Unveiling the Opportunities of Unexplored Use of Cover Crop in Mediterranean Agriculture through Systematic Review and Meta-Analysis

Abstract

Cover crops are multifunctional, and contribute to improving soil properties and reducing environmental impact compared to no-cover crops, thus could provide multiple soil, agricultural, and environmental benefits, and they are recognized as a valid strategy for the achievement of sustainable agriculture. However, cover crops’ impacts on subsequent cash crops and soil characteristics are dependent on several factors, such as pedoclimatic conditions, cover crop species, agricultural practices, method of termination, and irrigation management. The fact that cover crops are never applied as a single practice in the real agricultural sector, but are instead combined with other factors or agricultural practices, deeply affects their performance, but the scientific literature nevertheless keeps considering the use of cover crops alone. Moreover, the potential outcomes of cover crops that encompass other factors or agricultural practices affecting soil quality, weed control, and cash crops are still unclear. Additionally, cover crops are still poorly use in the Mediterranean type of climate. Therefore, this study reviewed the scientific literature to identify the most relevant factors or agricultural practices driving cover crops’ performance, and to outline future fields of investigation looking towards promising sustainable agriculture in Mediterranean areas with a view to minimizing the competition for soil water with the cropping systems and to reduce soil degradation. Furthermore, the investigation includes multiple factors or agricultural practices that better represent the real farming system, contributing to a comprehensive understanding of their interactions with cover crops, and suggests alternative strategies for reducing yield gap while seeking to achieve agricultural sustainability.

1. Introduction

Cropping systems that provide a variety of ecosystem services beyond crop production, such as cover crops, are gaining increasing interest from farmers, policy makers, and society [1,2]. In addition, water resources continue to dwindle, and agricultural droughts could be twice as likely at 1.5 °C global warming [3]. This therefore leads to an increase in potential yield gaps, thus affecting the sustainability of agriculture in the context of water scarcity. As a consequence, relevant mitigation measures oriented towards the adaptation of cultural practices could somehow alleviate the negative effects of those changes with the aim of boosting soil fertility. In this regard, the Soil Science Society of America (SSSA) defines cover crops as close-growing crops that provide soil protection, seeding protection, and soil improvement between periods of normal crop production, or between trees in orchards and vines in vineyards [4]. In annual cropping systems, cover crops are grown in that period after harvesting the first crops and before planting the subsequent crops [5], or they can also be intersown by dispersing the seed throughout the main crop [6]. To avoid competition with the cash crop for water and nutrients, cover crops generally terminate in late March or early April under the Mediterranean climate [7].

Cover crops, however, are limited in Mediterranean agriculture due to the concern around water and nutrient competition between swards and plants, which has led many researchers to include water irrigation practices as a strategy to perform a canopy crop establishment, as a consequence of improving water use efficiency. Thus, these practices have to evolve with the aim of prioritizing the improvement of soil-water balance for the crops, and to ensure productivity and crop growth under limited water conditions, as in Mediterranean areas. Indeed, the combination of cover crops with irrigation techniques for improving soil stability, increasing total porosity, and enhancing water-holding capacity is attracting increasing scientific interest [8].

On the contrary, the interest in more sustainable and holistic approaches nowadays (including organic farming systems and irrigation strategies) has been brought to people’s attention. As a potential of diversification, the use of tactical practices such as intercropping and service crops involving diversifying crops and agricultural management practices may be a solution to many issues.

In the scientific literature, cover cropping has been extensively assessed in a variety of soils and climate conditions across the world [9], and several researchers have defined cover crops as practices through which ecosystems, and the species that make them up, sustain mankind [10]. Nevertheless, cover cropping is still poorly adopted in “Hot-Spots” like the Mediterranean area, according to climate change projections [11] (Giorgi, 2006). Subsequently, this study wishes to highlight that cover cropping is not a common practice in Mediterranean agriculture, and to give a perspective on how its combination with further practices could contribute to improvements in limiting scenarios.

Any plant genus can be used as a cover crop, but leguminous, gramineous, and brassicaceous are the most used ones [12]. Regardless of the genus, cover crops have become largely recognized as an essential practice for sustainable agriculture [13]. Their use provides several benefits for soil physical, chemical, and biological properties [12,14], but these effects depend on the species used, period of cover crop adoption, biomass production, and cropping system [15,16,17]. The affected soil properties influence in their turn the soil’s ability to guarantee a variety of ecosystem services [18]. Specifically, integrating cover crops into cropping systems may: (i) reduce nitrate leaching risks by acting as a catch crop and scavenging nitrogen in the soil by using the fibrous root system of grass species [19,20], (ii) fix atmospheric nitrogen in the case of legume species [21], (iii) enhance soil mycorrhizal activity [22], (iv) increase soil organic carbon (SOC) stocks and adjust soil temperature [23,24], (v) foster pest control [25], and (vi) reduce soil erosion and runoff [26]. Moreover, some benefits are proportional to cover crop biomass production, like weed management that is best provided by cover crop species with high biomass production, rapid growth rates, and allelopathic chemical production [27], while some other benefits are specific to species’ functional traits [28]. Indeed, leguminous genera are well known for their atmospheric nitrogen fixation capacity in the soil [29]. Gramineous genera gain high biomass with persistent residues that slowly decompose, and their fine, fibrous, and well-adapted root systems are used for improving soil structure and delivering large amounts of carbon directly into the root zone [30]. Brassicas are an important cover crop group for weed and pest control through their release into the soil of some allelochemicals, and they relieve soil compaction and increase infiltration through their distinctive root system [31]. However, there is no single species that can provide all the desired benefits looked for at one time, therefore cover crop mixtures are used [28,32].

