Contribution of a Seeded Cover Crop Mixture on Biomass Production and Nutrition Status Compared to Natural Vegetation in a Mediterranean Olive Grove

Abstract

Intensive agricultural management practices (i.e., the burning of pruning residues, the absence of organic amendments) lead to a reduction in organic matter and nutrients in the soil resulting in agroecosystem vulnerability. Implementing a cover crop would provide soil organic matter while increasing nutrition levels in the soil. A mixture of cover crop trial in sandy loam soils under Mediterranean climatic conditions was conducted in a rainfed olive grove in Western Crete. In this study, the dry biomass, macro- and micronutrition, and carbon status of the seeded cover crops (legume and grass) were compared to natural plants in an olive grove. Seeded cover crops were conducted in two sustainable management systems (cover crops solely, and a combination of compost, pruning residues, and cover crops); natural plants were in a conventional system involving soil tillage. In combination with conservation tillage practices, the addition of carbon inputs may improve soil fertility. Results indicate that the dry biomass production and C content of cover crops under sustainable management systems was significantly higher than that of the control. The higher dry biomass production and C content found in cover crops compared to the natural vegetation indicates not only that this type of management provides enhanced carbon storage, but can also potentially lead to a future increase in soil organic matter through decomposition. Higher dry biomass is important in the context of carbon sequestration, and cover crops facilitated carbon storage in this study. In addition, this study suggests that sustainable agricultural management practices would provide significant benefits in terms of nutrient retention and CO₂ fixation, thus improving ecosystems in Mediterranean countries.

1. Introduction

Owing to its longevity, edible fruit, and storable oil, the olive is one of the most important socioeconomic tree crops, a domesticated and extensively cultivated fruit tree in the Mediterranean countries [1]. The area under olive trees in the European Union (EU) accounted for about 4.6 million ha, of which the most significant area under cultivation is located in Spain (55%), followed by Italy (23%), Greece (15%), and Portugal (7%), in 2017 [2]. EU Mediterranean countries supply approximately 95% of the world’s demand for olive oil [3]. After Spain and Italy, Greece is the third largest country in the world in olive oil production, with an average annual production of 225 t olive oil [4].

In olive orchards, inappropriate cultivation practices such as intensive tillage, inappropriate use of agrochemicals (herbicides, pesticides, mineral fertilizers), and burning of pruning residues in situ, in combination with the Mediterranean climate, lead to soil organic matter depletion, erosion, and biodiversity loss [5,6]. The effect of the Mediterranean climate (hot dry summers and irregular rainfall distribution) due to rising temperature and droughts has a direct impact on climate change [7,8], which leads to negative impacts on soil health [9]. It has been reported that strategies based on modification in soil management techniques could improve soil fertility, enhance soil carbon (C) storage, and reduce atmospheric CO₂ [10].

Furthermore, climate change mitigation through the adaptation of sustainable agricultural practices that enhance agroecosystem diversity and resilience is essential to avoid the negative consequences of a global temperature increase [11,12]. Therefore, the management of olive farms should be improved with the available adaptive practices, policies, and tools to build resilience to climate change in the Mediterranean basin. Carbon storage as soil organic matter (SOM) or plant biomass plays an essential role in mitigating climate change as C is removed from the atmosphere [13]. Moreover, enhancing C balance in olive groves can improve soil fertility and biodiversity, increasing agroecosystem resistance to adverse biotic and abiotic conditions [14].

The United Nations Sustainable Development Goals emphasize the importance of soils in order to achieve sustainability [15,16]. In this context, several authors have proved that the use of sustainable practices based on organic matter inputs, such as the use of cover crops, have restored and maintained soil quality in Mediterranean climate conditions [17,18], since these practices increase microbial biomass, activity, and complexity [19]. Legume species, nitrogen-fixing plants, have been implemented as green manures and considered a more ecologically sustainable alternative to synthetic fertilizers [20]. On the other hand, grasses provide a higher level of C; however, they offer less N (nitrogen) than legumes. For this reason, the use of two cover crop species together, usually a legume and grass, can provide varied benefits compared to monocultures [21]. Vetch and oat combination has been considered one of the most promising cover crops for biomass under irrigation conditions [22]. The production of a legume and grass potentially leads to a lower C/N ratio, resulting in improved availability of P (phosphorus) and synchronized N mobilization for the main crop [23]. Recently encouraging studies revealed that the presence of cover crops, particularly T. subterraneum, for a long period of time (3–4 years) improved SOM, macro- and microelement levels, and increased ammoniacal nitrogen, nitric nitrogen, and beneficial bacteria [24,25].

Cover crops can help to achieve good soil, which is important for production and plant health. However, cover crops are not commonly used by farmers. Additionally, no state-of-the-art approach that clarifies the impact of service crops has been established so far [26]. In this context, the objective of this field study was to compare the dry biomass production, C content, and nutrient content of seeded cover crops to those of natural plants grown in an olive orchard.

2. Materials and Methods

The study was implemented during 2020–2021. We used sustainably managed 50-year-old olive trees (Olea europaea L., Kalamon; 1.1 ha, distances between trees 7 × 7 m) in the experimental field of the Institute of Olive Tree, Subtropical Crops and Viticulture, Nerokourou (35° 28′ 36.76″ N, 24° 02′ 36.44″ E, 51 m). According to the Institute’s meteorological station, the yearly average air temperature was 20 °C, the relative humidity (RH) was 61%, and the annual rainfall was 574.20 mm during the year of implementation, October 2019–October 2021 (Figure 1). The soil at the study site is sandy loam (clay 6.8%, silt 28.0%, sand 65.2%) with a pH of 7.2. and with macroelement content of 7.24 mg kg⁻¹ NO^3- Ν, 8.53 mg kg⁻¹ available P, and 72 mg kg⁻¹ ex. K, at 0–40 cm depth.

Nine plots, each covering an area of approximately 200 m² of the field, including 4 trees with uniform olive tree canopy size and natural vegetation, were selected according to a completely randomized design (Figure 2). Three treatments with three replicates (n = 3) each were studied; cover crop solely (COVER), cover crops combined with recycling of chopped pruning residue, and olive mill byproduct compost (ALL), together with the control (CON). CON plots were characterized exclusively by a soil tillage practice, with no additions of organic material taking place, except for fallen olive leaves and twigs. Concerning treatments involving cover crops, we seeded 104.6 kg ha⁻¹ of pea (Pisum sativum L.) seeds, 52.3 kg ha⁻¹ of vetch (Vicia sativa L.) seeds, and 26.1 kg ha⁻¹ of oat (Avena sativa L.) seeds as a mix in December 2020. The sowing was done manually, and the seeds were incorporated into the soil by a tractor with a superficial rotary tiller pass. Regarding ALL, the chopped pruning residues from the olive trees from the same grove were incorporated into the soil in December 2020 as mulch without tillage. Pruning residues consisted of olive tree leaves and twigs with sizes up to 7 cm in length and containing 51–55% total C, 0.6–1.8% total N, 0.4–1.2% total K, and 0.4–1.2% total P, on a dry matter basis. The compost, derived from recycling olive mill byproducts from a three-phase olive mill mixing olive leaves, fruit pulp, stones, and liquid waste, was implemented in the soil without tillage at a 12.5 t ha⁻¹ in December 2020. The applied compost was characterized as follows: 18 C/N, pH 7.8, 49.76% total C, 2.77% total N, 2.26% total K, and 0.18% total P, on a dry matter basis.

The area covered by cover crops and the number of individual plants and species in 0.25 m² quadrats in the COVER and ALL plots were determined in April 2020. Consequently, the number of legume and grass cover crop seeds used per hectare was compared with the number of grown plants per hectare. Thus, the proportion of germination was obtained. Germination rate of both legume and grass cover crops in the ALL and COVER treatment during the year of measurement is presented in Figure S1.

Furthermore, the nutrition status of the natural plants in the CON plot and seeded cover crops in the ALL and COVER plots were analyzed to predict the evaluation of the cover crops in releasing nutrients in the soil compared to natural vegetation after they are terminated. The natural plants were the typical plants of the area, mainly Oxalis pes-caprae and a small percentage of Apera spica-venti and Hordeum murinum. Three random samples from each plot of all herbaceous plants in the CON, and cover crops in COVER, and ALL plots were taken by collecting all the plants within the 0.25 m² square from the center of the rows in April 2021, corresponding to the peak period of biomass for legumes [27]. The aboveground plants and roots were carefully cleaned from the soil, dried in an air forced Memmert oven at 65 °C for 2 days, and weighed as dry weight.

Once the materials were dried, they were ground for analyzing mineral nutrition content. The N (%) was determined with the standards and the sample solutions described by Evenhuis [28] and using a Hitachi 1100 visible–UV spectrophotometer at 662 nm. The potassium (K, %), phosphorus (P, %), calcium (Ca, %), magnesium (Mg, %), iron (Fe, mg kg⁻¹), zinc (Zn, mg kg⁻¹), manganese (Mn, mg kg⁻¹), and copper (Cu, mg kg⁻¹) contents were determined using a precalibrated inductively coupled plasma optical emission spectrometer (ICP-OES). Boron (B, mg kg⁻¹) concentration was determined by colorimetry using a photoLab 6100 VIS spectrophotometer. The C (kg C/ha) content was determined (t ha ̶¹ year ̶ ¹) by multiplying the dry weight of plant materials with the conversion factor 1.755, obtained considering the CO₂ molecular weight and the following relations [29]:

1 g of dry matter = 0.4782 g of C

(1)

1 g of C = 3.67 g of fixed CO₂

(2)

Differences between biomass and nutrient content of the cover crop and natural vegetation were analyzed using SPSS (SPSS Inc., