Effect of Sewage Sludge Compost and Urban Pruning Waste on Agronomic Parameters and Wine Composition in Arid Zones Under Climate Change

Abstract

Soil degradation is caused by climate change and some agricultural practices. The use of compost from organic waste can be a sustainable solution, but poses risks to soil, crops and fruit. This article examined vineyard yield, vine and wine composition when compost from sewage sludge and urban waste was applied to two soils. One rainfed plot received 80 UFN kg/ha, while two irrigated plots received 40 and 80 UFN kg/ha. Compared to mineral fertilizer, compost increased crop yield (+60% in rainfed conditions) and above-ground biomass (+15% in rainfed conditions). Aromatic series were obtained by grouping the aroma compounds according to their aroma descriptor. In both rainfed and irrigated trials, higher values were observed in the fruity, green, waxy and floral series in wines from vines fertilized with compost compared to the control and chemical fertilization. The compounds with a higher influence in such series were ethyl butanoate, ethyl hexanoate, ethyl octanoate, hexanal, phenylacetaldehyde and 2-phenylethanol. Organoleptically, wines from compost were preferred to those from mineral fertilizer, with ratings close to the control wine, particularly in aroma, flavor and overall impression. Although further studies are needed, compost fertilization appears on the one hand to improve wine quality and, on the other hand, is a suitable alternative that reduces municipal waste.

1. Introduction

Winegrowing is one of the most important agricultural sectors in the world, with a volume of 292 × 10⁶ hL produced in 2018, with France, Spain and Italy standing out with 40% of the world’s vineyards [1]. In recent years, the intensification of this production, added to the effects of climate change, has caused a significant environmental impact, which is being mitigated by replacing traditional farming systems with alternative production framed in the concept of sustainability [2,3]. In this sense, the incorporation into agricultural soils of the resources contained in numerous organic wastes (urban, agricultural, agro-industrial) is an interesting option that, on the one hand, avoids sending these wastes to landfills and, on the other, contributes to the circular economy of the agricultural and agro-industrial sector [4,5,6]. The treatment of this type of waste through composting processes, under appropriate operating conditions, makes it possible to obtain organic amendments that can be used in agriculture. This can minimize the problems of erosion, desertification and deficit of organic matter and nutrients that, in general, characterize soils, in particular those of the Mediterranean area [7]. These problems are exacerbated by the effects of climate change, including higher temperatures and lower but torrential rainfall [8]. The commitment to this type of technology helps to minimize the environmental problems associated with the acquisition of chemical fertilizers and their application to the soil [9]. However, for this alternative practice not to have a negative impact on crop yields and fruit quality, composting must be developed under conditions that result in a sanitized, stabilized, mature product, with a metal concentration within the limits allowed by legislation for agricultural use. The application must also be carried out in a safe manner, considering the physico-chemical conditions of the soil, the type of crop and the composition of the starting residual material [7,10].

A study of the effects of the continuous application of three types of organic amendments in vineyards for 13 years, i.e., pelletized compost from plant and animal waste and sewage sludge, OF-MSW compost, and sheep manure compost, showed that the continuous and long-term application of these types of amendments had a positive effect on soil quality, increasing microbial activity and moderately increasing carbon sequestration [11]. On the other hand, the application of compost made from vegetable residues (including horticultural and wheat straw) to vineyards over a three-year period provided evidence for positive effects on vegetative growth [12]. The application of compost from wine waste mixed with zeolite in different proportions was compared with commercial compost. The results showed that the contribution of the mixture of zeolite (10%) and compost from wine waste (90%) was that which best met the plant’s needs in terms of available nutrients [13]. Lastly, fertilization for 5 years with compost made from a mixture of vine shoots and cattle manure showed an increase in vegetative expression (root growth, shoot and pruning weight) and an increase in plant yield. An increase was also found in total soluble solids and Yeast Available Nitrogen (YAN) in the musts, although there was a decrease in the concentration of anthocyanins and flavonoids [14].

Sludge from urban wastewater treatment plants (WWTPs) is a waste that is generated in large quantities and has a significant environmental impact. The bibliography includes studies that have evaluated the potential of this type of waste for valorization through composting and the effects of its application in a vineyard as a total or partial substitute for commercial mineral fertilizers. The results are disparate, as they depend on the quality of the compost and the production techniques used. For example, some authors [15] analyzed the combined effect of two types of pruning (manual and mechanical) in a vineyard with the application of different organic amendments (biochar, manure, municipal solid waste, urban sludge). An increase in yield was observed when mechanical pruning and amendments were used, except in the case of biochar. However, the application of municipal sludge and compost reduced the concentration of soluble solids, anthocyanins and phenolics in grapes and caused an excess of N and P at the annual doses applied. Others [16] compared the application of sewage sludge and MSW compost and found that, although sludge application resulted in higher N availability, which led to higher plant biomass, grape yield and quality were reduced. Regarding the risk of accumulation of heavy metals in vineyard soils after the application of compost from WWTP sludge [17], it was observed that neither the total nor the available concentrations of heavy metals increased in the soil, possibly due to the phenomena of absorption and complexation by humic substances that occur during the composting process and, in addition, the fraction of available metals was less than 1%, a result that was related to the basic pH and the high percentage of clay in the soil.

Despite the extensive literature, there are few studies on the application of organic amendments in vineyards and their effects on the aroma composition of wine. In this regard, some authors [18,19,20] have analyzed the application of compost and vermicompost from wheat-straw spent mushroom substrate, grape pomace compost and sewage sludge, respectively.

Considering all the above, the present study deals with the use of compost obtained from sewage sludge mixed with urban pruning. The aim is to evaluate its effect on agronomic parameters of the vineyard under conditions of soils poor in organic matter and different water regimes. Wine composition and the organoleptic characteristics of the resulting wines will also be analyzed. The hypothesis is that the use of organic amendments will not have a negative impact on wine quality.

2. Materials and Methods

2.1. Composting and Compost

COPERO Environmental Facilities of EMASESA (Seville) supplied the sewage sludge. The composting was carried out following the open trapezoidal pile system (12 m³), consisting of a mixture of sludge and urban pruning in a volumetric ratio of 1:2. The pile was irrigated and mechanically turned at eight specific moments during the process. Under the imposed operating conditions, it was possible to maintain moisture from the material of between 40 and 70% during composting and a bi-oxidative phase of around 53 days (T > 40 °C). Figure 1 shows the evolution of internal and ambient temperatures during the composting period (91 days). It is observed that thermophilic temperatures are reached for more than 50 days (T max 67.9 °C) and thermal conditions (55.0 °C) for more than 15 days, which favors the sanitization of the material, according to EU Regulation 1009/2019 [21].

