4], although it does not always occur [
8,
9] due to the referred autoregulation mechanisms.
Mechanically pruned grapevines develop a higher leaf area in the beginning of the season [
9], nevertheless the differences are not always maintained until the end of the season [
10]. The higher the bud load difference is, the later the leaf area become equal between pruning systems [
11] and, at veraison, there are no differences [
2,
10]. Besides, photosynthetic rates are higher in free growing canopies [
12], which are more suitable for mechanical pruning, than in those forced between catch wires.
Mechanical pruning leads to an increase in the shoot number per plant and to a decrease of the individual shoot weight [
8,
13]. The total pruning weight per vine is usually lower in mechanically pruned grapevines [
10,
14], reflecting the higher competition of reproductive growth for available assimilates.
One of the limiting factors of vineyard productivity in Portugal is the lack of fertility of the soils, which is strictly correlated with the low organic matter content [
15]. In a climate change scenario, Mediterranean viticulture is facing serious risks and adaptation measures are needed, in order to face this problem. Among those measures, organic fertilization of soil is highlighted [
16] because organic matter plays an important role in long-term soil conservation and/or restoration by sustaining its fertility, due to the improvement of physical, chemical and biological properties of soils [
15].
In Europe, vineyards are one of the agricultural ecosystems with the lowest carbon content in the soil [
17] and the one with the highest risk of erosion [
18]. The decrease of carbon content in vineyard soils is intensified in Mediterranean regions, due to climatic conditions, namely high temperatures associated with large periods of drought, during late spring and summer, which are expected to worsen with the expected climate changes [
16].
No tillage, no removal of pruning wood from the vineyard soil or the usage of crop rotations, are some of the strategies that may be used to raise the soil organic matter content [
19]. However, soil fertility can also be improved with organic waste application which generally presents high contents of organic matter, substantial quantities of nutrients and their use in agriculture can contribute to close the natural ecological cycles [
19,
20].
The reduction of the availability of animal manures, which led to a decrease in the practice of replacing soil organic matter, is compensated by the increasing availability of “unconventional” organic fertilizers, obtained from alternative residues [
21]. There is a particular interest in composted municipal solid waste (MSWC) and sewage sludge (Sludge), due to their availability, and in coal resulting from the pyrolysis of biomass (biochar), due to its potential of carbon fixation in soil.
The chemical modifications of soil characteristics due to the application of organic amendments can play a significant role in the nutrient status of the vine and, consequently, affect yield [
22].
The effects of nitrogen (N) on the grapevine have already been extensively studied. Nitrogen fertilization tends to induce an increase in yield [
23] up to a certain level, that is around 40 and 56 kg N ha
−1 [
24,
25]. In agreement with these results, Morris et al. [
26], with 152 kg N ha
−1 and 228 kg N ha
−1, did not notice any increase in yield. However, the response to N is not always the same, as reported by Wolf and Pool [
27] who observed a tendential decrease in yield with the increase of N application rate from 39 to 84 kg N ha
−1. Usually the organic amendment that provides the larger amount of this nutrient is sewage sludge (Sludge), while municipal solid waste compost (MSWC) and cattle manure also provide it but in a lower extent [
21,
28,
29].
Organic amendments supply also phosphorus (P), particularly Sludge, which effects on grapevine are not widely studied, since the grapevine requires only small amounts of this nutrient. According to Schreiner et al. [
23], P fertilization did not affect growth and yield, while Conradie and Saayman [
24] observed an increase in yield and growth with only 9 kg P ha
−1 year
−1, in a soil that was originally deficient in this nutrient.
Potassium (K) is another of the macronutrients that is supplied by organic amendments. However, the results of its application are not consistent with some authors finding slight increases in yield [
24], while others found no differences [
30] or even small reductions [
31].