The successful performance of cover crops depends on several factors such as compatibility with growing environment, complementarity of functional traits, species life cycle, existing crop rotation, and tillage practices [30,33]. Furthermore, cover crop planting dates can also influence the amount of biomass that they can produce, nitrogen accumulation, and weed suppression [34,35], while their termination method is a key factor in improving soil fertility and weed control [36,37]. Cover cropping is an agricultural practice that has several demonstrated benefits and can represent a real support for farming systems vulnerable to the climatic conditions of the Mediterranean climate region [38]. Accordingly, tactical decisions and adaptive management alone may have the potential to strike an adequate balance between climate adaptation and agricultural yield. This can also deal with the 4 per 1000 Initiative (launched for the first time by France in 2015 at the COP 21), which aims to enhance global soil organic matter by 0.4% each year [39].

Overall, a real farming system is still unlikely to apply cover crops alone instead of as part of a more complex management strategy. Therefore, the systematic review here aims to investigate the standpoint adopted by the scientific community on cover crops under the Mediterranean climate. Moreover, the additional meta-analysis aims to deepen the factors (agricultural practices) commonly considered key in the performance of cover crops and the most crucial effects (studied parameters) of cover crops in the cropping system. Cover crops are recognized as a relevant agricultural practice for sustainable agriculture, but their positive impacts can be corroborated and boosted by the application of other practices. This paper seeks to identify the unexplored factors for advocating new fields of development, and to bring up the criticisms and highlight the opportunities to fill the existing gap in the literature, especially for the Mediterranean area.

2. Materials and Methods

2.1. Literature Review

The research question formulated for this work is: “What are the relevant factors and agricultural practices that drive the performance of cover crops, and how do these factors, when combined with cover crops, affect soil quality, weed control, and cash crop yields in comparison to cover crops used alone, in the context of achieving sustainable agriculture?”. It was formulated using the model proposed by [40]. Briefly, the model, called the PICO process, was created to streamline clinical research, but more recently it has been useful for any evidence-based scientific activity. This process relies on four elements: Patient/Population/Problem, Intervention, Comparison, and Outcome.

For the realization of this systematic literature review, peer-reviewed papers published between January 2000 and May 2024 were searched exclusively in the Web of Science database [41] on 30 May 2024. The observation start point of January 2000 was selected because of the increasing global concerns around environmental issues of that period. Indeed, international agreements like the Kyoto Protocol guide us towards sustainability, as do the Strategic Framework for FAO, or the package of legislative measures like Agenda 2000, and they had their first practical impacts in this year, despite having been signed/published before. On the other hand, the end point of the observation of May 2024 was chosen to expand the period of observation as much as possible. In the end, we excluded book chapters, review articles, proceedings papers, and limited the literature review to articles in the English language. The PICO process structure for this work, as well as the script, are described in

Table 1. PICO components and keywords used to build the research question.

PICO Components	Description	Script
Population (P)	Agricultural cropping systems under a Mediterranean climate using cover crops	(agriculture OR “organic farm*” OR “organic agriculture*” OR “organic horticulture” OR agroecology OR “conservation agriculture” OR “conservation farm*” OR “conservation horticulture” OR “regenerative agriculture” OR “regenerative farm*” OR “regenerative horticulture”)
Intervention (I)	Factors and agricultural practices that can be used in combination with cover crops (e.g., cover crop species, sowing time, termination time and methods, irrigation, crop rotation, tillage practices)	(specie* OR mixture* OR Irrigation OR “water management” OR “crop* rotation” OR “termination method*” OR “termination technique*” OR “termination time” OR “planting time”)
Comparison (C)	Cover crop species used alone	(“cover crop*” OR “catch crop*” OR “companion crop*” OR “green manure”)
Outcome (O)	Cover crop performance, soil quality, weed control, and cash crop yield parameters	(weed* OR “cash crop*” OR “subsequent crop*” OR “crop* yield” OR (Soil AND (fertility OR health OR quality OR “biological propert*” OR “chemical propert*” OR “nutrient* cycling” OR “physical propert*” OR structur* OR “organic carbon” OR “organic matter”))).

