Can Biochar Alleviate Machinery-Induced Soil Compaction? A Field Study in a Tuscan Vineyard

Abstract

Soil compaction from mechanized agriculture is a major concern, as frequent machinery use degrades soil structure, reduces porosity, and ultimately impairs crop productivity. Among potential mitigation strategies to enhance soil resilience to machinery-induced compaction, biochar has shown promise in laboratory settings but remains untested under real field conditions. To address this, we monitored soil in a Tuscan vineyard where biochar was applied at 16 and 32 Mg ha⁻¹, compared to control, on both flat and sloped plots. Soil compaction was induced by 20 passes of a wheeled orchard tractor. Soil bulk density (BD) was measured before, immediately after, and one year following the initial passes, during which 19 additional machine passes occurred as part of the vineyard’s routine agronomic management. Initial results showed a significant BD increase (up to 12.8%) across all treatments, though biochar significantly limited soil compaction, regardless of the applied dose. After one year, in which the soil underwent further compaction, BD further increased across all treatments (up to 20.2%), with the steepest increase observed on the sloped terrain. At this stage, the mitigating effect of biochar on soil compaction was no longer evident. Our findings suggest that biochar may offer some short-term relief from compaction, but further investigations are needed to clarify its long-term effectiveness under field conditions.

1. Introduction

Machine traffic due to agricultural activities certainly leads to soil degradation by causing soil compaction and/or displacement [1]. Soil compaction is generally considered one of the most serious soil degradation problems and a major determinant of crop productivity worldwide [2,3]. Understanding how to manage and mitigate soil compaction is essential to address future global food security challenges.

Compacted soil typically has a degraded structure with low porosity and oxygen content, high penetration resistance, low hydraulic conductivity, and reduced drainage, which increases risks of runoff and flooding [1,4,5,6]. Wheeled machine traffic can also cause rutting formation and soil displacement, resulting in surface deformation, puddling, and localized waterlogging [7]. All these physical changes can profoundly affect the microbial community structure, favoring certain groups over others and ultimately shifting the overall microbial composition [8,9]. Ultimately, soil compaction impairs key soil ecological functions, such as water retention and movement, gas exchange, heat regulation, and nutrient cycling, with negative effects that can last for decades [10].

Soil compaction is therefore a problem in modern agriculture due to the use of increasingly large and heavy field equipment [1,2,11]. When agricultural machinery is used during different types of processing such as seeding, spraying, and harvesting, the soil is stressed beyond its resistance, and soil compaction increases, causing compression and sinking of the surface layer [12]. Moderate compaction of the topsoil can also be beneficial for some soil types such as sandy soil [13], but it damages the soil in most cases. It is well known that excessive compaction negatively impacts physical, chemical, and biological processes in the soil [9] such as reducing water infiltration, root growth, water and nutrient uptake, and crop production [14]. Compaction also increases production costs (e.g., fuel use), exacerbating environmental concerns, such as increased greenhouse gas emissions [2]. Therefore, preventing soil compaction is more cost-effective than mitigating its effects.

The widespread use of machinery in a broad range of agricultural activities has therefore attracted the attention of both researchers and manufacturers. Mitigation strategies such as smoother transmission (hydrostatic) [15], low-pressure tires [16], and scheduling of soil accessibility based on the state of soil moisture content [17,18], are among the tools available to reduce the machine-induced impact on the growing substrate. Nevertheless, millions of hectares of arable land worldwide are now degraded due to soil compaction, especially in intensive agricultural production areas, caused by excessive use of agricultural machinery [19,20,21]. For example, agronomic practices for pest and disease control in vineyards dedicated to wine production in Mediterranean environments often require repeated tractor passes along the same tracks [22]. The number of treatments per year depends heavily on environmental conditions and the phenological stage of the grapevines [23] and can reach dozens of passes annually [24], leading to considerable soil compaction. In addition to the previously mentioned negative effects on soil properties, compaction in sloping vineyards can significantly exacerbate soil erosion [25]. This not only reduces productivity [26] but also contributes to the further degradation of soils that are already fragile and highly susceptible to disturbance [27].