The effects of Sludge and MSW amending on yield, vegetative growth and grape composition have been poorly studied, but an increase of yield was observed with the application of a sewage sludge compost in a Cabernet Sauvignon vineyard [
32], while, in a Merlot experiment, no differences were found in yield, but only a decrease in pruning weight [
33]. Concerning MSW application, no differences were found in vegetative and reproductive growth [
33,
34]. About soil amending with cattle manure the results are not always consistent, while in one trial significant increases in yield were observed since the second year of application [
35], in another study, no significant differences were found with a 28-year application [
22].
Biochar is a co-product of a thermochemical conversion of biomass that is recognized to be a beneficial soil amendment which when incorporated into the soil increases the retention and availability of water [
36] and nutrients [
37]. Few works have been made regarding biochar application in vineyards and its influence on yield and quality of grape and wine. Nevertheless, with the application of 22 Mg ha
−1 year
−1 of biochar, an increase in productivity was observed, mainly due to water availability, without loss of quality [
38]. However, in another work, biochar and biochar-compost treatments induced only small, economically irrelevant and mostly non-significant effects on yield and quality over the three years of the experiment [
39].
Mechanical pruning appears to be an interesting way to reduce annual production costs. On the other hand, organic amendments improve soil fertility and tends to increase productivity. However, the interaction between mechanical pruning and the organic amending of vineyard soil was not previously studied, to the best of our knowledge. The objective of this work is to study if the increase of soil fertility by the organic amending will reduce the impact of self-regulation mechanisms in mechanically pruned vines maintaining or increasing vegetative and reproductive growth.
2. Materials and Methods
2.1. Vineyard Sites and Experimental Layout
The trial, run over four years (2012 to 2015), was installed in two vineyards of Vitis vinifera L. cv. Shiraz. Quinta do Côro (QC) experimental site is located in Tejo wine region and the vines had been grafted on 99R in 1999 and spaced 1.0 m × 2.5 m, for a density of 4000 plants ha−1. In Quinta do Gradil (QG) experimental site, which was located in Lisboa wine region, the vines, grafted on 1103P, were planted in 2005 and spaced 1.0 m × 2.6 m, for a density of 3846 plants ha−1. Row orientation is N-S in both cases.
The soil in QC was a Hypereutric Regosol [
40], with a sandy-loam texture, a pH
H2O of 6.4, a low organic matter content (1.54%), an extractable K and P contents of 70.7 mg K kg
−1 and 59.8 mg P kg
−1 (ammonium lactate extraction—[
41]), respectively. In QG soil was also a Hypereutric Regosol [
40], with a sandy-loam textures, a pH
H2O of 5.9, a low organic matter content (1.07%), an extractable K and P contents of 167.0 mg K kg
−1 and 61.2 mg P kg
−1 (ammonium lactate extraction—[
41]), respectively.
The climate in QC is a Csa and in QG is a Csb, according to the Köppen-Geiger climate classification [
42]. Total rainfall and monthly mean air temperature data (
Figure 1 and
Figure 2), were collected in the IPMA’s weather station of Alvega (12 km from QC trial field) and in a weather station located in QG (200 m from the trial field), show differences between the two trial fields and among four years of the work. The mean air temperature amplitude was higher in QC, with cooler winters but warmer summers. Among the four years, the average of the highest mean air temperatures was 23.6 °C in QC while in QG was 20.3 °C. The warmer summer in QC led to a earlier grape maturation when compared to QG.
In what concerns to precipitation, the rainier experimental site was QG (
Table 1) with an average annual rainfall of 812 mm comparing to 603 mm in QC. The precipitation was the highest in both experimental sites in 2014, with 819 mm in QC and 1112 in QG, while 2012 was the driest year only with 269 mm in QC and 635 mm in QG.