After composting, the material was sieved through a 1 cm mesh to obtain compost. Table 1 shows the physico-chemical, biological and microbiological characteristics of the compost applied to the vineyard. The organic matter content was determined by calcination at 550 °C in a muffle (HOBERSAL-230, FornsHobersal, Barcelona, Spain). The total organic carbon (TOC) content was determined from the Waskman coefficient (1.72). The pH and electrical conductivity were analyzed in an aqueous extract at a ratio of 1:5 (m/v) using a pH meter (Hanna-5521, Hanna Instruments, Woonsocket, RI, USA) and a conductivity meter (XS-51+, Scharlab S.L., Barcelona, Spain).

The microbiological parameters for E. coli and Salmonella spp. were determined according to ISO 7251 [22] and ISO 6579 [23], respectively. The germination index was analyzed according to the method described by Palenzuela et al. (2023) [18] and the TKN content according to Rosal (2007) [24]. Finally, the analysis of P, K, Cd, Cu, Cr, Ni, Pb, Zn and Hg was carried out using an ICP-MS (Agilent 7800, Agilent Technologies, Santa Clara, CA, USA) after microwave-assisted acid digestion (CEM, Mars One model, CEM Corporation, Matthews, NC, USA).

Table 1. Agronomic characterization of compost.

Agronomic Parameters		Biologic and Microbiologic Parameters		Metal Content **
pH	7.98 ± 0.08	GI (%)	96.4	Cd (mg·kg−1)	0.8
EC (μS·cm−1)	2662 ± 10	Escherichia coli (NMP/g)	<1 (1000) *	Cu (mg·kg−1)	164
OM (%)	36.9 ± 1.4 (≥35%) *	Salmonella spp. (A/P/25 g)	A (Absent) *	Cr (mg·kg−1)	63
C/N	11.3 (<20) *			Ni (mg·kg−1)	19
TKN (%)	2.03 ± 0.15			Pb (mg·kg−1)	61
P2O5 (%)	4.26 ± 0.08			Zn (mg·kg−1)	525
K2O (%)	1.06 ± 0.03			Hg (mg·kg−1)	0.5

Results expressed on dry matter, except %OM (on wet matter) and microbiological parameters. EC, electrical conductivity; OM, organic matter; TKN, total Kjeldahl nitrogen; C/N obtained from TOC and NTK, GI, germination index; A/P, absence/presence. * Requirements for OM and C/N ratio according to RD 506/2013. ** Class C based on metal concentration, according to RD 506/2013 [25].

Table 1. Agronomic characterization of compost.

Agronomic Parameters		Biologic and Microbiologic Parameters		Metal Content **
pH	7.98 ± 0.08	GI (%)	96.4	Cd (mg·kg−1)	0.8
EC (μS·cm−1)	2662 ± 10	Escherichia coli (NMP/g)	<1 (1000) *	Cu (mg·kg−1)	164
OM (%)	36.9 ± 1.4 (≥35%) *	Salmonella spp. (A/P/25 g)	A (Absent) *	Cr (mg·kg−1)	63
C/N	11.3 (<20) *			Ni (mg·kg−1)	19
TKN (%)	2.03 ± 0.15			Pb (mg·kg−1)	61
P2O5 (%)	4.26 ± 0.08			Zn (mg·kg−1)	525
K2O (%)	1.06 ± 0.03			Hg (mg·kg−1)	0.5

Results expressed on dry matter, except %OM (on wet matter) and microbiological parameters. EC, electrical conductivity; OM, organic matter; TKN, total Kjeldahl nitrogen; C/N obtained from TOC and NTK, GI, germination index; A/P, absence/presence. * Requirements for OM and C/N ratio according to RD 506/2013. ** Class C based on metal concentration, according to RD 506/2013 [25].

Compost complied with the parameters required by Spanish legislation [25] for agricultural use in terms of OM, C/N ratio, metal concentration and pathogen content. The germination index was higher than 50%, the minimum limit considered as an indicator of the absence of phytotoxicity in the material [26].

2.2. Study Area and Test Conditions

The experiment was carried out in two plots of V. vinifera cv. Cayetana Blanca in Ribera del Fresno (Badajoz, Spain). One of the vineyards was under rainfed conditions (38°36′46″ N 6°15′53″ W) with a target yield of about 5 t/ha, while the other vineyard (38°39′23″ N 6°16′11″ W) was under deficit irrigation (700 m³/ha) with a target yield of 15 t/ha.

The productive load was 18,500 and 29,600 buds/ha distributed in two-budded spurs in a double Royat cordon in rainfed and irrigated conditions, respectively.

Fertilization of the agricultural soil in the experimental plots was carried out with a chisel plough at a depth of 30 cm.

In the rainfed vineyard, the usual dose of fertilizer and double the dose were applied according to the yields of the previous seasons [27]. That is, the fertilizer was calculated based on 40 and 80 nitrogen fertilizer units (UFN; kg) per ha, in addition to a control without fertilizer. Thus, the trials are control (Tr); compost at 2150 kg/ha (C1r); compost at 4300 kg/ha (C2r); and the usual mineral complex fertilizer 17-9-12 (N-P₂O₅-K₂O) at 230 kg/ha (Q1r) and at 460 kg/ha (Q2r).

In the case of the vineyard under deficit irrigation conditions, the usual doses of fertilizer for a vineyard with these characteristics were applied (80 UFN/ha) [27], in addition to a control. Thus, the trials are a control without fertilization (Tdi); compost at 4300 kg/ha (Cdi); and the usual mineral complex fertilizer 17-9-12 (N-P₂O₅-K₂O) at 460 kg/ha (Qdi).

2.3. Analysis of Agronomic Parameters and Must Composition

The agronomic parameters determined were the yield per plant and the measurement of the exposed leaf area as the sum of the area of the sides, considering the trellis as a parallelepiped [28]. The determination of the general parameters to the must (pH, titratable acidity, YAN) were carried out according to International Organization of Vine and Wine (OIV) official methods [29]. Ammonium was determined using a methodology based on the Kjeldahl method, without prior digestion of the sample, addition of NaOH until pH > 9.5 and distillation, collecting the distillate in sulfuric acid, which is then titrated with NaOH.

2.4. Vinification and Wine Analysis

To determine the effect of these experiments on the wines, the fermentations of each experiment were carried out in duplicate in 15 L stainless steel fermenters at a controlled temperature of 18 ± 1 °C. All the wines were inoculated with Lallemand Oenology’s ICV OKAY^® yeast at a dose of 25 g/hL.

After vinification, the wines were clarified with BENGEL^® bentonite from Agrovin S.A. (Agrovin, S.A., Alcázar de San Juan, Spain) at a dose of 25 g/hL and general analyses (pH, titratable acidity, volatile acidity, ethanol content and color index) were carried out using OIV official methods [29]. In addition, the malic and lactic acid contents were determined by reflectometry using Reflectoquant™ (Merck^®, Darmstadt, Germany).

