Conservation Practices Boost Soil-Protected Organic Carbon Stocks in Olive Orchards

Abstract

Carbon farming practices are pivotal for enhancing soil organic carbon (SOC) storage in agricultural systems. This study focuses on evaluating the effects of spontaneous cover crops as a conservation strategy compared to conventional management practices on total, non-protected, and protected SOC fractions, as well as carbon saturation, in olive groves across 13 paired sites (26 sites in total) in Andalucía, Spain. The research evaluates organic carbon concentrations in different soil fractions: non-protected (250–2000 µm), physically protected (53–250 µm), and chemically protected (<53 µm). The results reveal that olive groves managed with temporary spontaneous cover crops (CC) over the last 8–12 years generally exhibit higher SOC concentrations compared to those managed conventionally (BS), with significant differences observed across multiple sites. CC sites exhibited higher carbon stocks, with protected carbon averaging 42.6 Mg C ha⁻¹ compared to 29.7 Mg C ha⁻¹ in BS, and non-protected carbon at 10.3 Mg C ha⁻¹ versus 4.8 Mg C ha⁻¹. A direct relationship was identified between total SOC and both protected and non-protected carbon fractions, indicating that the soil of the studies olive orchards is far from being saturated in protected SOC. Moreover, the soil of the CC olive farms had a lower carbon saturation deficit (45.3%) compared to BS (67.2%). The findings show that maintaining the cover crops in olive orchards significantly contributed to carbon sequestration and reduced carbon saturation deficits by increasing the stocks of protected SOC.

1. Introduction

Beginning in 2021, the European Commission concentrated on advancing carbon farming within agriculture. This initiative focuses on encouraging conservation agricultural practices that enable long-term carbon storage in soil and biomass. Carbon farming practices for conservation encompass methods such as employing cover crops, crop rotation, incorporation of shredded pruning residues, manure application, peatland restoration, and the expansion of agroforestry systems, among others [1,2]. In Andalucía (southern Spain), approximately 47% of agricultural land (1.7 million hectares) is planted with olive trees [3]. Consequently, management practices that affect soil organic carbon (SOC) dynamics can have significant effects on regional organic carbon balance and the carbon footprint of the olive oil sector.

Agricultural systems that integrate trees, such as olive groves, have a superior capacity to sequester carbon compared to other agricultural systems [4]. This capacity can be further enhanced with soil management practices and agronomic strategies that boost CO₂ absorption, including practices like employing cover crops. These cover crops, mainly consisting of natural vegetation that emerges spontaneously in autumn and winter along the olive grove rows, have been shown to be effective in increasing soil carbon stocks [5,6,7] and reducing soil erosion risk [8,9]. Additionally, this conservation practice improves the biological, chemical, and physical properties of the soil through the increase in SOC [10]. However, most studies on carbon farming in olive groves tend to focus on total soil carbon, without differentiating between protected and non-protected carbon fractions. This leads to an incomplete understanding of long-term soil carbon stability. The importance of distinguishing between these fractions lies in their different behaviors and functions in the soil. Non-protected carbon represents approximately 50% of the total soil carbon and is susceptible to decomposition and release as CO₂, reducing its effectiveness as a carbon sink. In contrast, protected carbon can be associated with clay and silt soil particles and/or embedded in microaggregates, which shield it from microbial attack and enable long-term storage [11]. This distinction is essential for grasping the actual ability of agricultural soils to function as carbon sinks and can also serve as an early sign of alterations in soil conditions due to different management practices [11]. Determining which carbon fractions are protected and which are not will enhance our understanding of how soil aggregates stabilize and retain SOC [12]. Several studies have quantified the amount of protected and non-protected carbon in woody crops [13,14], but they generally focus on one or two experimental farms where one or two treatments for SOC increase have been applied over a short period of 2 to 3 years. Because of this, it is necessary to conduct studies that include a larger number of farms where the implementation of some practices that improve carbon sequestration has been in effect for a long period (>8 years). This broader and longer-term approach is essential to obtain a more representative and robust understanding of the effectiveness of conservation practices. Including a variety of farms will allow for the evaluation of spatial variability and site-specific conditions, while a prolonged implementation will ensure that the long-term effects of management practices are captured, providing more accurate and reliable data to develop effective carbon farming strategies.