The studied factors were the pruning system and organic amendment that were compared in a strip-plot design, with three replications, installed in both trial fields. Each block held eight adjacent rows where pruning treatment was randomly assigned, creating two groups of four adjacent rows each with a different pruning treatment. The 60 m rows were divided in five parts of twelve meters each, where organic amendments were randomly distributed. The organic amendments were spread all over the seven interrows, creating a stripe perpendicular to the row orientation. Each one of the 30 experimental units consisted of 48 vines.
In what concerns to pruning, two treatments were imposed: MAN—manual spur pruning, retaining six to seven 2-bud spurs per vine; MEC - mechanical pruning, simulating the pruning effect of four cutting bars (2 parallel and 2 perpendicular to the ground) working at a distance of 15 cm from the cordon. All wood with ventral insertion was removed. In MAN treatment, the training system was a spur pruned Royat cordon, established at 70 cm above soil surface, with vertical shoot positioning. Mobile wires were moved once in the season (right before bloom) in order to position shoots which were later trimmed in order to create a parallelepipedic canopy. In MEC treatment, the cordon was also established at 70 cm above soil surface and the wires stayed in the same position (40 cm and 80 cm above the cordon) the whole year, and shoots were not positioned. Thus, some shoots grabbed their tendrils to the wires and maintained a vertical position, while the others grew freely in oblique or horizontal positions, creating a larger and sparser canopy. Canopy management, in MEC, was limited to light and wide mechanical shoot trimming performed to promote upright growth and optimal machine operating conditions for the harvest and the next winter pruning. Shoots were tipped before bloom and veraison to retain approximately 9 to 10 leaves on the main shoots.
In relation to organic amendments, five treatments were imposed: Ctrl—no application of organic amendment neither fertilizer; Bioc—application of 8500 kg ha
−1 year
−1 of char dust; MSWC—application of 16,100 kg ha
−1 year
−1 of municipal solid waste compost; Manure—application of 24,000 kg ha
−1 year
−1 of cattle manure; Sludge—application of 34000 kg ha
−1 year
−1 of sewage sludge. The referred quantity of each organic amendment is expressed in fresh weight and its definition was based on the application of 5000 kg of dry organic matter per hectare and per year. The composition of the organic amendments is presented in
Table 1.
The organic amendments were spread over the soil and incorporated with a light disk harrow, before bud burst. The nutrients supplied by each organic amendment are the following:
Bioc—72.0 kg ha−1 year−1 of total N, 7.2 kg ha−1 year−1 of total P and 43.8 kg ha−1 year−1 of total K;
MSWC—226.0 kg ha−1 year−1 of total N, 110.4 kg ha−1 year−1 of total P and 124.9 kg ha−1 year−1 of total K;
Manure—179.4 kg ha−1 year−1 of total N, 101.9 kg ha−1 year−1 of total P and 432.2 kg ha−1 year−1 of total K;
Sludge—503.7 kg ha−1 year−1 of total N, 460.5 kg ha−1 year−1 of total P and 108.6 kg ha−1 year−1 of total K.
2.2. Reproductive and Vegetative Growth
In order to determine yield components, the number of clusters per vine and their weight were assessed at harvest in six previously selected vines in each replication. Berry weight was assessed in 100 berries samples, randomly collected in each experimental unit, and the number of berries per cluster was estimated by the ratio between cluster and berry weight. During dormant pruning, the number of canes per vine, including water sprouts, and their weight was measured, in order to evaluate the effect of the different treatments on vegetative growth. The dry matter production was calculated as proposed by Carbonneau and Cargnello [
43]: DMP = 0.2 × yield + 0.5 × pruning weight. The Ravaz index was calculated by dividing the yield per vine by the dormant pruning weight of the same vine in the following winter. Right after pruning, the bud load was assessed in the same six vines. The vines where the pruning data was collected were the same that were evaluated at harvest.
2.3. Statistical Analysis
Data were tested to verify if the assumptions of analysis of variance (ANOVA) were met using Shapiro-Wilk’s test and then subjected to three-way ANOVA (site × pruning × organic amendment), using the general linear procedure for strip-split–plot design and F-test. The significance level was set at α = 0.05 and means were separated using Tukey’s honestly significant difference test. The statistical analysis was performed using Statistix software package (version 9.0; Analytical Software, Tallahassee, FL, USA).