The addition of soil amendments is considered one of the best practices in soil management and prevention of soil compaction as it can improve infiltration, water drainage, aeration and, most importantly, soil pore structure [28,29,30,31]. Biochar, a carbon-rich byproduct derived from biomass pyrolysis or gasification, is increasingly recognized as a promising soil amendment due to its environmental benefits, including carbon sequestration, resistance to microbial breakdown, the capacity to trap environmental contaminants (e.g., anti-biotics, heavy metals), and many other beneficial aspects, such as enhancing soil microbiota biomass and activity [32,33,34,35,36,37,38]. In vineyard soils, typically characterized by low soil fertility, organic matter content, and water holding capacity, biochar may help alleviate some of the soil health deficiencies [39,40,41,42,43] that challenge grape production [39,40,41].

The addition of biochar can improve soil physical characteristics [44,45] and could be a potential strategy to manage soil compaction. Indeed, biochar bulk density (<0.6 Mg m⁻³) is substantially lower than the average density of mineral arable soils, which typically ranges from <1.25 to >1.65 Mg m⁻³, depending mainly on texture and organic carbon content [46]. Consequently, mixing biochar with soil can reduce the overall density of the bulk soil through a dilution effect [44]. Moreover, in the long term, biochar interacts with soil particles to form aggregates and rebuild soil structure in degraded soils, potentially improving the overall resilience of soil against compaction forces [47].

Several studies have discussed the effects of biochar on soil physical properties, indicating that biochar application reduces soil bulk density, one of the main indicators of soil compaction [48,49,50]. However, these studies are not specifically focused on soil compaction. Other studies evaluated the effect of biochar addition on soil compactability as determined using the Proctor test, a common lab test used in geotechnical engineering to determine the moisture content at which a given soil type will achieve its maximum density when compacted [51,52,53]. All these studies found a significant reduction of Proctor maximum bulk density (ranging from 1% to 17%) in soils amended with different amounts of biochar (from 12.5 to 325 Mg ha⁻¹). However, to our knowledge, no study has examined real open-field cases to evaluate the effect of biochar addition against soil compaction induced by agricultural machinery.

In this general framework, a field experiment was set up in a vineyard in Tuscany, Central Italy, to evaluate the effect of biochar application on reducing soil compaction, hypothesizing that biochar would reduce soil compactability. Specifically, we investigated (i) the immediate effect of biochar on soil bulk density (BD) following repeated tractor passes and (ii) the residual effect of biochar on compaction resistance one year later, as the same soil plots were subjected to additional tractor trafficking as part of routine vineyard management. The field experiment was conducted on plots with two slope conditions and three treatments: no biochar and biochar applied at two different doses (16 and 32 Mg ha⁻¹).

2. Materials and Methods

2.1. Experimental Site

The study was carried out in the vineyard “Felsina Estate” (www.felsina.it, accessed on 15 July 2025) located in Castelnuovo Berardenga (Tuscany, Central Italy, 43°21′57′′ N 11°29′06′′ E, 345 m a.s.l.).

The climate of the study area is typically Mediterranean based on the meteorological parameters collected hourly by an automatic weather station installed close to the experimental field. During 2023 and 2024, the years of the field measurements campaign, the total rainfall was 808 mm and 789 mm, and the mean air temperature was 15.7 °C and 15.9 °C, respectively. Seasonal rainfall distribution was 27%, 32%, 13%, and 28% for winter, spring, summer, and autumn, respectively. The morphology of the terrain is characterized by rounded tops and linear slopes, ranging from moderately to strongly sloping, and is subject to moderate to strong and widespread superficial water erosion, primarily of a channeled type. The soils, which developed on marine Pliocene sands, are classified according to their position along the slopes. On more stable, rounded tops, they are identified as Typic Haplustepts (coarse loamy, mixed, mesic) with an Ap–Bw–C profile. When occurring along the slopes more prone to erosion processes, the soils are classified as shallower Typic Ustorthents (coarse loamy, mixed calcareous, mesic) (USDA, 2022) with an Ap–Bw–Cr profile. In both cases, soils are moderately gravelly, with a loam to sandy loam texture, moderately calcareous to very calcareous, from weakly to moderately alkaline, and well drained.