.

2.2. Literature Selection

The selection of the peer-reviewed papers was carried out by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) technique [42,43,44]. PRISMA techniques include four stages: identification of research questions (performed according to the PICO model), screening, eligibility (considering exclusion criteria), and inclusion (quantitative analysis).

After searching the literature, all peer-reviewed papers were imported into the systematic review tool Rayyan [45], and abstracts were screened to check their relevance to the search question using the following inclusion criteria:

• Peer-reviewed papers with experimental studies (only relevant field studies that reported at least one intervention and outcome components described in Table 1 were selected);
• Peer-reviewed papers conducted in field conditions;
• Peer-reviewed papers not involving cover crops in the intercropping with other crops;
• Peer-reviewed papers conducted under Mediterranean climate conditions.

Later, the peer-reviewed papers selected against the pre-defined criteria were retrieved and the full texts screened. The following exclusion criteria were applied to assess the eligibility of the studies and to decide on their inclusion or exclusion for this work:

• Reason 1: Peer-reviewed papers comparing cover crops and no cover crops, or comparing cover crop species; performances;
• Reason 2: Peer-reviewed papers whose sites do not belong to the Mediterranean climate conditions (and are not specified in the abstract, otherwise they would be excluded in the first step);
• Reason 3: Peer-reviewed papers comparing agricultural management where cover crops are included as part of, or as a treatment of, the studied factors (management system);
• Reason 4: Peer-reviewed papers with no field experiment, no statement of the experimental design, multi-region experiment, or where results are represented as a model.

2.3. Data Extraction and Qualitative Analysis

All the selected peer-reviewed papers have been thoroughly studied and considered suitable for the construction of three datasets of variables. The first one considered the specific characteristics of each peer-reviewed paper as year of publication and site of the experiment. The second dataset explored several distinctive features defining the experiments as: cropping system, season of cover copping, adopted cash crop (if applicable), duration of the experiment, used cover crop type and genus, and eventual application of other factors (as other agricultural practices) in the totality of the treatments. In the end, the last dataset outlined the effects of cover crops considered as more relevant-indeed, it included the studied parameters as classified in 6 categories: recording of experimental site’s climatic condition, measurement of cover crop-related parameters, detection of weed growth, assessment of soil improvement, quanti–qualitative evaluation of cash crop, and registration of irrigation data. Data are presented in graphs and tables, and a forest plot was used to demonstrate some results of the meta-analysis, including individual studies, and thus achieving an overall result, irrespective of the disparity of the results. The meta-analysis included studies from different experimental studies, a random-effect approach was adopted, and a 95% confidence interval was determined to compare the obtained data [46].

3. Results and Discussion

3.1. Literature Selection

The literature search was carried out according to the PICO model, resulting in the identification of 874 peer-reviewed papers, and after the application of the first step of the PRISMA technique no duplicate was identified. Then the screening procedure led to the eligibility of 103 peer-reviewed papers and the consequential exclusion of 771 records. This exclusion occurred entirely during the first screening step, while among the 103 eligible peer-reviewed papers 68 were excluded in accordance with the reasons described above during the third and last screening steps. In the end, only 35 peer-reviewed papers were included in this work and used for data extraction and meta-analysis. A flowchart of the detailed study-selection process based on the PRISMA technique is presented in Figure 1.

3.2. Overview of the 35 Peer-Reviewed Papers

The 35 selected peer-reviewed papers and some of their characteristics are listed in Table 2. According to the previously described criteria, all the peer-reviewed papers: (i) are published in peer-reviewed journals during the considered period of observation, (ii) were conducted in the field under Mediterranean conditions, (iii) include cover crop treatments combined with at least one other agronomic practice.

The geographical distribution of the included peer-reviewed papers consisted of data from eight countries of Mediterranean basin regions, but also other sites with a Mediterranean climate such as Cape-town (South Africa) and California (USA). More than 60% of studies were conducted in Italy, followed by Spain (14%), Turkey, and the USA (6% each), and only one study in each of Portugal, France, Tunisia, and South Africa. It is interesting to observe that only 3 out of 35 experiments were carried out far from the Mediterranean basin, despite the variability of temperature and precipitation that had already impacted on the natural resources and cropping systems, thus threatening farmers’ income and food security in many other areas under the Mediterranean climate [38]. In light of the proven benefits provided by cover cropping, many governmental agencies encourage the adoption of this practice [47,48,49].

Table 2. Summary of the studies included in the meta-analysis.