In the following tables the presented values are averages of both trial fields.
3. Results
3.1. Shoot and Cluster Number
On
Table 2 are presented the results of bud load as well as shoot, water sprout and cluster number per vine. The bud load as well as the number of shoots per vine was significantly higher in MEC. The budburst percentage was lower in MEC (60%) when compared to MAN (100%). The differences between pruning systems in what concerns to bud load were higher than to shoots number per vine (74% and 37% average, respectively), due to the higher water sprout number in MAN. While the number of water sprouts in MEC was residual (between 1 and 3%), in MAN it accounted for almost 41% of the total number of shoots per vine. In result of the higher number of shoots, the number of clusters per vine in MEC was also significantly higher than in MAN (52%). Even though the interaction between pruning and experimental site was significant in three years in each site the cluster number per vine was always significantly higher in MEC.
Organic amendments had no significant effects, neither on bud load nor on water sprout number per vine. However, the shoot number was affected in the last two years of the trial, being higher in MSWC, Manure and Sludge. In consequence of the higher shoots number per vine, these treatments also had more clusters per vine.
In shoot number per vine, the interaction between the two studied factors was significant in the last two years of the trial, because shoot number was significantly affected by organic amendments only in MEC. The interaction between pruning and organic amendment was significant for cluster number per vine in 2015, also because it was significantly affected by organic amendment only in mechanically pruned treatments.
3.2. Cluster and Berry Weight
Table 3 presents the results of berry and cluster weight as well as the berry number per cluster. Cluster weight was, consistently, significantly lower in MEC, with a 39% difference between treatments. In 2014 the cluster weight was the highest of the four years of trial in both treatments and, among years, cluster weight was more stable in MAN than in MEC.
Regarding the effect of the organic amendments on cluster weight, significant differences were observed only in 2013 and 2014. In 2013, the difference was significant only between Sludge and Ctrl (22%). In 2014, Sludge was significantly higher than Bioc and Ctrl, being the differences of 12% and 15% respectively. In 2012 and 2015 there were no significant differences in cluster weight between organic amendments.
The berry weight and its number per cluster were assessed in 2012, 2013 and in 2014 and the results show that the decrease in cluster weight in MEC is due to a significant decrease in berry weight as well as in berry number per cluster in the three years. The effect of the organic amendments is not so consistent since in 2013 the berry weight was not affected by this factor while the berry number was significantly increased by Sludge. On the other hand, in 2014 was the berry weight that was increased by Sludge, only in QG, while the berry weight was not significantly affected in any of the trial fields. In this year, in QG, the lowest berry weight was obtained in Ctrl and in Bioc (2.00 g in both treatments) and it was significantly lower than Sludge (2.31 g).
3.3. Yield
In
Table 4 is presented the effect of the studied factors in yield per plant. Yield was significantly increased by MEC only in 2014. In the other three years a not significant tendency for higher yield in MEC was observed. Globally, MEC produced 24% more than MAN. The organic amendments had a significant effect on yield since the second year of the trial. Sludge was the organic amendment with a faster effect on yield, while MSWC and Manure took more time to take effect. In 2013, Sludge produced more than Ctrl, Bioc and MSWC. In 2014, Sludge had a significantly higher yield when compared to Ctrl and Bioc. In 2015, Sludge, manure and MSWC produced more than Ctrl and Bioc.
One interesting fact is that in the three years when the organic amendments originated significant effects on yield, there was a significant interaction between this factor and the pruning system. The analysis of the interaction between both factors (
Figure 3) reveals that the effect of the organic amendments was significant only in MEC, while in MAN the organic amendments had no significant effect on yield.