The vineyard was planted in 2006 (cv. Sangiovese), and the trellis system is a single curtain with a plant and row spacing of 0.8 m and 2.3 m, respectively. The soil agronomic management strategy adopted by the firm is keeping a cover crop only every second inter-row using barley (Hordeum vulgare L.) or horse bean (Vicia faba L.) alternatively and making the other inter-rows free from tillage and available for tractor trafficking. The crop covered (tilled) and non-tilled (bare) inter-rows change every year. Tractor trafficking is then evenly distributed along all the inter-rows in a turnover of two years to allow 1 year of resting for the cover cropped inter-rows.

2.2. Experimental Design and Soil Management

The soil properties of the experimental vineyard before biochar addition are described in detail in

Table 1. Soil characteristics before biochar addition.

	Unit	Value
Sand a (2–0.05 mm)	g kg−1	606
Silt (0.05–0.002 mm)	g kg−1	242
Clay (<0.002 mm)	g kg−1	152
Coarse fragments	g kg−1	360
BD bulk soil	Mg m−3	1.39
Total carbon b	g kg−1	6.56 ± 0.22
Total organic carbon (TOC) c	g kg−1	3.00 ± 0.23
AWC d Ksat e	mm m−1 mm h−1	58.08 93.99 ± 42.14
pH f		7.4

^a It refers to the fine-texture fraction (<2 mm). ^b Total carbon content was determined using a CHN auto-analyzer (CHN 1500, Carlo Erba). ^c Total organic carbon (TOC) was determined using the Walkley and Black method. ^d Available water capacity (AWC) was determined using a pressure plate apparatus [54] and calculated as the difference between water content at field capacity (pressure −0.33 MPa) and wilting point (−15 MPa). ^e Saturated hydraulic conductivity (Ksat) using a Guelph permeameter (Soil Moisture Equipment Corp. 2012). ^f The pH was measured in a 1:2.5 (mass/vol) soil solution.

. A plot experiment with three treatments and two slope conditions was started in April 2023, applying biochar to soil (Figures S1 and S2). Each plot, six in total, had a surface area of 60 m² (5 m in width and 12 m in length), including one vineyard inter-row and two rows. Treatments were a single application of biochar at a rate of 16 Mg ha⁻¹ (B), double applications of biochar at a rate of 32 Mg ha⁻¹ (BB), and untreated control plots (CRL). The average length of rows was 196 m. Biochar was applied in the inter-row space of the vineyard using a spreader and incorporated into the soil to a depth of 0.3 m using a chisel plow. Concurrently, control rows (without biochar) were also tilled with the same machine. The slope rate changed along each row. The plots have been drawn in two distinct and homogeneous slope gradients: “slope terrain” around 15% and “flat terrain” around 3% slope gradients. The two plots on the same row were separated by 50 m.

The first soil BD sample campaign was conducted in June 2023 (Figure S3). The compaction took place immediately after the first soil sampling using a wheeled tractor vehicle, conducted at an average speed of 6 km h⁻¹ for a total of twenty passages (10 uphill and 10 downhill) (Figure S4). We considered the induced trafficking to cause a “severe compaction” event because 20 passages corresponded to 70% of the yearly recurrence (passages) for ordinary agronomical management, on average.

In the time period from November 2023 to July 2024, before the second field campaign (Figure S5), a wheeled tractor was used for the following operations: spray application (15 passages), grape vine suckering (2 passages), grapevine pruning (1 passage), and shredding (1 passage). No soil tillage took place in the inter-rows between the first and second field campaign. The vehicles used by the farm for agronomical management are small double traction wheeled tractors that are used for ordinary orchard management with the following features: 67 kW engine powered, double traction (DT), “New Holland”, model TN-95F.

To ensure experimental repeatability and attempt to parameterize the stress exerted on the soil, detailed characteristics and real weight data of the tractor were recorded immediately after the first compaction event, which occurred on the same day as the soil sampling. Weight measurements were conducted using a professional portable scale designed for trucks while making the tractor drive over the portable scale plates, one axle per time. To avoid a change in barycenter direction, an even distribution of the rear and front axle was provided by adding two wooden steps of identical width to the scale hardware (metal plate) (Figure 1). These were placed on the axle that was not being measured at the time, applying the methodology described in Spinelli et al. (2015) [55]. The tractor was equipped with radial tires 420/70 R28 and 280/70 R20 for the front and rear wheels, respectively. The total effective mass of the tractor vehicle during the field experiment was 3946 kg, unevenly distributed along front and back axles (2508 kg and 1438 kg, respectively) during the induction of stress where a “front borne” shredder, which is a heavy tool, was attached to the frontal three-point hitch of the tractor.

However, during the agronomic tractor trafficking events between November 2023 and July 2024 (referred to as ‘re-compaction’ in the experimental design), no portable scales were available. As a result, the compaction stress during the second year could not be precisely quantified. Additionally, no data were collected on soil moisture content or the direction of the tractor movement (i.e., uphill or downhill) during these operations.

2.3. Biochar

Biochar was obtained from orchard pruning biomass using a slow pyrolysis process at 500 °C in a transportable ring kiln of 2.2 m in diameter and holding around 2 tons of feedstock. The biochar at the end of the pyrolysis was crushed into particles < 5 cm in diameter before soil application. Biochar porosity was determined with a mercury intrusion porosimeter equipped with a macropore unit (Pascal 140 and 240 series, Thermo Finnigan, Waltham, MA, USA). Surface area measurements of biochar samples were performed using the dynamic Brunauer–Emmett–Teller (BET) method using a Micromeritics Flowsorb 2309 apparatus (Dunstable, UK) with N₂ as adsorbate. The samples were oven-dried at 250 °C for 30 min prior to BET analysis. The biochar pH was measured in 1:5 biochar: water extracts using a portable meter (PC7, Hydro Tech, Rosolini). Biochar bulk density was determined from oven dried cores as mass per volume of oven dried biochar using 260 × 220 mm cylindrical cores. Total biochar elemental concentration was measured using microwave-assisted acid digestion (CEM, MARS) using 0.5 of biochar. Cation exchange capacity (CEC, cmolc kg⁻¹) was determined using the NH₄OAc method. Maximum water absorption (WHC) was measured using the following method: DM 1/08/97 SO n. 173 GU 204 2/09/1997 Met.4. Total carbon (C) and nitrogen (N) contents (%) of biochar were determined using a CHN Elemental Analyzer (Carlo Erba Instruments, mod 1500 series 2). The complete chemical and physical properties of the biochar are given in [39,41]. Selected chemical and physical properties of the biochar are reported in

Table 2. Chemical and physical characteristics of pure biochar applied in the field experiment.

	Unit	Value
C	%	77.81
N	%	0.91
C/N	-	63.53
Ca	%	2.5
K	%	1.4
Mg	%	0.91
P	%	1.3
pH	-	9.8
CEC	cmolc kg−1	101
Bulk density	Mg m−3	0.4
Specific surface area (BET)	m2 g−1	410 ± 6
Maximum water absorption (WHC)	% (w w−1)	162.2
Total porosity	mm3 g−1	2722

.

2.4. Soil Sampling and Measurements

Soil was sampled three times, twice on 27 June 2023, just before and soon after the tractor passage, and the third time one year later, on 10 July 2024 (Figures S3–S5). The experimental design involved soil sampling within the wheel ruts from the top 10 cm of the mineral soil using a steel cylinder (4.25 cm inner diameter and 10 cm height). Ten soil samples were collected for each plot, five samples for each wheel rut, 2 m apart each other along the trail. At each sampling time, 60 soil samples were collected, for a total of 180 soil samples.

The soil moisture content was calculated using the gravimetric method in which the samples’ water content is calculated by subtracting the oven dried weight (at 105 °C for 48 h) from the field condition weight. The coarse fraction (skeleton) and fine earth were separated by sieving soil samples at 2 mm. The two fractions were weighed after drying in the oven at 105 °C to determine the bulk density value adjusted for coarse elements content using the calculations suggested by Torri et al. (1994) [56,57]:

BD_{fine earth} = BD (1 − 1.67 R_m^3.39)

(1)

where BD_{fine earth} is the bulk density of the fine earth fraction adjusted for coarse particles content (Mg m⁻³), and R_m is the fraction of the coarse particles as a weight % of the collected sample, i.e., the ratio of the coarse fraction mass to the total mass of the soil sample.