Carbon protection mechanisms in the soil include the formation of stable aggregates, interaction with clay minerals, and incorporation into recalcitrant organic matter. Each of these mechanisms contributes to carbon stability and its capacity to be stored for extended periods. Additionally, soil carbon saturation, which is the point at which the soil can no longer adsorb more carbon without losing its ability to retain it long-term, is a critical factor often overlooked in total carbon studies. Carbon saturation directly affects SOC stability; once reached, the soil cannot effectively store more carbon, increasing the risk of CO₂ release. Therefore, understanding carbon saturation is fundamental to evaluating the true potential of agricultural soils as carbon sinks and to developing management strategies that maximize carbon sequestration capacity. According to Steward et al. [15], there is a limit to the potential for stabilizing SOC. When SOC nears its saturation point, additional increases in SOC reserves become less pronounced, even with elevated carbon inputs. Additionally, they noted that SOC saturation is a real phenomenon, although it may not always be apparent in agricultural field experiments due to often insufficient carbon input levels to clearly demonstrate such saturation. Other studies, such as those by Chung et al. [16] and Six et al. [11], also support the concept that soils can reach a state of carbon saturation. To assess the impact of cover crops on SOC sequestration in fruit tree cultivation systems like olive groves at a regional level, predictive models might be required. It is crucial to determine whether the relationship between protected SOC fractions and carbon input derived from cover crops is linear or has a saturation level to accurately forecast the carbon sequestration potential under this agricultural management approach.

The main hypothesis of this research was that spontaneous cover crops in olive orchards contribute to the increase not only in soil organic carbon content and stocks, but also in the long-standing protected soil organic carbon pool, thereby enhancing the contribution of olive cultivation to climate change mitigation. Therefore, protected, non-protected, and total soil organic carbon contents and stocks were analyzed in 13 pairs of olive orchards. In each pair, one orchard had implemented cover crops for at least the last 8 years, while the other orchard maintained bare soil free of spontaneous cover crops. Additionally, the carbon saturation deficit in the soil was assessed, providing a more comprehensive understanding of the soils’ capacity to act as carbon sinks under different management regimes. This knowledge is crucial for developing soil management strategies that maximize carbon sequestration and optimize carbon farming, effectively contributing to climate change mitigation.

2. Materials and Methods

2.1. Study Sites and Experimental Design

The study was conducted on 13 pairs of commercial olive groves situated in both Western (Seville province) and Eastern Andalucía (Córdoba, Jaén, and Granada provinces) (Figure 1). This area features a Mediterranean semi-arid climate, featuring prolonged, very dry summers and precipitation primarily occurring during winter and early spring. The average annual rainfall in Eastern Andalucía is 516 mm y⁻¹, while in Western Andalucía, it is 482 mm y⁻¹. Mean annual temperatures are approximately 15.6 °C in Eastern Andalucía and 17.0 °C in Western Andalucía (based on historical data spanning from 1991 to 2023, sourced from [17]).

At each location, a pair of commercial olive groves was selected, with each pair consisting of two nearby groves (<50 m apart) that were comparable in terms of orientation, soil type, and weather conditions. In one olive grove of each pair, a seasonal cover crop made up of wild annual herbs was maintained during the rainy season. These cover crops had been established for a minimum of 8 to 12 years prior to the commencement of this study. In these olive groves, the cover crops generally emerge in early autumn and are managed until the following April to prevent competition with the trees for water and nutrients. Conversely, the second olive grove in each pair was left without vegetation during this period, using pre- and post-emergence herbicides and/or tillage (BS), in line with conventional olive grove practices in the region. Consequently, a total of 26 olive groves were selected, with 13 including cover crops (CC) and 13 maintained as bare soil (BS).

The soils in both the CC and BS plots were fairly uniform. These are calcic Regosols (38.4% of the olive groves), calcic Cambisols (36% of the olive groves), calcic Luvisol and Leptosol (27%), and chromic Vertisol and calcic Fluvisol (7.7%) [18] (Table 1). The clay and sand composition in the soil showed minimal variation across the area, averaging 35% and 28.1% for CC and 40.2% and 24% for BS olive groves, respectively (Table 1). The soil pH averaged 8.3 in both cases. Cation exchange capacity (CEC) averaged 25.1 cmol (+) kg⁻¹ and 25 cmol (+) kg⁻¹, for CC and BS olive groves, respectively (Table 1).