2.5. Analysis of Volatile Compounds

The volatile compounds present in must and wine can be divided into two groups according to their concentration: major volatile compounds (≥10 mg/L) and minor volatile compounds (<10 mg/L). The analysis was carried out in three biological replicates for each fermentation.

2.5.1. Major Volatile Compounds

Quantification of the main volatile compounds and polyols was carried out using an Agilent Technologies HP 6890 Series II gas chromatograph (Palo Alto, CA, USA) equipped with a CP-WAX 57 CB capillary column (50 m long, 0.25 mm internal diameter and 0.4 µm thick) and a flame ionization detector (FID), according to the protocol described by Peinado et al. [30].

For the analysis, 0.5 µL aliquots of wine samples (10 mL), previously prepared with 1 mL 4-methyl-2-pentanol as internal standard (1024 mg/L) were injected. Prior to injection, tartaric acid was removed by precipitation with 0.2 g calcium carbonate followed by centrifugation at 300× g.

The chromatographic conditions were set as follows: 30:1 separation ratio, FID detector, and a temperature program starting at 50 °C for 15 min, followed by an increase of 4 °C/min to 190 °C, where it was maintained for 35 min. The injector and detector temperatures were 270 and 300 °C, respectively. The carrier gas used was helium, with an initial flow rate of 0.7 mL/min for 16 min, followed by an increase of 0.2 mL/min to a final flow rate of 1.1 mL/min, maintained for 52 min. Identification and quantification of the compounds analyzed were performed by injection of standards under the same conditions as the samples. Additional information on the linear retention index (LRI) used for the identification of volatile compounds is provided in Table S1.

2.5.2. Minor Volatile Compounds

The analysis of these compounds was carried out in two stages, both previously described in detail [31].

In the first stage, an extraction procedure based on the use of stir bars covered with PDMS (with a film thickness of 0.5 mm and a length of 10 mm, Gerstel GmbH, Mülheim an der Ruhr, Germany) was used. These were placed in a vial with 10 mL of 1:10 diluted sample and 0.1 mL of ethyl nonanoate (0.4464 mg/L) as internal standard. After 100 min of shaking at 1500 rpm, the Twistter^® were removed and placed in a desorption tube for subsequent chromatographic analysis.

In the second stage, the volatile compounds were analyzed by gas chromatography coupled to mass spectrometry (GC–MS) using a Gerstel TDS 2 thermal desorption system. The Twistter^® contained in the desorption tubes were exposed to a temperature of 280 °C to release the volatile compounds in a CIS 4 PTV cooling system programmed at 25 °C and containing a Tenax adsorption tube. The CIS was then heated to transfer the volatiles to the GC–MS, which used an Agilent 19091S capillary column (30 m × 0.25 mm internal diameter and 0.25 µm film thickness). The mass detector was operated in scanning mode at 1850 V, analyzing a mass range between 39 and 300 amu.

For identification of volatile compounds, analytical standards were injected under the same chromatographic conditions as the samples and retention times were compared with the Wiley spectral library. Quantification was performed using calibration curves. Additional information on the linear retention index (LRI) for the identification of volatile compounds is provided in Table S1.

2.5.3. Calculation of Aromatic Series

The Odorant Activity Values (OAV) of volatile compounds were calculated from the ratio between their concentration and their olfactory perception threshold.

An aromatic series group’s volatile compounds, with similar odor descriptors and values, is obtained by summing the OAVs of its constituent compounds. Nine aromatic series have been established: chemical, green, citrus, creamy, floral, fruity, green fruit, honey and waxy. The same volatile compound may belong to one or more aroma series, depending on its sensory characteristics (Table S2).

2.6. Organoleptic Characterization

The organoleptic characterization was carried out by a panel of 8 people, 4 men and 4 women, who were experts in wine tasting.

The tasting was carried out according to the AENOR guidelines [32], with 4 characteristics being rated on a scale of 1–10: appearance, smell, flavor and overall perception. In addition, the panel had to select the best rated wine from the irrigated vineyard trials (3 wines) and rank the first, second and third favorites among the 5 rainfed vineyard wines.

The results are expressed as the deviation from the average score of all wines tasted for each irrigation condition.

2.7. Statistical Analysis

ANOVA statistical analysis was carried out separately for each group of trials (rainfed and deficit irrigated) as they are not comparable plots. In addition, when significant differences were found, a post hoc Tukey analysis (p < 0.05) was carried out to separate them into homogeneous groups, with capital letters for the irrigated condition and normal letters for the rainfed condition. A principal component analysis was also carried out.

All the analysis were carried out bin triplicate and the dates were treated using IBM SPSS 25 software (Armonk, New York, NY, USA).

3. Results and Discussion

3.1. Agronomic Parameters and Must Composition

Table 2. Yield and vegetative expression of different vineyard treatments.

	Yield (kg/Plant)	SA 1 (m2/ha)
Tr	2.3 ± 0.8 a	5500 ± 916 ab
C1r	2.3 ± 0.8 a	6148 ± 809 a
C2r	1.6 ± 0.7 b	5741 ± 819 ab
Q1r	1.3 ± 0.5 b	5148 ± 417 b
Q2r	1.4 ± 0.7 b	4870 ± 592 b
Tdi	8.4 ± 0.6	7731 ± 440 a
Cdi	8 ± 1	6690 ± 730 b
Qdi	8 ± 1	7130 ± 454 ab

¹ Surface Area; Tr: control rainfed; C1r: compost at 2150 kg/ha rainfed; C2r: compost at 4300 kg/ha rainfed; Q1r: mineral fertilizer at 230 kg/ha rainfed; Q2r: mineral fertilizer at 460 kg/ha rainfed; Tdi: control deficit irrigated; C2di: compost at 4300 kg/ha deficit irrigated; Q2r: mineral fertilizer at 460 kg/ha deficit irrigated. Different letters indicate significant differences at 95% confidence level.

shows that the trials with chemical fertilizers generally had lower yields than those without fertilizers or those fertilized with compost at lower doses. It should be noted that the 2023 campaign was an extremely dry year between the month of January and the time of budburst (time of compost application), with rainfall of 37 mm instead of the usual 108 mm in the last 10 campaigns [33]. This may have resulted in increased water stress due to increased point salinity when chemical fertilizer was applied close to the roots [34]. In addition, a reduction in the leaf area produced by the plants was observed under both application conditions.

The increase in crop productivity is significant in the case of rainfed vineyards, while no differences were found in the case of irrigated vineyards. The increase is substantial, at 60%, because of a better response of the plant in dry conditions in an extremely dry year, as mentioned above. Similarly, a better plant performance in rainfed conditions was observed in the compost-fertilized or unfertilized plant, due to a greater production of aerial biomass, measured as exposed leaf area (surface area), which may be related to lower water stress of the plant.