In 2024, starting in March, sensors for continuous soil moisture (m³/m³) monitoring, measuring the volumetric soil water content, were installed at two different depths (15 and 30 cm) in each plot of the experimental design. The sensors were connected to a data logger, which recorded measurements every 30 min, ensuring consistent and detailed tracking of soil moisture variations over time.

2.5. Statistical Analysis

To avoid distortion of statistical analyses as well as measurement errors and anomalies that would lead to misleading conclusions, we checked the database to identify and remove outliers. Assuming an expected bulk density range between 1.1 and 1.6 Mg/m³ for mineral soils, values < 1.0 or >1.7 were manually reviewed. Outlier detection and removal was then based on using a group-wise statistical method (IQR) and domain knowledge (typical bulk density ranges). Considering combinations of machinery/topography/biochar/year, outliers were flagged if the value < (Q1 − 1.5 × IQR) or value > (Q3 + 1.5 × IQR), where Q1 and Q3 are the first and the third quartile, respectively, and IQR is the interquartile range (Q3-Q1). Outlier detection resulted in the removal of 11.1% of the data, leaving 134 observations available for further analysis. The descriptive statistics of the data set are presented in

Table 3. Descriptive statistics of the dataset retained after outlier detection.

Variable (N = 134)	Mean	Std. Dev.	Std. Error	Min	Q25	Median	Q75	Max
Soil moisture a, %	11.41	3.69	0.32	0.22	10.39	11.42	13.00	29.15
Coarse fragments, %	27.76	13.74	1.19	1.10	16.80	29.80	38.80	53.00
Bulk density, Mg m−3	1.381	0.136	0.012	1.028	1.279	1.393	1.486	1.627

^a Gravimetric water content.

.

Normality of data and homogeneity of variance were assessed using the Shapiro–Francia test and Levene’s test, respectively. A two-way analysis of variance (ANOVA) was conducted on BD values of soils under the initial conditions, i.e., before compaction, to determine significant differences between the plots. Additionally, a three-way analysis of variance was performed on the BD values of soil soon after compaction to evaluate differences in BD increase among the plots and on BD values one year after compaction to evaluate soil re-compaction. In the first case, slope (flat and slope terrain conditions), compaction (before and after), and treatment (no biochar, single dose and double dose of biochar added [CRL, B and BB, respectively]) were the fixed factors. In the second case, slope, treatment, and re-compaction were the fixed factors. Significant differences between means for multiple comparisons were determined using Tukey’s post-hoc significance test (p < 0.05). The statistical analysis was primarily carried out using SPSS software, version 29 (IBM Corp., Armonk, NY, USA).

Statistical analysis for soil moisture measurements data was performed using Statistica (Release 12, StatSoft, Inc. 1984–2014). Sample normality was tested using the Shapiro–Wilk test. Analysis of variance (ANOVA) was used to compare treatment effects. The coefficient of variation (CV) was calculated as the ratio of the standard deviation to mean to assess the degree of variation of the parameter. Prior to ANOVA, Bartlett’s test was used on the data to test the homogeneity of variance. Means were separated pairwise using the post-hoc Tukey test, with a level of significance set to p < 0.05.

3. Results

Topsoil coarse fragments were common to abundant in all plots (27.8%), and their content did not differ significantly between the different treatments considered in this study (i.e., biochar addition and slope conditions). Soil moisture immediately before the first compaction event was significantly higher in the double biochar treatment (BB, 13.9%) compared to the control (CRL, 9%). The single biochar treatment (B, 12.2%) showed an intermediate value, which was not significantly different from either BB or CRL.

Slope condition did not show a significant impact on initial soil moisture content and soil BD. Even biochar addition, at any dose, did not significantly affect the BD of the analyzed soils before compaction (

Table 4. Results of the analysis of variance using treatment (biochar dose addition at none, single dose, and double dose), slope (3 and 15%), and compaction or time (before compaction, soon after compaction, and 1 year after compaction) as fixed factors on bulk density values. The statistical analysis compared all the treatments with reference to: (i) initial soil BD conditions; (ii) soil BD values soon after compaction; and (iii) soil re-compaction after one year. Values in bold highlight a level of significance of p < 0.05.

Comparison	Fixed Factor	F	p-Value
Initial soil conditions	Treatment	0.558	0.557
	Slope	0.108	0.744
	Treatment*×Slope	2.219	0.124
Soil compaction	Treatment	3.695	0.030
(T1)	Slope	1.348	0.249
	Compaction	20.160	<0.001
	Treatment*Slope	2.264	0.111
	Treatment*Compaction	0.686	0.507
	Slope*Compaction	0.433	0.512
	Treatment*Slope*Compaction	1.009	0.369
Soil re-compaction	Treatment	3.037	0.054
(T2)	Slope	0.569	0.453
	Re-compaction	6.036	0.016
	Treatment*Slope	0.584	0.560
	Treatment*Re-compaction	2.314	0.105
	Slope*Re-compaction	7.749	0.007
	Treatment*Slope*Re-compaction	0.581	0.562

, Figure 2). Therefore, the average BD of all the treatments was quite homogeneous, ranging from 1.21 to 1.35 Mg m^−3,, although with a clear decreasing trend in mean values, in any slope condition, from CRL (1.31 ± 0.06 Mg m⁻³) to B (1.29 ± 0.06 Mg m⁻³) and BB (1.25 ± 0.069 Mg m⁻³).

The repeated machine passage in 2023 led to a significant BD increase in all treatments with an average increase of 8.4% (Table 4, Figure 2). Nonetheless, biochar addition, independently of the dose, significantly limited soil compaction by an average of 4% in 2023.

In the year following the initial compaction, average soil water content (0–15 cm depth) increased with the addition of biochar, with variations influenced by slope conditions (Figure 3). CRL treatment exhibited the lowest average moisture (approximately 12–15%), while B treatment showed intermediate values (approximately 21–23%). Finally, BB treatment achieved the highest moisture content, particularly in flat conditions, with values ranging between 21 and 28% depending on slope. Figure 4 displays the monthly soil moisture (0–15 cm depth) from March to August 2024 for the three treatments, CRL, B, and BB, with data averaged across both flat and slope conditions. Soil moisture peaked in the spring months (April and May), with BB exceeding 30%, while a progressive decline was observed in the summer months. The control treatment (CRL) experienced the most pronounced drop, falling below 10% in August. Over all months, treatments B and BB consistently showed higher soil moisture levels compared to the control, with BB maintaining the highest values throughout the period.

After one year, during which the soil underwent 19 additional tractor passes as part of routine vineyard management, bulk density further increased across all soil treatments, on average by an additional 13.3%, with the most pronounced rise observed on the slope (Table 4, Figure 2). At this stage, the mitigating effect of biochar on soil compaction was no longer significant, although an average reduction of 2% was still observed.

4. Discussion

The soils from the three vineyards rows, both on slope and flat conditions, were quite stony. In general, a high stone content is positive in terms of minimizing soil compaction with machine compression as soil stones can act as a barrier that reduces direct compression of the finer soil particles by spreading and redistributing the load from the tractor [58]. Soil skeleton content did not vary significantly among treatments and, therefore, did not contribute to differences in soil compaction. Even biochar addition, at any dose, did not significantly affect the BD of the analyzed soils before compaction (Table 4, Figure 2), although a decreasing trend with increasing biochar addition was observed. Usually, biochar addition to soil is reported to lower soil BD given that biochar is a low-density material [59]. Nonetheless, the vineyard rows analyzed were ploughed some months before the first sampling campaign. Indeed, the barley cover crop and the biochar were buried using arrows at 15 cm deep in spring (April 2023). As a result, the initial average BD of all the treatments plots was quite homogeneous, ranging from 1.21 to 1.35 Mg m⁻³.

Repeated machines passes in 2023 significantly increased bulk density across all treatments, with an average increase of 8.4% relative to the initial conditions (Table 4, Figure 2). Biochar addition reduced soil compaction with an average decrease of 7.5% compared to CRL. This reduction is likely due to biochar’s low density and highly porous structure, which enhances soil resistance to compaction [59]. Previous laboratory studies have shown that soils treated with biochar tend to exhibit reduced compressibility, lower settlement, and improved stability under the same compaction force compared to untreated soils [51,52,53,60]. This effect was particularly notable given the higher moisture content in the biochar-amended soils in our study area, at least for the higher application rate (BB), where the average increase in soil moisture was 35%. This is relevant because soils are generally more and more susceptible to compaction at increasing moisture levels, up to a certain threshold. Even a modest increase in water content (as 5%)—such as from 9% to 13.9% in our case—has been shown to markedly increase compaction under the same applied pressure [51,53]. Indeed, other studies reported that biochar application appears to raise the threshold moisture level at which soils become susceptible to compaction, suggesting that fields amended with biochar can better tolerate machinery traffic under wetter conditions compared to unamended soils [51,52,53,61,62].

Contrary to expectation, the reduction in soil compaction was independent of the applied biochar dose. Previous studies evaluating the compatibility of soil added with biochar using the Proctor test highlighted reduced soil compaction with increasing biochar content [51,53]. A synthesis of data from various studies using the Proctor test revealed that soil compaction reduction starts being effective with a minimum application of 20 Mg ha⁻¹ of biochar, while only an application of 50 Mg ha⁻¹ leads to a substantial reduction in compaction [59]. The inherent complexity of soil–biochar interactions under field conditions makes it challenging to fully interpret the results obtained in this study. However, the experimental conditions of the cited studies were not the same (laboratory vs. real-world field conditions). It is therefore possible that the Proctor test does not adequately capture the complexity of the forces exerted by a tractor wheel on soil, as also pointed out by Reichert et al. [63]. Therefore, this field experiment was essential for testing our hypotheses under real-world conditions, providing more valid and applicable insights into biochar’s role in mitigating soil compaction than could be achieved using laboratory studies alone. Under field conditions, several uncontrolled or hard-to-standardize factors can influence increases in soil bulk density. These may include operator driving patterns and inherent field variability, such as spatial heterogeneity in moisture content, microtopography, and biological activity [64]. Additionally, the size of the biochar particles applied may have also influenced soil compaction [60,65]. Indeed, the compressibility behavior of soil amended with biochar can be affected by biochar’s brittle nature, which causes it to physically fracture under increasing loads beyond a certain threshold [60]. If this behavior is also linked to the amount of biochar applied, it could help explain the lack of significant differences observed between treatments with varying application rates.

Slope conditions did not play a significant role in the initial soil compaction event (Table 4, Figure 2). The effect of slope conditions on soil compaction has been widely reported in previous studies [66,67] and can be explained by a different load distribution on the two axles of the tractor [68]. Several studies reported that slopes, particularly those above 10% or 20% gradients and in uphill directions, exhibit more marked increases in bulk density [69,70,71]. Indeed, on flat terrain, soil compaction mainly results from the direct vertical load pressure, while on slopes, the combination of mechanical pressure and soil disturbance due to machinery movement can exacerbate compaction effects. However, some studies have found no significant difference in compaction between flat and sloped trails, e.g., [67,72,73], highlighting the complexity of soil compaction processes and suggesting that factors such as soil type, moisture content, machinery, and operational practices may be more influential than slope gradient alone. Last, machine movement across the hillslope would be more impactful than movement in the downhill direction [66,72,74], but the tractor in our study area moved half of the time in a hillslope direction and half downslope in all the rows.