2.2. Soil Analysis and Organic Carbon Contents of the Soil

Between mid-March and mid-April 2021, soil samplings were performed. In both the CC and BS olive groves, four soil samples were randomly collected from the top 30 cm between the trees, particularly in the 1–5 m zone where temporary cover crops had naturally emerged. The samples were placed in labeled plastic bags and transported to the lab on the same day. In the lab, the soil samples were allowed to air-dry over the course of a week before being meticulously passed through a sieve with apertures less than 2 mm in size. The fraction of soil particles that passed through the sieve and those that were held back by the 2 mm screen was documented. The soil particles with a size less than 2 mm were subsequently dried in an oven at 80 °C for a duration of 48 h, after which the organic carbon content was analyzed. The organic carbon content was assessed using an acid digestion method with potassium dichromate, as outlined by Anderson et al. [19]. Bulk density was measured at three points per plot in the olive grove inter-row. For this, two samples were taken using Kopecky cylinders at depths of 0–5 cm and 15–20 cm. The material was placed in a sealed bag and dried in the laboratory at 105 °C.

2.3. Measurements of Soil Organic Carbon Fractionation

The process of separating soil carbon fractions was performed in triplicate for soil samples from 13 pairs of CC and BS olive groves, totaling 26 samples. The different soil C pools were separated using a physical wet fractionation method of Stewart et al. [20]. Fifty grams of soil, air-dried, were mixed with distilled water, and the mixtures were strained through two successive sieves with mesh sizes of 250 and 53 µm. The soil particles that were held back by the 250 and 53 µm sieves, as well as those that passed through the 53 µm sieve, were dried at 105 °C for 48 h before being weighed. The organic carbon levels in the soil fractions of 250–2000 µm, 53–250 µm, and <53 µm were analyzed using the previously outlined method. These fractions corresponded to coarse particulate organic matter (hereafter non-protected carbon), microaggregate carbon or physically protected carbon, and silt and clay that are easily dispersed or chemically protected organic carbon, respectively.

2.4. Data Analysis

The calculation of organic carbon in the soil and per hectare (to a depth of 30 cm) was performed using Equation (1).

Stock SOC (Mg C ha⁻¹ top 30 cm) = SOC × 0.3 m × 10,000 m² × BD × (1 − f)(1)

where SOC stands for soil organic carbon content (g C g⁻¹ soil), BD denotes the bulk density of the soil samples, and f indicates the fractions (over 1) of soil particles larger than 2 mm.

The calculation of the maximum carbon protection capacity through chemical stabilization of soil (to a depth of 30 cm) was conducted using Equation (2) from [11]:

C Protection Capacity (g C kg⁻¹ soil) = (0.21 × (%clay + %silt)) + 14.76(2)

Carbon protection capacity, along with the total carbon content in each olive farm, was used to calculate SOC saturation. Carbon saturation deficit was calculated as the difference between SOC saturation and measured organic carbon content.

2.5. Statistic Analysis

Two-way ANOVA (considering management and site) was used to evaluate the impact of temporary spontaneous cover crops on total SOC, SOC fractions, carbon saturation deficit, and SOC stock. Prior to this, assessments of normality and homoscedasticity were conducted. To normalize non-protected carbon (250–2000 µm) and chemically protected carbon (<53 µm) measured in mg C g⁻¹ fraction, a logarithmic transformation was applied. When significant, differences between treatments were determined using a Tukey post-test at a 0.05 level of significance.

The relationship between total SOC and protected soil carbon, as well as between total SOC and non-protected soil carbon, and the statistical analyses were conducted with R (version 4.3.0; R Foundation for Statistical Computing, Vienna, Austria).

3. Results

3.1. Soil Organic Carbon Fractions of Olive Orchards with Temporarily Spontaneous Cover Crops and Bare Soils

Among the 13 pairs of olive groves analyzed for CC and BS, the concentration of organic carbon (mg C g⁻¹ fraction) in the non-protected C fraction (250–2000 µm soil particles) was notably greater under CC compared to BS at 5 out of the 13 locations (Figure 2a). For physically protected C (53–250 µm), the organic carbon concentration was significantly higher under CC in 4 out of the 13 sites (Figure 2b). The organic carbon concentration in the silt plus clay soil fraction (chemically protected, <53 µm) was significantly higher under CC in 9 out of the 13 sites (Figure 2c). Per gram of soil, the organic carbon in the 250–2000 µm, 53–250 µm, and <53 µm fractions was significantly higher under CC in 3, 3, and 9 sites out of the 13, respectively (Figure 2d–f).