As for the oenological parameters of the must (

Table 3. Characterization of the must from the different vineyard treatments.

	pH	Titratable Acidity (g/L TH2)	YAN (mg/L)	NH4 (mg/L)
Tr	3.96 ± 0.02 c	3.7 ± 0.1 a	175 ± 19 c	110 ± 3 b
C1r	3.98 ± 0.02 c	3.54 ± 0.03 a	212 ± 3 b	110 ± 3 b
C2r	4.09 ± 0.01 b	3.15 ± 0.09 b	225 ± 2 b	124 ± 3 a
Q1r	4.23 ± 0.03 a	2.98 ± 0.06 b	263 ± 14 a	112 ± 3 b
Q2r	4.18 ± 0.02 a	3.2 ± 0.2 b	262 ± 6 a	122 ± 3 a
Tdi	4.01 ± 0.02 a	2.9 ± 0.02	175 ± 7	69 ± 3
Cdi	4.01 ± 0.02 a	2.83 ± 0.09	166 ± 11	71 ± 3
Qdi	3.94 ± 0.03 b	2.96 ± 0.04	156 ± 10	65 ± 3

g/L TH2: g/L Tartaric Acid; YAN: Yeast assimilable nitrogen; Tr: control rainfed; C1r: compost at 2150 kg/ha rainfed; C2r: compost at 4300 kg/ha rainfed; Q1r: mineral fertilizer at 230 kg/ha rain-fed; Q2r: mineral fertilizer at 460 kg/ha rainfed; Tdi: control deficit irrigated; C2di: compost at 4300 kg/ha deficit irrigated; Q2r: mineral fertilizer at 460 kg/ha deficit irrigated. Different letters indicate significant differences at 95% confidence level.

), statistically significant differences were obtained only in the case of those from the drying trials. For these, an increase in pH and a decrease in titratable acidity were observed for those obtained after chemical fertilization compared to compost and to the control. The opposite is true for the YAN of the must, where the chemical fertilization trials showed an increase in YAN compared to the compost and the latter compared to the control. The increase in pH and the decrease in acidity are strongly related to fertilization, because the increase in K+ content of the must causes the tartaric acid to salify into tartrate and potassium bitartrate, increasing pH and decreasing acidity [35].

3.2. Vinifications

In the rainfed trials as

Table 4. Oenological variables and color parameters of wine obtained after different fertilization and irrigation regimes of the vineyard.

	pH	Titratable Acidity (g/L TH2)	Ethanol (% v/v)	Volatile Acidity (g/L AcH)	Malic Acid (g/L)	Color Index
Tr	3.41 ± 0.05 b	5.7 ± 0.09 a	12.16 ± 0.05 c	0.43 ± 0.02 a	1.64 ± 0.02 a	0.08 ± 0.01
C1r	3.31 ± 0.02 c	5.4 ± 0.05 b	12.6 ± 0.2 b	0.33 ± 0.03 b	1.4 ± 0.2 ab	0.08 ± 0.01
C2r	3.54 ± 0.02 a	4.81 ± 0.06 c	12.9 ± 0.1 ab	0.25 ± 0.02 c	1.27 ± 0.05 b	0.3 ± 0.4
Q1r	3.47 ± 0.05 ab	5.2 ± 0.04 b	12.9 ± 0.2 ab	0.4 ± 0.02 a	1.48 ± 0.09 ab	0.1 ± 0.01
Q2r	3.49 ± 0.01 a	4.8 ± 0.1 c	13.1 ± 0.2 a	0.44 ± 0.03 a	1.37 ± 0.04 b	0.12 ± 0.01
Tdi	3.31 ± 0.01 a	5.16 ± 0.02 a	11.73 ± 0.06 b	0.31 ± 0.02 a	1.29 ± 0.1 ab	0.06 ± 0.01 a
Cdi	3.33 ± 0.02 a	5.1 ± 0.3 a	11.9 ± 0.1 ab	0.28 ± 0.02 a	1.15 ± 0.07 b	0.06 ± 0.01 a
Qdi	3.32 ± 0.01 a	5.5 ± 0.1 a	12.1 ± 0.2 a	0.4 ± 0.1 a	1.43 ± 0.02 a	0.06 ± 0.02 a

g/L TH2: g/L Tartaric Acid; g/L AcH: g/L Acetic Acid; Tr: control rainfed; C1r: compost at 2150 kg/ha rainfed; C2r: compost at 4300 kg/ha rainfed; Q1r: mineral fertilizer at 230 kg/ha rainfed; Q2r: mineral fertilizer at 460 kg/ha rainfed; Tdi: control deficit irrigated; C2di: compost at 4300 kg/ha deficit irrigated; Q2r: mineral fertilizer at 460 kg/ha deficit irrigated; Different letters indicate significant differences at 95% confidence level.

shows, the pH and titratable acidity of the wines showed a similar behavior to that of the starting musts. The must with the lowest pH and highest acidity were the control wine, followed by both fertilizers, compost and chemical, at lower doses, and finally the highest doses of both fertilizers. This is explained by the correlation between pH and acidity and the concentration of K+ in the must and wine from fertilization [36].

However, ethanol shows the opposite trend, as wines with the highest concentration are those fertilized with high doses, followed by medium doses and finally the control. This may be due to fertilization itself. In this sense, some authors [37] found an increase in sugar content in the must, and therefore ethanol in the wine, when different fertilizers were applied compared to an unfertilized control. This increase could also be due to the yield reduction itself in these trials, indirectly caused by fertilization, in agreement with other studies [38], which found that a lower number of clusters led to lower yields and higher sugar accumulation.

As for the volatile acidity, all the wines showed low values, so that it can be affirmed that there was no microbial contamination during fermentation [39]. Finally, the higher malic acid content in wines from fertilized vineyards is probably due to a higher accumulation of K, although other authors report the opposite dynamics [37]. Lactic acid levels were practically zero (all wines below 0.03 g/L), as malolactic fermentation was inhibited by sulfite [40].

3.3. Volatile Composition

3.3.1. Chemical Families

Analyzing the results by chemical families (

Table 5. Concentration of volatile aroma compounds from rainfed wines.

	Tr	C1r	C2r	Q1r	Q2r
Alcohols
Mayor Alcohols (mg/L)	436 ± 1 b	470 ± 7 a	291.7 ± 0.4 d	372 ± 2 c	389 ± 3 c
Methanol	91 ± 4 ab	101 ± 17 a	25 ± 1 c	26 ± 4 c	71 ± 8 b
Propanol	37 ± 1 b	49 ± 7 a	46.1 ± 0.8 a	34 ± 1 b	42 ± 1 ab
Iso-butanol	25.3 ± 0.5 bc	28 ± 2 b	14 ± 0.2 d	24.1 ± 0.5 c	34.1 ± 0.3 a
2-methylbutanol	33 ± 1 bc	35 ± 2 ab	30.9 ± 0.1 c	38 ± 1 a	34.4 ± 0.6 ab
3-methylbutanol	233 ± 4 a	241 ± 17 a	152 ± 1 c	221 ± 5 a	190 ± 4 b
2-phenylethanol	17 ± 1 c	17 ± 1 c	23.4 ± 0.3 b	29 ± 1 a	17.9 ± 0.9 c
Minor Alcohols (µg/L)	906 ± 96 a	794 ± 26 ab	578 ± 16 bc	482 ± 43 c	567 ± 41 bc
Hexanol	856 ± 83 a	758 ± 26 ab	546 ± 15 bc	436 ± 40 c	538 ± 39 bc
2-ethyl-1-hexanol	49 ± 13 a	36 ± 4 ab	32 ± 2 ab	45 ± 4 ab	29 ± 3 b
Dodecanol	0.41 ± 0.01 a	0.37 ± 0.01 a	0.38 ± 0.06 a	0.37 ± 0.01 a	0.39 ± 0.02 a
Esters
Mayor Esters (mg/L)	127 ± 1 b	132 ± 4 a	79 ± 1 c	57 ± 1 d	72 ± 1 c
Ethyl acetate	50 ± 4 a	37 ± 0.8 b	55 ± 3 a	23 ± 4 c	42 ± 3 b
Ethyl lactate	63 ± 3 b	86 ± 9 a	15.4 ± 0.5 c	16 ± 0.2 c	12.3 ± 0.5 c
Diethyl succinate	14 ± 1 b	9 ± 0.7 c	8.57 ± 0.06 c	18 ± 2 a	17.6 ± 0.9 a
Minor Esters (µg/L)	5755 ± 122 b	5492 ± 134 b	8834 ± 561 a	3045 ± 123 c	2604 ± 178 c
Ethyl iso-butanoate	24 ± 2 c	27 ± 2 c	27 ± 3 c	82.1 ± 0.9 a	75 ± 0.7 b
Ethyl butanoate	140 ± 4 b	149 ± 5 b	216 ± 5 a	140 ± 7 b	114 ± 4 c
Butyl acetate	1.1 ± 0.2 a	0.9 ± 0.1 a	1.17 ± 0.05 a	1.1 ± 0.2 a	1.1 ± 0.1 a
Ethyl 2-methylbutanoate	2 ± 0.2 a	1.07 ± 0.07 b	0.44 ± 0.06 c	1.9 ± 0.2 a	1.3 ± 0.1 b
Ethyl 3-methylbutanoate	4.8 ± 0.4 a	3.4 ± 0.1 b	2.3 ± 0.1 cd	2.6 ± 0.2 c	1.8 ± 0.1 d
Isoamyl acetate	3515 ± 57 b	3395 ± 90 b	5200 ± 517 a	2386 ± 96 c	2006 ± 191 c
Ethyl hexanoate	714 ± 13 b	605 ± 17 c	884 ± 10 a	142 ± 11 d	97 ± 5 e
Hexyl acetate	67 ± 2 a	64 ± 1 a	54.9 ± 0.5 b	2.1 ± 0.2 c	2.3 ± 0.2 c
Ethyl heptanoate	1.47 ± 0.04 a	0.29 ± 0.01 b	0.12 ± 0.01 c	0.04 ± 0 d	0.05 ± 0 d
Z-3-hexenylbutyrate	86 ± 3 b	66 ± 2 c	129 ± 5 a	4.2 ± 0.5 d	4.8 ± 0.5 d
Ethyl octanoate	612 ± 43 b	471 ± 11 c	946 ± 39 a	89 ± 1 d	93 ± 2 d
2-phenylethyl acetate	483 ± 91 a	577 ± 18 a	567 ± 15 a	161 ± 17 b	120 ± 8 b
Ethyl do-decanaote	139 ± 21 b	116 ± 4 b	778 ± 14 a	23.2 ± 0.6 d	76 ± 1 c
Phenethyl hexanoate	0.22 ± 0.02 ab	N.D.	0.23 ± 0.01 a	0.2 ± 0.01 b	N.D.
Ethyl tetra-decanoate	5 ± 2 b	5.6 ± 0.4 b	12.3 ± 0.8 a	3.9 ± 0.2 b	3.8 ± 0.5 b
Phenethyl benzoate	0.56 ± 0.05 b	0.75 ± 0.02 a	0.74 ± 0.03 a	0.72 ± 0.01 a	0.73 ± 0.04 a
Ethyl hexa-decanoate	6 ± 4 c	11 ± 1 b	15.7 ± 0.3 a	5.2 ± 0.5 c	5.4 ± 0.6 c
Aldehydes
Mayor aldehydes (mg/L)	108 ± 5 a	64 ± 4 c	79 ± 4 b	93 ± 7 b	122 ± 6 a
Acetaldehyde	108 ± 5 a	64 ± 4 c	79 ± 4 b	93 ± 7 b	122 ± 6 a
Minor aldehydes (µg/L)	34 ± 3 a	16 ± 2 c	23.7 ± 0.9 b	20.1 ± 0.4 bc	18 ± 1 c
Benzaldehyde	4.3 ± 0.1 a	1.7 ± 0.2 d	4.1 ± 0.3 a	2.2 ± 0.2 c	3.4 ± 0.2 b
Hexanal	4.6 ± 0.3 b	6.8 ± 0.8 a	7.3 ± 0.8 a	6.8 ± 0.3 a	6.6 ± 0.4 a
Nonanal	0.1 ± 0.02 c	0.6 ± 0.1 b	1.3 ± 0.2 a	0.14 ± 0.02 c	1.1 ± 0.1 a
Phenylacetaldehyde	25 ± 3 a	7 ± 0.9 c	9.9 ± 0.7 bc	10.8 ± 0.3 b	6.5 ± 0.5 c
(E,E)-2,4-Decadienal	0.21 ± 0.03 a	0.17 ± 0.02 ab	0.18 ± 0.02 ab	0.16 ± 0.02 b	0.15 ± 0.01 b
Ketones
Mayor ketones (mg/L)	48 ± 6 a	42 ± 4 ab	32.8 ± 0.4 b	43 ± 6 ab	46 ± 4 a
Acetoin	48 ± 6 a	42 ± 4 ab	32.8 ± 0.4 b	43 ± 6 ab	46 ± 4 a
Minor ketones (µg/L)	0.09 ± 0.02 cd	0.2 ± 0.03 b	0.15 ± 0.03 bc	0.51 ± 0.06 a	0.06 ± 0.01 d
3-Heptanone	0.09 ± 0.02 cd	0.2 ± 0.03 b	0.15 ± 0.03 bc	0.51 ± 0.06 a	0.06 ± 0.01 d
Lactones (µg/L)	3.02 ± 0.05 c	7.7 ± 0.9 b	6 ± 0.6 b	18 ± 2 a	7.4 ± 0.5 b
γ-Nonalactone	3.02 ± 0.05 c	7.7 ± 0.9 b	6 ± 0.6 b	18 ± 2 a	7.4 ± 0.5 b
Terpenoids (µg/L)	22 ± 4 b	37 ± 2 a	35 ± 2 a	6 ± 0.2 c	7.5 ± 0.5 c
E-Nerolidol	18 ± 4 b	35 ± 2 a	31 ± 2 a	3.7 ± 0.2 c	5.3 ± 0.5 c
Z-Geranyl acetone	1.5 ± 0.1 a	1.53 ± 0.09 a	1.39 ± 0.03 a	1.5 ± 0.1 a	1.44 ± 0.08 a
E-Methyl-dihydro-jasmonate	1.87 ± 0.08 a	0.97 ± 0.04 b	2 ± 0.5 a	0.76 ± 0.05 b	0.7 ± 0.1 b

N.D.: Not detected; Tr: control rainfed; C1r: compost at 2150 kg/ha rainfed; C2r: compost at 4300 kg/ha rainfed; Q1r: mineral fertilizer at 230 kg/ha rainfed; Q2r: mineral fertilizer at 460 kg/ha rainfed. Different letters indicate significant differences at 95% confidence level.

and

Table 6. Concentration of volatile aroma compounds from deficit irrigation wines.

	Tdi	C2di	Q2di
Alcohols
Mayor Alcohols (mg/L)	480 ± 6 a	455 ± 4 b	443 ± 2 c
Methanol	79 ± 14 a	83 ± 2 a	73 ± 4 a
Propanol	42 ± 3 a	40 ± 1 a	37 ± 2 a
Iso-butanol	28 ± 1 a	24 ± 2 b	25.92 ± 0.02 ab
2-methylbutanol	42.3 ± 0.9 a	36 ± 2 b	44 ± 2 a
3-methylbutanol	265 ± 14 a	253 ± 12 a	241 ± 7 a
2-phenylethanol	23 ± 2 a	19 ± 1 b	22 ± 2 ab
Minor Alcohols	482 ± 36	441 ± 34	432 ± 22
Hexanol	441 ± 39	397 ± 31	393 ± 19
2-ethyl-1-hexanol	40 ± 4 a	37 ± 4 a	37 ± 3 a
Octanol	n.d.	6.1 ± 0.2 a	n.d.
Farnesol	0.62 ± 0.04 c	1.15 ± 0.04 a	1.07 ± 0.01 c
Esters
Mayor Esters (mg/L)	128 ± 2 a	134 ± 2 a	107 ± 1 b
Ethyl acetate	46 ± 6 a	37.7 ± 0.7 a	39 ± 3 a
Ethyl lactate	69 ± 4 b	90 ± 4 a	61 ± 4 b
Diethyl succinate	14 ± 1 a	6.8 ± 0.3 b	6.8 ± 0.4 b
Minor Esters (µg/L)	4289 ± 232 b	5701 ± 215 a	4143 ± 242 b
Ethyl iso-butanoate	28 ± 2 a	23.4 ± 0.9 b	27 ± 2 ab
Ethyl butanoate	153 ± 9 a	157 ± 14 a	122 ± 8 b
Butyl acetate	0.9 ± 0.1 b	1.3 ± 0.2 a	1 ± 0.1 ab
Ethyl 2-methylbutanoate	1.3 ± 0.1 a	1.1 ± 0.2 a	1.3 ± 0.1 a
Ethyl 3-methylbutanoate	3.5 ± 0.4 a	2.8 ± 0.2 b	3.4 ± 0.1 ab
Isoamyl acetate	2176 ± 151 b	2957 ± 170 a	2152 ± 233 b
Ethyl hexanoate	603 ± 46 b	792 ± 5 a	482 ± 20 c
Hexyl acetate	22.2 ± 0.5 b	44 ± 2 a	20.6 ± 0.3 b
Ethyl heptanoate	0.23 ± 0.01 b	0.2 ± 0.01 b	0.35 ± 0.02 a
Z-3-hexenylbutyrate	75 ± 2 b	84 ± 1 a	72 ± 2 b
Ethyl octanoate	543 ± 25 b	617 ± 11 a	521 ± 11 b
2-phenylethyl acetate	437 ± 7 b	682 ± 33 a	489 ± 27 b
Ethyl do-decanaote	234 ± 7 b	329 ± 8 a	243 ± 4 b
Phenethyl hexanoate	0.24 ± 0.01 b	0.26 ± 0.01 a	0.23 ± 0.01 b
Ethyl tetra-decanoate	4.4 ± 0.1 a	4.7 ± 0.2 a	4.3 ± 0.1 a
Phenethyl benzoate	0.71 ± 0.01 a	0.74 ± 0.03 a	0.71 ± 0.02 a
Ethyl hexa-decanoate	5.3 ± 0.4 a	5.1 ± 0.4 a	4.7 ± 0.2 a
Aldehydes
Mayor aldehydes (mg/L)	104 ± 8 a	35 ± 1 b	48 ± 4 b
Acetaldehyde	104 ± 8 a	35 ± 1 b	48 ± 4 b
Minor aldehydes (µg/L)	20 ± 2 a	16.8 ± 0.6 b	20 ± 0.5 a
Benzaldehyde	1.19 ± 0.08 c	1.8 ± 0.3 b	2.6 ± 0.2 a
Hexanal	6.8 ± 0.4 a	5.4 ± 0.4 b	6.3 ± 0.4 ab
Heptanal	n.d.	0.37 ± 0.03 b	0.76 ± 0.08 a
Nonanal	1.3 ± 0.2 a	1.36 ± 0.09 a	0.74 ± 0.07 b
Phenylacetaldehyde	10 ± 2 a	7.3 ± 0.4 a	9 ± 0.5 a
4-methylbenzaldehyde	0.5 ± 0.07 b	0.52 ± 0.08 b	0.7 ± 0.1 a
Ketones
Mayor ketones (mg/L)	61 ± 5 a	23 ± 2 b	23 ± 2 b
Acetoin	61 ± 5 a	23 ± 2 b	23 ± 2 b
Minor ketones (µg/L)	2.9 ± 0.4 b	5.3 ± 0.4 a	2.6 ± 0.3 b
Benzophenone	0.32 ± 0.03	0.37 ± 0.02	0.36 ± 0.03
3-Heptanone	0.31 ± 0.03 b	2.6 ± 0.1 a	0.2 ± 0.1 b
Acetophenone	2.3 ± 0.4 a	2.3 ± 0.3 a	2.1 ± 0.2 a
Lactones (µg/L)	8.8 ± 0.2 b	13 ± 1 a	8 ± 1 b
γ-Nona-lactone	7.7 ± 0.4 ab	8.5 ± 0.9 a	6.2 ± 0.7 b
γ-Deca-lactone	1.1 ± 0.2 b	4.3 ± 0.4 a	1.4 ± 0.3 b
Terpenoids (µg/L)	46 ± 2 a	22 ± 1 b	25.8 ± 0.8 b
Limonene	17 ± 2 a	2.4 ± 0.4 b	14 ± 1 a
E-Nerolidol	27.2 ± 0.7 a	17 ± 1 b	9.9 ± 0.2 c
Z-Geranyl acetone	1.34 ± 0.03 a	1.35 ± 0.02 a	1.34 ± 0.02 a
E-Methyl-dihydro-jasmonate	0.71 ± 0.06 a	0.7 ± 0.1 a	0.11 ± 0.01 b

n.d.: not detected; Tdi: control deficit irrigated; C2di: compost at 4300 kg/ha deficit irrigated; Q2di: mineral fertilizer at 460 kg/ha deficit irrigated. Different letters indicate significant differences at 95% confidence level.