In MEC, the higher yield was obtained in Sludge, followed by MSWC and Manure, while Bioc did not significantly differ from Ctrl in any year. MEC*Ctrl and MEC*Bioc were not significantly different from MAN treatments in any year.
3.4. Vegetative Growth
Table 5 presents the results of shoot weight and of the pruning weight per vine. Mechanical pruning significantly reduced the shoot individual weight. In the first year, the reduction in shoot weight was lower (37%) than in the next three years (between 51% and 61%).
Pruning weight per vine was not significantly different between pruning systems in the first three years of trial. In the last year, pruning weight was significantly lower in MEC. Although the differences were not significant, in all the three first years there was a tendency for lower pruning weight in mechanically pruned vines. The difference between pruning systems oscillated between 103 g (13%), in the first year, and 345 g per plant (36%), in the last one.
The organic amendments started to have effect in vegetative growth only from 2014 on. Sludge and MSWC treatments were those where shoot and pruning weight were higher, while Ctrl and Manure were the lowest. Bioc had an intermediate behavior and did not significantly differ from all the other treatments, in terms of shoot weight, and, like Ctrl and Manure, had a significantly lower pruning weight when compared to Sludge and MSWC.
3.5. Dry Matter Production and Ravaz Index
Dry matter production and Ravaz Index results are presented in
Table 6. Mechanical pruning induced a significant increase of 26% in dry matter production, in 2014. Dry matter production tended to be higher also in 2012 and in 2013, nevertheless, in 2015, both treatments had equal values of this variable. On the other hand, Ravaz Index was consistently increased by mechanical pruning, due to the previously referred rise in yield and reduction of the pruning weight. The difference among pruning systems was higher in 2014 (55%) and the higher values of this index were obtained in 2015, in both pruning systems.
Organic amendments started to have effect on dry matter production from the second year on. Sludge was significantly higher than Ctrl and Bioc since the second year of the trial. MSWC significantly increased dry matter production, when compared to Ctrl, from 2013 on and Manure only induced significant effects in the last year of the trial. Bioc was never significantly different from Ctrl. Organic amendments had no significant effects on Ravaz Index along the trial.
The interaction between the pruning system and organic amendment effects in dry matter production is presented in
Figure 4. The differences among organic amendments, in MAN, are not significant with the exception of Sludge that, in 2013, had more dry matter production than Manure and, in 2015, than Manure, Bioc and Ctrl. However, even though the differences were statistically significant, the maximum dry matter production difference among MAN treatments was 20% (between Sludge and Ctrl in 2015). On the other hand, in MEC, the dry matter differences, between organic amendments, was higher. The lower difference between Sludge and Ctrl was 34% and the higher was 42%. MSWC and Manure were significantly different from Ctrl every year and 2015 was the only season in which these two treatments were significantly different from Sludge. Bioc was never different from Ctrl.
The only year in which MEC*Ctrl and MEC*Bioc were lower than MAN*Ctrl and MAN*Bioc was 2015, with a difference of 21% and 19% respectively.
5. Conclusions
Mechanical pruning and the organic amendments had a positive effect in shoot and cluster number per vine. However, while mechanical pruning significantly reduced shoot and cluster weight, the organic amendments had an opposite effect
Although the present work was performed in non-irrigated vineyards and the results showed a good performance, even under drought conditions, mechanically pruned vines tend to consume more water. Therefore, the use of irrigation in this system should be considered.
The obtained results show that the interaction between mechanical pruning and soil organic amending successfully increases yield, without decreasing vegetative growth. Both tools, when combined, create a high yielding production system with low hand-labor input, increasing the activity profitability.
The growth in waste generation all over the world has quickly become one of the major challenges for today’s economy and is a key issue for circular economy. This work shows that the use of organic amendments together with mechanical pruning are an effective way to valorize human residues, to reduce the dependence of winegrowers on the increasingly scarce hand labor and increase vineyards profitability.