Aggregating data from the 13 pairs of CC and 13 BS sites, the organic carbon concentration (mg C g⁻¹ fraction) in the particle size fractions of 250–2000 µm, 53–250 µm, and <53 µm was 40.7%, 100.3%, and 67.2% significantly higher in CC than in the BS olive groves. Additionally, the organic carbon content in the non-protected, physically protected, and chemically protected fractions of one gram of soil was 2.56, 2.27, and 1.49 times higher, respectively, in olive groves with cover crops (Figure 3).

Figure 4 shows the stock of non-protected and protected soil organic carbon (Mg C ha⁻¹ top 30 cm of soil) in the selected 13 pairs of olive groves with CC or BS, as well as the average for CC and BS olive groves. The results showed that in 7 of the 13 sites, the stocks of protected carbon (including both physically and chemically protected carbon) were significantly higher in CC compared to BS. In contrast, only one site showed a significantly higher stock of protected carbon in BS compared to CC. For the non-protected carbon fractions, no site showed significant differences in SOC stocks between CC and BS. For the average of the 13 sites, significant differences were observed in both carbon fractions (protected and non-protected) between CC and BS. On average, the stocks of protected carbon in the first 30 cm of the soil were 42.5 and 29.7 Mg C ha⁻¹ in CC and BS, respectively. For the non-protected carbon stocks, the values were 10.3 Mg C ha⁻¹ in CC and 4.8 Mg C ha⁻¹ in BS (Figure 4).

3.2. Linear Regression between Total SOC and Unprotected and Protected SOC, and SOC Saturation Deficit

The relationship between whole SOC (mg C g⁻¹ soil) and the different fractions of SOC content (mg C g⁻¹ soil) was analyzed by pooling data from all sites and management practices. A significant positive linear relationship was identified between total SOC and protected soil carbon, explaining 90.9% of the variance in total SOC (R² = 0.90, p < 0.05, n = 78) (Figure 5a). Furthermore, a significant positive linear relationship was found between total SOC and non-protected soil carbon, explaining 34.3% of the variance in total SOC (R² = 0.34.3, p < 0.05, n = 78) (Figure 5b).

Pooling data from the 13 CC olive farms, the average soil organic carbon saturation deficit was 45.3% at a depth of 30 cm, whereas for the 13 BS olive farms, it was 67.2%, with significant differences observed between them (Figure 6).

4. Discussion

4.1. Effects of Temporarily Spontaneous Cover Crop on Soil Organic Carbon in Olive Groves

The use of cover crops in olive groves significantly increased the soil organic carbon content compared to olive farms with bare soils, confirming their role in carbon sequestration in agricultural soils. This increase can be directly linked to the biomass input from the cover crop residues after they are cleared during April and May. However, the amount of sequestered soil organic carbon with cover crops depends on the quantity but also depends on the quality and the decomposition of residues [21]. Biomass in spontaneous cover crops is not particularly high compared to other agricultural systems, around 200 g m⁻² [22], but its C content is relatively high and the entry of spontaneous cover crop residues-derived carbon to the soil ranges from 420 to 560 kg C ha⁻¹ y⁻¹ [22,23]. However, its decomposition is expected to be relatively high due to the low C/N ratio (17 on average) in cover crops [24]. Nevertheless, the increase in SOC might not be exclusively linked to the inputs of the cover crop residues. Indeed, it has been demonstrated that cover crops play a crucial role in reducing soil erosion [9], which can also avoid significant losses of organic C from the soil compared to bare soils. The overall positive effect of cover crops on soil organic carbon was not observed in all pairs of olive farms studied, finding recently noted by Pareja–Sánchez et al. [25]. This discrepancy could be attributed to low biomass in specific sites or varying pedoclimatic conditions. Moreover, soil samples were taken from a depth of 0 to 30 cm, which might not be precise enough to evaluate the impact of cover crops on SOC. Several studies have reported how the incorporation of plant residues increases SOC in the topsoil, with the highest SOC content observed from 5 to 15 cm. Beyond this depth, the effect of plant residues on SOC diminishes [26].