), the higher alcohol contents showed higher values in the trials without fertilization and/or with compost fertilization. These compounds are synthesized in the amino acid assimilation pathway by the Elrich pathway [41] and mainly by decarboxylation and reduction of keto acids from sugar metabolism. Therefore, in wines made from musts with lower NH₄ levels, the synthesis of amino acids from keto acids is not possible and an accumulation of keto acids occurs [42]. This statement coincides with the other studies [43], which found that the addition of different ammonium sources to the must at different times of fermentation caused a general decrease in the synthesis of higher alcohols, although this is also influenced by the yeast strain used.

In the case of esters, both majority and minority, significantly higher values were observed in wines from vines fertilized with compost and/or the control compared to those fertilized with chemical fertilizers. The most abundant esters in the case of minority aromas were isoamyl and 2-phenylethanol acetates, with fruity (banana) and floral aromas, respectively, and ethyl hexanoate and octanoate, with fruity aromas [44].

Control wines generally stand out from the rest in the case of aldehydes, with the most prominent minority aldehyde being phenylacetaldehyde, with green and honey aromas [42].

In the lactone family, the concentration observed in fertilized wines is higher than that of control, with γ-nonalactone being the most abundant. This compound contributes coconut and fruit (peach) aromas [42]. γ-nonalactone is synthesized by S. cerevisiae during alcoholic fermentation from 4-oxononanoic acid, derived from the oxidation of linoleic acid, and is associated with more mature grape harvests [45], which may be due to fertilization, since the ethanol content of wines from fertilized vines is higher, a sign of the maturity of the grape.

Different behaviors were observed in rainfed and irrigated vineyards when terpenes and norisoprenoids were analyzed. In the former, compost fertilization stimulated a higher concentration of these compounds whereas, in the irrigated vineyards, the highest concentration occurred in the control vineyards. The most important compounds in this group were E-nerolidol in rainfed vineyards and E-nerolidol and limonene in irrigated vineyards. E-nerolidol is a sesquiterpene that can be formed in two ways. On the one hand, it is formed in the leaves and berries during grape ripening, especially when these leaves have a high UV-B radiation [46], which would explain why wines from plants with more leaves (Table 1, SA) have a higher concentration of this compound. On the other hand, it can be formed by S. cerevisiae during alcoholic fermentation from the amino acid leucine, favored by a medium low in YAN and a low pH [47], which would also explain why compost and control (lower amounts of YAN) have higher amounts of this compound.

3.3.2. Aromatic Series

Volatile aroma compounds were grouped in aromatic series as described in the Materials and Methods section. Many authors have used this procedure as a way of reducing the number of variables and for a simpler and more intuitive understanding of the results [18,48,49,50,51,52]. Nine aromatic series have been obtained: fruity, green fruit, green, creamy, citrus, chemistry, honey, waxy and floral.

Regarding the fruity and green fruit series, in both rainfed (

Table 7. Values of the aromatic series in wines from rainfed vines.

	Tr	C1r	C2r	Q1r	Q2r
Fruity	25.3 ± 0.6 b	20.3 ± 0.3 c	36 ± 1 a	12.5 ± 0.4 d	11.4 ± 0.1 d
Green fruit	11.9 ± 0.1 b	9.1 ± 0.1 c	13.7 ± 0.3 a	2.2 ± 0.2 d	1.62 ± 0.03 e
Green	7.3 ± 0.7 a	3.3 ± 0.4 c	4.4 ± 0.2 b	4.2 ± 0.1 bc	3.2 ± 0.2 c
Creamy	0.84 ± 0.03 b	1.11 ± 0.08 a	0.52 ± 0.02 a	1 ± 0.05 c	0.64 ± 0.05 c
Citrus	0.05 ± 0.01 c	0.24 ± 0.06 b	0.5 ± 0.1 a	0.06 ± 0.01 c	0.45 ± 0.04 a
Chemistry	19.1 ± 0.8 a	17 ± 1 ab	15.1 ± 0.5 cd	13.7 ± 0.7 d	16.5 ± 0.4 bc
Honey	8.2 ± 0.3 a	4 ± 0.2 c	4.8 ± 0.1 b	3.3 ± 0.1 d	2.1 ± 0.1 e
Waxy	13 ± 1 b	10 ± 0.2 c	22.8 ± 0.8 a	1.9 ± 0.02 d	2.24 ± 0.04 d
Floral	3.7 ± 0.3 bc	4.1 ± 0.1 b	4.7 ± 0.1 a	3.54 ± 0.05 c	2.3 ± 0.1 d

Tr: control; C1r: compost at 2150 kg/ha; C2r: compost at 4300 kg/ha; Q1r: mineral fertilizer at 230 kg/ha; Q2r: mineral fertilizer at 460 kg/ha. Different letters indicate significant differences at 95% confidence level.

) and irrigated (

Table 8. Values of the aromatic series in wines from deficit irrigated vines.

	Tdi	C2di	Q2di
Fruity	22 ± 1 b	24.9 ± 0.3 a	20.4 ± 0.6 b
Green fruit	9.4 ± 0.5 a	11 ± 0.1 b	8.4 ± 0.3 c
Green	4 ± 0.5 a	3.4 ± 0.1 a	3.8 ± 0.1 a
Creamy	1.15 ± 0.04 a	1.14 ± 0.05 a	0.8 ± 0.04 b
Citrus	2.2 ± 0.2 a	0.8 ± 0.1 c	1.7 ± 0.1 b
Chemistry	21.3 ± 0.6 a	17.6 ± 0.4 b	18.7 ± 0.7 b
Honey	4.2 ± 0.4 a	4.5 ± 0.1 a	4.2 ± 0.2 a
Waxy	12 ± 0.5 b	14 ± 0.2 a	11.6 ± 0.2 b
Floral	4.2 ± 0.1 b	4.8 ± 0.1 a	4.3 ± 0.2 b

Tdi: control; C2di: compost at 4300 kg/ha; Q2di: mineral fertilizer at 460 kg/ha; Different letters indicate significant differences at 95% confidence level.