	Reference	Study Site
1	Njeru et al. (2017) [50]	Pise, Italy
2	Campiglia et al. (2015) [51]	Tuscia, Italy
3	Mancinelli et al. (2015) [52]	Viterbo, Italy
4	Abou Chehade et al. (2023) [53]	Pise, Italy
5	Antichi et al. (2022) [54]	Pise, Italy
6	Njeru et al. (2014) [55]	Pise, Italy
7	Farina et al. (2018) [56]	Monsampolo, Italy
8	Manici et al. (2018) [57]	Monsampolo, Italy
9	Ciaccia et al. (2020) [58]	Monsampolo, Italy
10	Ciaccia et al. (2016) [59]	Monsampolo, Italy
11	Campanelli et al. (2019) [60]	Monsampolo, Italy
12	Sartori et al. (2022) [61]	Legnaro, Italy
13	Mauromicale et al. (2010) [62]	Sicily, Italy
14	Testani et al. (2023) [63]	Metaponto, Italy
15	Diacono et al. (2018) [64]	Metaponto, Italy
16	Diacono et al. (2016) [65]	Metaponto, Italy
17	Campiglia et al. (2009) [66]	Viterbo, Italy
18	Radicetti et al. (2016a) [67]	Viterbo, Italy
19	Campiglia et al. (2012) [68]	Viterbo, Italy
20	Campiglia et al. (2010) [69]	Viterbo, Italy
21	Radicetti et al. (2016b) [70]	Viterbo, Italy
22	Radicetti et al. (2013) [71]	Viterbo, Italy
23	Alonso-Ayuso et al. (2018) [72]	Aranjuez, Spain
24	Alonso-Ayuso et al. (2020) [73]	Aranjuez, Spain
25	Cabrera-Pérez et al. (2024) [74]	Raimat, Spain
26	Baldivieso-Freitas et al. (2018) [75]	Gallecs, Spain
27	Ordóñez-Fernández et al. (2018) [76]	Cordoba, Spain
28	Ngouajio et al. (2003) [77]	California, USA
29	Araya et al. (2022) [78]	California, USA
30	Burak et al. (2024) [79]	Dipni, Turkey
31	Coruh et al. (2016) [80]	Erzurum, Turkey
32	Perdigao et al. (2021) [81]	Viseu, Portugal
33	Garcia et al. (2024) [82]	Montpellier, France
34	Saadani et al. (2019) [83]	Monastir, Tunisia
35	Le Roux and Swanepoel (2023) [84]	Cape-town, South Africa

Experiments using synthetic inputs, or those with no statement that the field was under organic management, or those in a transition period are considered as conventional systems.

Table 2. Summary of the studies included in the meta-analysis.

	Reference	Study Site
1	Njeru et al. (2017) [50]	Pise, Italy
2	Campiglia et al. (2015) [51]	Tuscia, Italy
3	Mancinelli et al. (2015) [52]	Viterbo, Italy
4	Abou Chehade et al. (2023) [53]	Pise, Italy
5	Antichi et al. (2022) [54]	Pise, Italy
6	Njeru et al. (2014) [55]	Pise, Italy
7	Farina et al. (2018) [56]	Monsampolo, Italy
8	Manici et al. (2018) [57]	Monsampolo, Italy
9	Ciaccia et al. (2020) [58]	Monsampolo, Italy
10	Ciaccia et al. (2016) [59]	Monsampolo, Italy
11	Campanelli et al. (2019) [60]	Monsampolo, Italy
12	Sartori et al. (2022) [61]	Legnaro, Italy
13	Mauromicale et al. (2010) [62]	Sicily, Italy
14	Testani et al. (2023) [63]	Metaponto, Italy
15	Diacono et al. (2018) [64]	Metaponto, Italy
16	Diacono et al. (2016) [65]	Metaponto, Italy
17	Campiglia et al. (2009) [66]	Viterbo, Italy
18	Radicetti et al. (2016a) [67]	Viterbo, Italy
19	Campiglia et al. (2012) [68]	Viterbo, Italy
20	Campiglia et al. (2010) [69]	Viterbo, Italy
21	Radicetti et al. (2016b) [70]	Viterbo, Italy
22	Radicetti et al. (2013) [71]	Viterbo, Italy
23	Alonso-Ayuso et al. (2018) [72]	Aranjuez, Spain
24	Alonso-Ayuso et al. (2020) [73]	Aranjuez, Spain
25	Cabrera-Pérez et al. (2024) [74]	Raimat, Spain
26	Baldivieso-Freitas et al. (2018) [75]	Gallecs, Spain
27	Ordóñez-Fernández et al. (2018) [76]	Cordoba, Spain
28	Ngouajio et al. (2003) [77]	California, USA
29	Araya et al. (2022) [78]	California, USA
30	Burak et al. (2024) [79]	Dipni, Turkey
31	Coruh et al. (2016) [80]	Erzurum, Turkey
32	Perdigao et al. (2021) [81]	Viseu, Portugal
33	Garcia et al. (2024) [82]	Montpellier, France
34	Saadani et al. (2019) [83]	Monastir, Tunisia
35	Le Roux and Swanepoel (2023) [84]	Cape-town, South Africa

Experiments using synthetic inputs, or those with no statement that the field was under organic management, or those in a transition period are considered as conventional systems.