During the year after the first compaction event, monitoring of soil moisture highlighted the effectiveness of biochar in improving soil water retention, particularly in dry summer conditions (July and August) and with increasing biochar dose in the soil. Both B and BB treatments consistently showed higher soil moisture than the control, with the largest differences occurring during the summer months when water availability is typically lower. This supports the well-documented role of biochar as a “water buffer” under drought conditions, gradually releasing stored moisture and reducing crop water stress [50,75,76]. Moreover, the stronger response observed in the BB treatment compared to B suggests a dose-dependent effect, which is frequently reported in biochar research [77,78]. These findings agree with several studies, e.g., [77,79,80] showing that biochar can significantly improve soil’s ability to retain water due to its porous structure, high surface area, and its capacity to increase total soil porosity [81].

After one year during which the soil was further subjected to additional machine passes, soil BD increased in all the treatments, on average by 13.3% compared to the initial conditions, particularly on slopes. The marked increase in BD on slope plots is likely related to erosion processes that appear to have occurred, a common occurrence in managed vineyards [82], despite the absence of clear visual signs, such as the accumulation of lightweight biochar particles at the base of the slope. Indeed, since BD typically increases with depth, surface soil erosion can result in an apparent increase in BD when changes are monitored over time. Unexpectedly, but not entirely surprisingly given the topsoil sand content exceeding 60%, biochar addition did not mitigate erosion on slopes at any application rate. This contrasts with previous findings showing that biochar can effectively reduce rainfall-induced soil erosion, even in the short term [47,83]. Furthermore, in both slope and flat conditions, biochar application did not result in a statistically significant reduction in soil compaction during the second year. The positive effect of biochar may have been hindered by the substantially higher moisture content in the B and BB soils compared to the CRL, which likely increased the soil’s susceptibility to compaction, although biochar-treated soils can better withstand machinery traffic under higher moisture conditions than untreated soils [59]. Notably, during 2024, the tractor made frequent passes over moist soil in spring and early summer, conditions that were consistently wetter than those during the compaction event of June 2023. Additionally, repeated machinery passes may have crushed or fragmented the biochar particles, diminishing their effectiveness in resisting compaction. The resulting smaller fragments could have even increased the soil’s susceptibility to compaction [65].

It would be challenging to infer some conclusions about how soil compaction evolves over the medium and long term in real field conditions. As biochar particles gradually interact with soil particles over time, they are expected to contribute to improved soil structure, increased soil organic matter, and the formation of stable aggregates [84]. Based on this, one could hypothesize that the impact of biochar on reducing BD may become more pronounced in the long term compared to the short term. However, the current biochar-related literature offers limited terms of comparison, as the (few) studies refer to laboratory conditions assessing soil–biochar mixtures immediately after incorporation, without accounting for biochar decomposition or the dynamic interactions that develop under real field conditions [59]. Nonetheless, in his data synthesis, Blanco-Canqui [59] did not find any evidence of bulk density decrease with time (1–5 years) after biochar application, although the author reported that the studies on this subject are limited and do not provide a rigorous analysis.

5. Conclusions

This study is the first to evaluate the effectiveness of biochar in mitigating machinery-induced soil compaction under real field conditions in a Mediterranean vineyard. This study’s main findings can be summarized as follows:

• Repeated machinery passes significantly compacted the soil across all treatments, both initially and over time.
• Biochar amendment reduced compaction in the short-term, regardless of application rate, with an average 7.5% reduction in bulk density after the first compaction event.
• Over the following year, the mitigating effect of biochar gradually diminished after additional tractor traffic. It is likely that the positive effect of biochar may have been hindered by the much higher soil moisture content in the biochar-amended plots during springtime, which probably increased soil compaction susceptibility. Moreover, repeated machinery passes may have crushed the biochar particles, thereby reducing their efficacy in mitigating compaction. Biochar, however, consistently improved soil water retention, particularly at higher doses, an increasingly relevant effect in Mediterranean agriculture.

The implications of our findings, along with potential directions for future research, are as follows:

• Short-term laboratory assessments would not accurately represent real-world conditions; therefore, field studies are crucial to fully understand biochar behavior in agricultural systems.
• The long-term impact of biochar on soil structure and compaction mitigation remains unclear.
• Further research should explore particle size effects and biochar–soil interactions over multiple growing seasons and across a wider range of soil types to fully leverage biochar for sustainable soil and water management, particularly in climate-vulnerable regions.