Cover crops significantly increased the content of non-protected organic carbon both per gram of fraction and per gram of soil compared to soil without cover crops. This result is consistent with findings in olive groves under different pedoclimatic conditions as reported by Pareja–Sánchez et al. [25]. These results were not unexpected as non-protected carbon includes small residues of the plant cover that are already partially decomposed, along with seeds and microbial residues, including fungal hyphae and spores, that are not linked to soil minerals [11]. Interestingly, the difference in organic carbon between CC and BS olive farms in the non-protected fraction, when expressed per gram of soil, was greater than the difference per gram of fraction. This is a clear indication of the increase in the amount of small residues from the plant cover in the CC olive farms. Non-protected carbon cannot be considered sequestered carbon because a portion of it will be mineralized relatively quickly after its incorporation into the soil. However, another fraction of the non-protected carbon is less decomposable and will eventually become part of soil microaggregates (physically protected) or even bound to silt and clay particles (chemically protected) in the medium to long term.

There was a general trend of higher physically protected carbon in farms where cover crops had been grown in the inter-row areas of the olive groves compared to bare soils. This effect was significantly positive for six olive groves with cover crops compared to their counterparts without cover crops. The increase in the physically protected carbon in the CC olive farms was not only due to a mean increase of 21% in the number of microaggregates per gram of soil but, more importantly, due to a 103% increase in the organic carbon concentration per gram of fraction. In contrast, Pareja–Sánchez et al. [25] found that although physically protected carbon was on average 20% higher in soils of olive groves with cover crops, the differences were not significant. This discrepancy may be explained by the relatively low aboveground biomass of cover crops in the latter study, which covered only a small proportion of the inter-row area. Physically protected C comprises organic C that is part of the macro- and microaggregates. The incorporation of cover crop residues into soil is expected to enhance the formation of soil aggregated and improve soil structure [27,28,29]. Incorporating crop residues into the soil enhances microbial activity, which leads to the production of organic acid and polysaccharides during the breakdown of organic matter, thereby fostering soil aggregation [30,31]. The ability of soil macro- and microaggregates to protect organic carbon physically is linked to their function in isolating the organic carbon substrate and microbial biomass [32] and to the potential decrease in oxygen diffusion within macroaggregate and particularly microaggregates, leading to reduced microbial activity inside these aggregates [33]. Therefore, the presence of cover crops and the input of their residues increased the amount of physically protected organic carbon, which is of utmost importance for long-term organic carbon sequestration.

On average, olive groves with cover crops showed significantly higher levels of chemically protected C. This effect was significant for six pairs of olive groves, confirming that cover crops can enhance the formation of C association with silt and clay. The increase in chemically protected carbon in the CC olive farms was primarily due to the rise in the organic carbon concentration of the silt + clay fraction, as the percentage of soil particles <53 µm was similar in both groups of olive farms (62.7% and 64.3% in CC and BS olive farms, respectively). A higher concentration of chemically protected C in soil with cover crops is beneficial for C sequestration in soil because these C associations have long turnover times and are highly stable [34]. Similar results were also observed by Garcia–Franco et al. [27] in almond orchard soil amended with green manure. Clay particle surfaces are negatively charged (especially in basic soils), while organic matter has strong positive charges allowing them to form strong bonds, and protecting organic matter from microbial attack. In fact, Six et al. [11] discovered a correlation between clay content and soil organic carbon. Therefore, the higher concentration of chemically protected soil organic carbon quantified in olive groves with cover crops demonstrates the role of cover on carbon sequestration, by enhancing the pool of physically and chemically protected carbon, which remains stable in the medium to long term.

4.2. Soil Organic C Stock in Olive Groves

On average, between 8–12 years of cover cropping increased soil organic C stocks in olive groves to 52.9 Mg C ha⁻¹, compared to 34.5 Mg C ha⁻¹ in bare soils. Additionally, the increase in protected carbon in soils with cover crops was 1.43 times higher (42.5 Mg C ha⁻¹) compared to olive farms with bare soils (29.7 Mg C ha⁻¹), confirming the role of cover crops in enhancing long-term carbon sequestration by increasing the pool of protected carbon in the soil. Recent meta-analyses have demonstrated the positive effects of cover crops on increasing soil carbon stock in agroecosystems under different climate conditions [35,36]. Furthermore, cover crops provide additional ecosystem services and benefits to both farmers. They can significantly contribute to nutrient retention, enhance soil fertility, and reduce water and soil losses, as demonstrated by Torrús-Castillo et al. [7], García–Ruiz, et al. [37], and Cerdà et al. [38], respectively, without compromising yield, as shown by Zuazo et al. [39].