The included peer-reviewed papers span from 2003 to 2024, with a linear upward trend (Figure 2). Despite the low number of overall papers, the trend reflects a growing interest of research on cover crop combination with other agronomic practices under Mediterranean conditions. This trend mirrors the increase in the number of publications on agricultural sustainability transitions [85]. Indeed, cover crops are nowadays recognized as valid tools for increasing crop resilience to extreme weather-related losses [86], reducing yield gap [87], providing ecosystem services [14], as well as ameliorating soil health [88]. Moreover, in general it is interesting to highlight that peaks of these publications coincide with some crucial events rousing public concern around environmental issues, such as COP 15 (late 2009), the Paris agreement (2016), and the Intergovernmental Panel on Climate Change 1.5 °C report (2018).

3.3. Distinctive Features of the 35 Peer-Reviewed Papers

The information characteristics for each of the experiments set up in the 35 peer-reviewed papers are summarized in

Table 3. Cropping system, cover crop season, cash crops, and experimental duration in the 35 considered peer-reviewed papers.

Number of Peer-Reviewed Paper	Cropping System	Cover Crop Season	Cash Crop	Experiment Duration
1	Organic	Winter	Tomato	3
2	Conventional *	Winter	Tomato	2
3	Conventional	Winter	Tomato	2
4	Organic	Winter	Tomato	2
5	Conventional	Winter	Sunflower	3
6	Organic	Winter	Maize	3
7	Organic	Winter	Melon	2
8	Organic	Winter	Tomato/Zucchini	2
9	Organic	Winter	Melon	2
10	Organic	Winter	Zucchini	2
11	Organic	Winter	Melon	2
12	Conventional	Winter	Maize	2
13	Conventional	Winter	Lemon/Grapes	2
14	Organic	Winter	Tomato	2
15	Organic	Winter	Tomato	2
16	Organic	Winter	Cauliflower/Tomato	1
17	Organic	Winter	Potato	2
18	Conventional	Winter	Eggplant	2
19	Conventional	Winter	Pepper	2
20	Conventional	Winter	Tomato	2
21	Conventional	Winter	Eggplant	2
22	Conventional	Winter	Pepper	2
23	Conventional	Winter	Maize/Sunflower	2
24	Conventional	Winter	Maize	3
25	Organic	Winter	Grapes	2
26	Organic	Winter	4 years rotation	4
27	Organic	Winter	Olives	4
28	Conventional	Summer	Lettuce	2
29	Conventional	Winter	Sorghum	8
30	Conventional	Winter	-	1
31	Conventional	Summer	-	2
32	Organic	Winter	Maize	3
33	Conventional	Winter	Grapes	2
34	Conventional	Winter	Lettuce	2
35	Conventional	Summer	Wheat	1

* Experiments that used synthetic inputs or had no statement that the field was under organic management or in transition period are considered conventional system.

. Cover crops were implemented in both organic and conventional cropping systems, with a predominance of the conventional system (54%), which suggests a strong interest in adopting this practice for sustainable agriculture, regardless of the cropping system. Cover crops are recognized as a valid tool for sustainable agriculture [89,90], and their use is advised in organic farming but can be fruitful for conventional systems too. According to [91], about 80% of US farmers not adopting cover crops have stated they are open to considering this practice. Among the reasons that encourage the adoption of cover cropping, these authors cited incentive payments, tax breaks, crop insurance discounts, and soil carbon payments. Indeed, the US incentive program appears to facilitate the adoption of cover crops and results in double the average cropland treated with cover crops (from 50.7 ha to 101.0 ha) [92]. On the other hand, in Europe, eco-schemes have been designed for supporting farmers towards more sustainable farming models by the adoption of practices minimizing the negative agricultural impact on the environment and climate. This aim falls under the CAP 2023-27 incentive program. Despite cover crops’ adoption representing an additional cost, their impact on the farm system, including farm profits, cash crop yields, or both, are sufficient reasons for their use [93]. Looking at cover crops from a wider perspective, according to [94] the positive externalities that cover crops provide are more relevant than the social (institutional) costs needed for their large spread. An example has shown how, by adopting winter cover crops for irrigated cotton, producers had greater productivity compared to conventionally tilled and no-till cotton without a cover crop [95].