) vineyards, it was observed that wine obtained after compost fertilization had higher values than the controls. Similarly, those that received conventional fertilization showed lower values in these series compared to the control and compost. These series mainly contribute esters, highlighting the OAV of ethyl butanoate, ethyl hexanoate, ethyl octanoate and ethyl acetate. Possibly, the more complex and balanced fertilization of the plant provided by the compost may promote alternative metabolic pathways in the grapes, leading to more complex musts and greater synthesis of these compounds by the yeasts.

In the case of the green series, where green and herbaceous aromas predominate, the unfertilized trials showed the highest values when rainfed wines were compared (Table 7). This may be due to insufficient grape ripening, where compounds such as hexanol, hexanal, formed from fatty acids from the skin cell walls extracted during pressing [36], or phenylacetaldehyde stand out. On the other hand, in deficit irrigated wines (Table 8), no differences are observed. A similar behavior is observed in the chemical series, mainly influenced by the OAV of 3-methylbutanol and 2-methylbutanol. In both experiences (rainfed and deficit irrigated), the control trials stand out from the fertilized experimental wines, especially in the case of irrigation.

Finally, an important series is the floral. In all cases, the wines from the compost-fertilized trials stand out from the rest. In this series, the compounds that contribute most are 2-phenylethanol, a higher alcohol formed from the amino acid phenylalanine, and the α-ketoacid phenylpyruvate [42]. Therefore, very high levels of YAN lead to reduced synthesis, which would explain why wines from musts with lower YAN have higher concentrations of this compound.

Considering the overall value of the aromatic series for a given wine, vines fertilized with compost at the higher dose in rainfed trials showed the highest value, whereas wines obtained after chemical fertilization show the lowest. As was observed in other parameters, the differences are attenuated in wines from irrigated vines although, again, wines from vines chemically fertilized show the lowest values.

Apart from the compounds described above, Z-3-hexenylbutyrate, 2-phneylethyl acetate, isoamyl acetate, ethyl iso-butanoate, 3-methyl butanoate, ethyl decanoate and (E,E)-2,4-decadienal show OAV higher than 1, so it can be assumed that these volatiles contribute significantly to wine aroma [18,53]. Summarizing, sixteen aroma compounds represent, on average, 73% of the sum of all the aromatic series for a given wine.

3.4. Relationship Between ∑VAO and Fertilisation

In rainfed wines, a clear decrease in the total content of aromatic compounds can be observed with increasing YAN concentration in the initial must (Figure 2a). The same occurs with the sums of the aromatic series (Figure 2b), which could be similar to the aromatic intensity of the wines obtained [54] These relationships show a decrease in the concentration of aromas and aromatic intensity with increasing YAN in the starting musts, provided that the concentrations are sufficient to carry out fermentation (>150 mg/L) [49]. In this sense, it has been reported [55] that, the higher the concentration or supplementation of nitrogen in the must, the lower the concentration of higher alcohols in the wine, without finding a clear trend in the case of esters, which is consistent with the results of this work.

3.5. Principal Component Analysis

A Principal Component Analysis (PCA) was performed to identify which variables contribute the most to distinguishing the treatments. This statistical method is used to reduce the dimensionality of a dataset by converting the original variables into new, uncorrelated variables. Ideally, a low number of components should explain most of the data’s variability. The analyst must then associate the chosen components with one of the sources of variation. Here, aromatic series and the values of assimilable nitrogen and ammonium were chosen as classifying variables.

Figure 3a shows the results of the PCA in wines from rainfed vines. Two components were obtained that explain 79.4% of the variability. The first clearly differentiates wines from unfertilized and fertilized vines with compost from the wines obtained after chemical fertilization. This component is mainly influenced by the fruity, green fruit and honey series and by the assimilable nitrogen. The second principal component seems to differentiate the dose of fertilizer. The higher dose is located in the positive values of the component and the lower in the negative values. The series that contribute the most to this differentiation is citrus and creamy, which also contribute the values of ammonium.

Regarding the wines from the deficit irrigated vineyard (Figure 3b), two principal components explain 76.3% of the variability. The first (55.05%) differentiates the wine from vines fertilized with compost from the others. The second component differentiates between unfertilized and fertilizer wines. The first component is mainly influenced by the fruity, green fruit, waxy and citrus series, whereas the second seems to depend on chemistry and creamy series and on the ammonium and assimilable nitrogen concentration.

Analyzing the variables that influence the most to discrimination among wines, both analyses always highlight the fruity, green fruit creamy and citrus series and the values related with nitrogen accumulation in the must.

3.6. Organoleptic Analysis

The organoleptic analysis was carried out to determine various parameters, such as the appearance, aroma and taste of the wines, as well as the overall impression.

The results are in line with what was observed from an analytical point of view, i.e., that the wines from the control and compost trials have a better aroma than the fertilized wines, especially in the case of the rainfed wines (Figure 4a,b). In the visual phase, the control and chemical fertilized wines stood out positively, since the wines fertilized with compost showed a very slight turbidity, despite having undergone the same clarification and filtering in all cases. In the tasting phase, the control and organic fertilized wines were rated higher due to a better acidity than the chemical fertilized wines, which were flatter.

4. Conclusions

The fertilization with compost increased crop yield by 60% in rainfed vines, and above-ground biomass by 15%, compared with mineral fertilizer. Compost also increases the assimilable nitrogen and sugar contents of the must compared to the control. These differences are attenuated when the trials were conducted in deficit irrigated vines. In addition, there was a clear negative correlation between the assimilable nitrogen of the must and the concentration of aroma compounds in wines from the rainfed trials.

By grouping, volatile compounds according to their aroma descriptor have been obtained in nine aromatic series. Fruity, green fruit, waxy and floral series showed higher values in wines from vines fertilized with compost in both rainfed and irrigated trials compared to control and chemical fertilization.

The sum of the aromatic series for a given wine provided evidence that wines from vines treated with the highest dose of compost in rainfed trials exhibit the highest values, while those produced after chemical fertilization have the lowest. The differences among wines become less pronounced in wines from irrigated vines, although wines from chemically fertilized vines display the lowest values.

Among the forty-eight volatiles analyzed, fourteen, i.e., ethyl butanoate, ethyl iso-butanoate, ethyl 3-methylbutanoate, ethyl hexanoate, ethyl octanoate, ethyl decanoate, ethyl acetate, isoamyl acetate, 2-phenylethyl acetate, Z-3-hexenylbutyrate, isoamyl alcohols, 2-phenylethanol, phenylacetaldehyde, and (E;E)-2,4-decadienal, contribute more than 70% to the overall aroma of the wine.

Organoleptically, wines from unfertilized vines were preferred, followed by those fertilized with compost, with scores like, and much higher than, those from vines treated with chemical fertilizers. However, all the differences were more pronounced in the rainfed trials than in the irrigated trials.

Summarizing, the use of organic amendment is a suitable alternative to traditional fertilization without affecting the analytical and organoleptic quality of the wine. However, to confirm these results, multi-year studies are needed, and those are already being carried out on the same study plots.