4.3. C Saturation Deficit Is Lowered by the Temporary Spontaneous Cover Crops

In this study, SOC stocks were assumed to serve as a proxy for soil C input. Stewart et al. [40] mathematically demonstrated how SOC concentrations in individual soil fractions are related to the total SOC concentration, enabling C saturation to be represented as a function of SOC concentration instead of soil C input. Nevertheless, we recognize the constraints of this assumption, particularly due to the inclusion of soils from diverse environments, which may not uniformly represent steady-state conditions.

According to this assumption, if the total SOC concentration and the protected SOC fraction exhibit a direct proportionality, it would suggest that C saturation behavior is absent. Throughout the range of total SOC concentration observed in this study (4.7–28.8 mg C g⁻¹), there was a tendency for a linear relationship between the non-protected SOC and the whole SOC for the combined site data (r² = 0.34, p < 0.05, n = 78). However, the variability in the non-protected organic carbon content explained by the whole SOC was relatively low (34%). The lack of a robust linear relationship could be mainly due to two factors. First, the relatively low annual cover crop residue-derived carbon in the area [25], compared to the overall SOC stock, coupled with a relatively high SOC deficit, might result in only a slight increase in the stock of non-protected carbon along the entire SOC stock gradient. Rodrigues et al. [41] found that litter-derived C incorporated into the non-protected fraction increased linearly with decreasing soil carbon saturation deficit. Secondly, a fraction of this non-protected carbon or other secondary organic carbon compounds resulting from microbiological processing is continuously transferred to physically and/or chemically protected carbon in areas with high SOC deficit, preventing a clearer linear increase in the non-protected carbon. Nevertheless, this linear tendency between non-protected SOC and the total SOC pool is in line with that found by Peng et al. [42], who showed in eight long-term agroecosystems experiments along with adjacent grassland or forest areas, it was demonstrated that non-protected SOC data were most accurately described by a linear model.

For the entire dataset of plots (n = 78), the relationship between total SOC and the concentration of both physically and chemically protected pools was most accurately represented by a linear function (r² = 0.90, p < 0.05). This result does not agree with other findings. It has been proposed that the connection between organic carbon inputs and the concentration of physically and chemically protected SOC should follow a saturation curve [40]. The amount of silt + clay particles and the potential for forming macro- and microaggregates in soil are finite, which implies that the amount of SOC protected through these mechanisms is also limited [11]. Stewart et al. [15] discovered that once the soil’s protective capacity was surpassed, additional organic carbon was no longer stabilized by these protective processes. The concept of C-saturation, which relates the maximum SOC storage capacity to a particle-size fraction, was recently supported by Guillaume et al. [43] in permanent grassland. However, these authors suggest that using silt + clay content as the sole factor to determine the maximum capacity of soils to store SOC may be an oversimplification. Nevertheless, the absence of saturation in the physically and chemically protected SOC in our study may be attributed to the relatively narrow range of total SOC observed in our research compared to the broader range reported by Stewart et al. [15] (i.e., 5.1–96.1 mg C g⁻¹). Therefore, while SOC saturation in these fractions may indeed occur, it might not always be apparent in agricultural field studies due to the typically limited range of organic carbon inputs used, which may be insufficient to reveal saturation trends. The lack of a saturation relationship in the studied olive groves indicates the high capacity to fill the soil with protected organic carbon.

The organic carbon physically protected within soil microaggregates and that associated with clay and silt have on average a longer residence time compared to bulk SOC [44]. Therefore, assessing the C saturation deficit in the protected SOC fraction could offer insights into the potential for increasing SOC stocks with prolonged residence times as suggested by Wiesmeier et al. [45] or McNally et al. [46]. Nevertheless, the precise quantitative relationship between SOC deficits and the soil’s capacity to retain organic carbon over multiple decades remains unclear and requires additional investigation.

The mean SOC saturation deficit of 56.2% for the soils of the 26 studied olive groves is comparable to the 62% found for croplands in Switzerland devoted mainly to cereals and rapeseed [47], but it is slightly higher than the 49% observed in traditional rainfed systems where no cover crops were allowed due to a combination of tillage and herbicide application [48]. The relatively low mean saturation deficit of the studied olive groves indicates their great potential to sequester organic carbon.

The 32.5% reduction in the SOC saturation deficit in the olive groves with cover crops demonstrates that: (i) a significant fraction of the cover crops derived C ends as protected organic carbon in the soil, and (ii) this conservation practice is a promising strategy to sequester organic carbon at the long term. However, setting a target for SOC levels based on maximum potential SOC storage might be impractical for olive groves due to the current limitations in plant C inputs and the availability of organic amendments under existing agricultural practices. Indeed, the relatively low net primary production of the cover crops [7,25] is mainly due to the relatively low annual precipitation (<500 mm). Additionally, cover crops typically grow only in a fraction of the inter-row area of the orchard, and they are controlled during April to minimize potential competition for water and nutrients. These factors prevent a further increase in the stock of protected SOC, thereby hindering a further reduction in the carbon saturation deficit. However, expanding the area covered by spontaneous cover crops could further enhance the potential benefits of C sequestration of this management practice, as demonstrated by Torrús-Castillo et al. [22].

5. Conclusions

This study has demonstrated that temporary spontaneous cover crops in a sufficient range of commercial olive groves in Andalusia significantly increase SOC content and SOC stocks (top 30 cm) compared to bare soils. This increase is primarily attributed to biomass inputs, as well as soil erosion reduction, thereby promoting soil conservation in olive orchards. Cover crops enhance the amount of non-protected organic carbon, which can be converted into protected carbon by integrating into soil aggregates. Additionally, they improve physically protected carbon by forming macro- and microaggregates, and increase chemically protected carbon, which is highly stable and has long residence times, contributing to carbon sequestration.

On average, olive groves with cover crops for more than 8 years showed a significant increase in SOC reservoir, particularly in protected carbon fractions, reducing the carbon saturation deficit by 32.5%. This underscores the effectiveness of cover crops as a strategy for soil conservation and carbon sequestration in olive orchards. However, the magnitude of the SOC increase varied across locations, indicating that further research is needed to establish a clear link between soil organic carbon deficit and long-term carbon storage capacity.

While cover crops are an effective practice for increasing carbon sequestration in olive orchards, their implementation must consider net primary production limitations and the effective management of water and nutrients to maximize benefits. This study provides a robust foundation for promoting the use of cover crops in sustainable soil conservation in olive orchards and contributing to climate change mitigation through soil carbon sequestration.

Author Contributions

E.P.-S.: writing—original draft preparation, review, and editing, statistical analysis, formal analysis; P.D.: writing—review and editing, statistical analysis; B.G.-M.: methodology, writing—review and editing; M.T.H.-L.: experimental work; R.G.-R.: funding acquisition, methodology, supervision, writing—review and editing, conceptualization, data curation. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The data discussed in this study can be obtained upon request from the corresponding author.

Conflicts of Interest

The authors state that they have no financial conflicts of interest or personal relationships that could have potentially affected the outcomes reported in this paper.

Footnotes

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Figures and Table

Figure 1. Placement of the olive orchards selected in Andalucía (Spain).

Figure 2. Soil organic carbon concentration per gram of fraction (mg C g−1 fraction) (a–c) and per gram of soil (mg C g−1 soil) (d–f) of different fractions (non-protected C 250–2000 µm (a,d); physically protected 53–250 µm (b,e) and chemically protected <53 µm (c,f)) as affected by management (CC; spontaneous cover crops and BS; bare soil) at different sites. Vertical bars indicate standard errors. The asterisk denotes significant differences between the CC and BS olive groves in each pair (p [less than] 0.05).

Figure 3. Average of soil organic carbon concentration per gram of fraction (mg C g−1 fraction) (a–c) and per gram of soil (mg C g−1 soil) (d–f) of different fractions (non-protected C 250–2000 µm (a,d); physically protected 53–250 µm (b,e) and chemically protected <53 µm) (c,f) as affected by management (CC; spontaneous cover crops and BS; bare soil). Vertical bars indicate standard errors. Distinct lowercase letters signify significant differences between management treatments at p [less than] 0.05.

Figure 4. Stock of soil organic carbon (stock SOC) (Mg C ha−1 top 30 cm of soil) as affected by management (CC; spontaneous cover crops and BS; bare soil) of different fractions (protected and non-protected soil carbon) at the site of field experiments and average (Average) of the groups of CC and BS olive groves. Vertical bars indicate standard errors. The asterisk denotes significant differences between the CC and BS olive groves in each pair (p [less than] 0.05) for both non-protected (upper asterisk) and protected SOC (lower asterisk).

Figure 5. Linear regression between total soil organic carbon (total SOC) (mg g−1 soil) and (a) protected soil carbon (mg g−1 soil) and (b) non-protected soil carbon (mg g−1 soil). Each point represents the values of each replicate for each site.

Figure 6. Box-plot representation of carbon saturation deficit (% of the current SOC content (mg C g−1) relative to the SOC content (mg C g−1) at carbon saturation) in each soil management (CC; spontaneous cover crops and BS; bare soil). Dots are the values of each group. The edges of the boxes nearest to and farthest from zero represent the 25th and 75th percentiles, respectively. The thin lines inside the boxes denote the median, while the ‘X’ symbol indicates the mean. The bars extending above and below the box correspond to the 90th and 10th percentiles. Outliers are shown as black dots. Distinct lowercase letters denote statistically significant differences between soil management practices (CC and BS) at p [less than] 0.05.

Table 1

Mean soil properties in olive groves with temporary spontaneous cover crops (CC) and with bare soil (BS). The number of years since the implementation of temporary spontaneous cover crops is also indicated. Cal Reg, Cal Cam, Cal Luv, Cal Lep, Ver Cam, Cal Flu, and Chr Ver stand for calcic Regosol, calcic Cambisol, calcic Luvisol, calcic Leptosol, vertic Cambisol, calcic Fluvisol, and chromic Vertisol, respectively.

Sites	Olive Groves	Clay%	Sand%	Texture	pHH2O	Bulk Density(g cm−3)	CEC(cmol (+) kg−1)	Soil Type	Cover Crop Implementation (Number of Years)	Municipality
CH	CC	37	28	Clay loam	8.47	1.27	26.3	Cal Reg	8–12	Estepa
	BS	49	26	Clay	8.28	1.41	30.3	Cal Reg		Estepa
MR	CC	37	34	Clay loam	8.25	1.32	25	Cal Luv	8	Estepa
	BS	45	24	Clay	8.08	1.44	27	Cal Luv		Estepa
GA	CC	38	35	Clay loam	8.15	1.27	24.3	Cal Cam	8	Estepa
	BS	46	31	Clay	8.35	1.30	30.3	Cal Cam		Estepa
CR	CC	43	24	Clay	8.32	1.36	27	Cal Reg	10	Herrera
	BS	40	31	Clay loam	8.53	1.41	22.6	Cal Reg		Herrera
JT	CC	36	35	Clay loam	8.39	1.30	27	Cal Lep	11	Pegalajar
	BS	48	23	Clay	8.61	1.32	26.3	Cal Lep		Pegalajar
CT	CC	34	43	Clay loam	8.35	1.66	20.3	Chr Vert	8–12	Lora de Estepa
	BS	38	34	Clay loam	8.05	1.46	21	Cal Flu		Lora de Estepa
PA	CC	42	30	Clay	8.47	1.32	28.7	Cal Cam	9	Lora de Estepa
	BS	44	33	Clay	8.17	1.27	28	Cal Cam		Lora de Estepa
PV	CC	35	44	Clay loam	8.16	1.44	24	Ver Cam	12	Mogón
	BS	48	25	Clay	8.49	1.35	27.3	Ver Cam		Mogón
DE	CC	42	32	Clay loam	8.21	1.30	30.3	Cal Cam	10	Deifontes
	BS	48	27	Clay loam	8.41	1.41	29	Cal Lep		Deifontes
ISA	CC	37	32	Clay loam	8.05	1.37	25.3	Cal Reg	9	Estepa
	BS	42	29	Clay	8.21	1.46	24	Cal Reg		Estepa
LH	CC	20	13	Clay loam	8.26	1.28	22.5	Cal Reg	12	La Zubia
	BS	22	15	Clay loam	8.42	1.33	18.5	Cal Reg		La Zubia
PRI	CC	27	9	Silty clay loam	8.34	1.33	23.5	Cal Cam	11	Priego de Córdoba
	BS	29	7	Silty clay loam	8.42	1.46	19.5	Cal Cam		Priego de Córdoba
ES	CC	27	7	Silty clay loam	8.23	1.21	23.1	Cal Reg	11	Escúzar
	BS	24	8	Silty clay loam	8.36	1.35	22.0	Cal Reg		Escúzar

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