Out of the 35 peer-reviewed papers, 33 (91%) experiments spanned from one to three years. Two peer-reviewed papers (6%) were from experiments lasting four to six years, and only (3%) one peer-reviewed paper covered over six years. Experiments over three years represented only 9% of papers, and according to [96] the dominance of short-term experiments (>3 years) is typical of the academic funding/graduation duration but still insufficient for investigating many of the cover crops’ long-terms effects. Additionally, the use of cover crops is not without drawbacks, since the acquisition of a specific level of biomass and retain adequate N in biomass to observe marked effects on soil may be constrained by the ability of the cover crops to grow immediately after harvesting the first crop, thus allowing the cover crops enough time to grow before termination. Thus, the timeframe of cover crop growth may depend on varied factors, specifically unsuitable agricultural practices or weather constraints obstructing the performance of cover crop effects on soil fertility and subsequent plant production [97].

Most of the experiments (almost 91%) focused on winter cover crops. Indeed, using winter cover crops under a Mediterranean climate can help to reduce nitrates, sediment, and total P losses [98], increase N fertility [81], and water content [99], foster the same hydraulic properties [100], and improve organic soil carbon [101]. On the contrary, summer cover crops are not highlighted in these studies. However, summer cover crops could be new opportunities in the Mediterranean area, where crops benefit during drought periods, to reduce leaching during wet periods and to help farmers deal with droughts.

Tomato is the most frequently studied cash crop (25%), likely due to its economic importance to Mediterranean agriculture [102]. The second most often studied cash crop in the considered experiments is maize (14%), despite cereals overall covering only 20 (%) of the total studied cash crops. Among cereals, maize is one of the most widespread, and the ecological intensification provided by this cover crop in intensive farming system has been proven [103]. Multi-year crops represent 11% of the total studied cash crops, and this is quite surprisingly since the use of cover crops in Mediterranean areas has been attempted to reduce the environmental problems caused by “simplified landscape” agronomic strategies in orchards [104].

3.4. Cover Crop Types and Genera in the 35 Peer-Reviewed Papers

Figure 3 shows the list of the cover crop species and categories in the database. Cover crop types were categorized into legumes, grasses, non-legume broadleaves, and mixtures. Legume cover crops are the most frequently used species, and this can be justified because of their high capacity to fix atmospheric nitrogen [20]. Among all the species, hairy vetch is the most widely used, and according to [71], hairy vetch is a well-suited cover crop for the temperate climate of the Mediterranean environment. Moreover, hairy vetch is a frost-resistant species that produces a high amount of above-ground biomass and strongly reduces weed density and richness [51]. Grasses are used 29 times; indeed, plants of this genus gain high biomass that decomposes slowly, thus delivering large amounts of carbon directly into the root [30]. Among the grass CC species, barley (Hordeum vulgare L.) had the greatest frequency, and this can be justified by the high adaptability of barley to the Mediterranean climate. Brassica species had the greatest frequency among the non-legume broadleaf CC species, most probably because of the natural production of allelochemicals useful in weed and pest control [31]. Regarding mixtures, grasses and legumes are dominant, and vetch and barley is the most common mixture.

3.5. Cover Crop Combinations with Other Agricultural Practices

Regarding the combination of practices with cover crops, the total number of considered experiments is 36, since [65] included two parallel experimental designs with different treatments, thus resulting in 29 two-factor experiments and 7 three-factor experiments. Overall, cover crops were combined with 10 other agronomic practices, in detail:

• ✓ 14 experiments combine CC with different termination methods.
• ✓ 8 experiments combine CC with some weed management techniques.
• ✓ 8 experiments combine CC with fertilization plans.
• ✓ 4 experiments combine CC with soil management/tillage procedures.
• ✓ 4 experiments combine CC with the effects on different cash crop types.
• ✓ 3 experiments combine CC with their sowing schedules.
• ✓ 3 experiments combine CC with their termination-time planning.
• ✓ 3 experiments combine CC with beneficial micro-organisms.
• ✓ 1 experiment combines CC with orchard-shading density.
• ✓ 1 experiment combines CC with cash crop irrigation system.

Cover crop termination methods explored in these 36 experiments included: leaving cover crop residues on the soil surface as mulch and incorporating them into the soil as green manure. Moreover, different mechanisms within the same termination type were explored as deep incorporation, shallow incorporation, no tillage, or shredding and roller crimper, and even different combined termination techniques were investigated (mechanical versus mechanical and chemical). Weed management practices were also examined in 22% of the experiments, with treatments comparing mulch, manual, chemical, mechanical, and combinations of some of these methods (e.g., mulch and mechanical, mulch and chemical, and manual and mechanical) versus weedy conditions. Fertilization plans included studies assessing the effects of different doses of the same fertilizer, different chemically organic fertilizers, amendments, and fertilization versus no fertilization. Soil management practices involved various tillage equipment (e.g., moldboard, chisel, or rototiller), types of tillage (e.g., conventional, minimum, no-till), and management systems (organic versus conventional farming system). Cash crop treatments included comparisons between the presence versus absence of a cash crop, consequent or parallel cash crops, or between different cultivars of the same cash crop species. Cover crop sowing date and termination time treatments focused on early versus late sowing dates for meeting better climatic conditions and different cover crop termination stages for assessing the potential benefits for soil of weed and pest control performances. Lastly, irrigation and orchard shading treatments involved comparisons between different levels/system of water supply, and different plant density or foliar coverage.

Combining cover crops with different termination methods has three main agricultural purposes, namely investigating the effects on soil properties, weed development, and water storage capacity [5,105,106]. Despite cover crops appearing to be an effective practice for weed management [5], their effects do not seem to carry over to the subsequent cash crops [107], hence leading to the need for additional weed management strategies. The fertilization plan is often associated with the cover crops’ practice application because cover crops can provide nutrients to soil (especially N), but if not properly arranged they can compete with the cash crop for water and nutrients [12]. Soil fertility, including nutrients availability, as well as weeds, have been considered among the most impacting factors of the yield gap. Therefore, it is also rational that fertilization, termination of the cover crops, and weed management are the most studied agricultural practices in combination with cover crop use.

In addition, the absence of proper management of irrigation could have negative effects on the soil’s physical properties, which in turn could impact the growth of crops and their productivity. In this regard, [8] showed positive results in an experiment that combined a cover crop, tillage, and irrigation, observing the lowest bulk density, highest porosity, and soil saturated hydraulic. Therefore, this proves the relevance of combining an appropriate irrigation method to agronomic practices, which could be an efficient manner to distribute water consumption while achieving the highest water use efficiency, keeping good physical soil properties, and providing optimal conditions for plant growth too [8].

3.6. Studied Parameters in the 35 Peer-Reviewed Papers

The observed parameters that the included articles focus on fall within six main categories summarized in Figure 4. Specifically, the effects of the treatments on cover crop-, weed-, soil-, and cash crop-related parameters were measured, while climate and irrigation were recorded to explore their impacts on the experimental target. Sixty-three percent of the 35 peer-reviewed papers considered climate on cover crop performances and treatments effects as crucial, while 43% of the 35 peer-reviewed papers recorded information about irrigation systems, inferring other measured parameters. Parameters related to CC were determined in 22 out of 36 experiments, and the most often investigated were the biomass provided as above-ground dry weight (82%) and the N provided (55%). In this sense, the specific capacity to apport organic matter and N were considered more important than CC density and physiological traits. Indeed, it is well established that the capacity of CC to increase organic soil matter and N, even in short-term management [108] and despite the quantity of augmentation, is species-, climate-, site-, and management-dependent. Forty-nine percent of the 35 peer-reviewed papers considered weed measurements and weed biomass (as above-ground and dry weight—71%), and weed density (77%). In experiments considering the agronomic benefits provided by cover crops, it is essential to study weed growth and development to understand the effectiveness of a species or a mixture in control. However, to the environmental sustainability and efficacy of this practice in controlling weeds must be added the economical consideration that is not always in favor of cover crops [109]. Considering the weed biomass, the list of peer-reviewed papers does not comply with the one investigating CC biomass, and in both cases (and in most of the considered experiments) below-ground biomass was neglected. This led to a huge underestimation of the root contribution to organic matter and N, especially in the case of legumes, and to the provision of ecosystem services [110]. Soil parameters were investigated in 13 out of 36 experiments, and the most investigated parameter is soil N (85%), followed by organic soil carbon (46%) and soil water content (39%). Organic soil carbon changes occur through drastic modification or during a long-lasting period. In the case of cover crops, the application under a mediterranean climate can take even more than 8 years [111]. Among the considered peer-reviewed papers, only 9% (Table 3) cover long periods of observation (≤4 years), nevertheless, this parameter was investigated in five-times more experiments. Soil water content was investigated in five experiments, despite irrigation being recorded in 15. This highlights the role of cover crops in the conservation and effective use of water under a Mediterranean climate, even in rain-fed conditions [112]. Regarding soil N, the number of peer-reviewed papers is almost similar to the number of those investigating N and provided by cover crops (directly by their biomass or indirectly by N-fixation action). Hence, suggesting an interest in focusing on the contribution of cover crops for balancing the costs of this essential element’s supply to plants [113]. In the end 60% of the peer-reviewed papers measured parameters related to cash crops’ quantity and quality. Specifically, 95% of the experiments considered the yield as a crucial characteristic affected by cover crops in combination with other agricultural practices, and 57% investigated some qualitative species-specific characteristics. In both cases, and in addition to the peculiar qualitative parameters, the intention not to miss the main farm target of producing more and better, while considering in parallel all the other environmental benefits provided by cover crops, was evident [15,19,21,26,50,114]. When considering the benefits of CCs on soil properties, soil bulk density and increased water infiltration was shown as compared to soil with no CCs [115]. A further study showed a mixture of cover crops improving the soil’s physical properties by decreasing the soil’s bulk density up to 5%. Yet, the multi-winter cover crops (https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/winter-cover-crops, accessed on 18 July 2024) improved soil physical properties in relation to monocrop in a no-tillage system, and increased crop yield [116]. Nearly half of the studies featured a brief comment about irrigation practices, such as the type of irrigation system used (citing that it was drip-crop irrigated, for example), but not specifying the irrigation regime. Nevertheless, by considering that the availability of soil water influences soil processes, irrigation management may play a relevant role to better control crop requirements and chemical exchanges, and the transport of nutrients and water. As a consequence, designing proper irrigation management to cover crop growth could help soils hold water, keeping moisture in the root zone and enhancing soil’s physical properties. However, most studies commonly explore various benefits of cover crops only on soil’s physical properties, not including the irrigation impact. Therefore, the combination of irrigation practices and CCs as agronomical strategies could contribute to showing further positive results.

3.7. Forest Plot of Two among All the Studied Parameters in the 35 Peer-Reviewed Papers

A comparison among the results of two studied parameters is shown in Figure 5. In detail, only N in cover crops and N in soil were parameters studied (with comparable methods) in a sufficient number of experiments to produce these graphs. Specifically, in Figure 5A are reported the N amounts (%) in the different cover crops alone and in mixtures. Most of the reported results (9 out of 12) displayed an N content between 1 and 7.29%. Considering Figure 5B, N in soil revealed a heterogenous distribution, but 5 out of 11 experiments revealed a soil N content between 2.5 and 7.29. Moreover, 7 out of the 12 involved are coherent among the two graphs, suggesting the interest of the scientific community in exploring the N dynamics from cover crops to soil, and possibly due to the relevance of this nutrient associated with its high costs [113]. In both graphs, the two orange markers represent the average (circle) and standard deviation (diamond), respectively, to all analyzed studies.

4. Conclusions

Cover crops are gaining increasing attention from the scientific community as a recognized sustainable agricultural practice, able to balance environmental aspects, economic costs, and agricultural benefits. These have demonstrated good performance, even under a Mediterranean climate, where the agricultural sector’s vulnerability is exacerbated by climate change effects. However, cover crops are never applied as a single practice in the real agricultural sector, but rather they are combined with other agricultural practices (factors), which deeply affects their performance. Thus, this systematic review was aimed at exploring the published peer-reviewed papers for identifying the viewpoints of the scientific community around cover crops under a Mediterranean climate. A meta-analysis was carried out to infer the agricultural practices commonly associated with cover crops and the subsequent effects (studied parameters) considered more relevant. To sum up the systematic review results, despite extensive research on the importance of cover crops in agricultural systems, very few studies have explored their effects in combination with other agronomic practices, especially in the Mediterranean region. Only 35 peer-reviewed papers out of 874 were recognized as eligible for the meta-analysis after the application of the PRISMA method. Most of the experiments applied cover crops during the winter season, regardless of the cropping system. Tomato was the most studied cash crop, and most of the experiments covered a short period of time (<3 years). Legumes and grasses, alone and in mixtures, were the most studied genera. Altogether, these characteristics portray typically Mediterranean farm management. Among the studied agricultural practices in combination with cover crop termination methods, weed management techniques and a fertilization plan were the most studied factors, hence revealing an interest of the scientific community in the capacity of cover crops to contribute to nutrient management, weed control, and soil amelioration—three of the heaviest burdens in the yield gap formation. With regard to the considered effects of cover crops in combination with other practices (studied parameters), there is a wide range of considered parameters as having affected or affecting results. However, it should be highlighted that there is a notable research gap in three examinations of factors as irrigation effects, organic matter/carbon dynamics, and physical soil properties, and their combination. Little is known about irrigation practices used, especially the effects of irrigation water distribution on cover crop performance. Systematic review and meta-analysis are robust tools for the exploration of the state of the art. We intended to show here the current results of the benefits of cover crops for improving soil’s physical and chemical properties, while acknowledging that Mediterranean agriculture is not well explored, and observing certain scientific gaps that could be filled by also including synergies between different agricultural practices of diverse cropping systems. Adopting this strategy in a Mediterranean type of climate can contribute to reducing soil degradation while improving the soil’s physical properties, thus increasing the ability of soils to hold water and nutrients. Thus, encouraging investment in irrigation system management is needed to obtain an appropriate crop growth environment, and simultaneously maintaining soil quality through the role played by the cover crop in improving the soil properties. On the other hand, this analysis reveals that only a few studies are currently available for the understanding of how cover crops could have synergistic effects with other practices on soil and yield in Mediterranean agriculture. To conclude, future research should focus on these aspects to develop a more effective combination of practices with cover crops, and to better understand effects on soil in long-lasting experiments. Cover crops have demonstrated numerous benefits for Mediterranean agriculture, and for many other agricultural systems a deeper exploration of the opportunities they provide is mandatory for sustainable agriculture development.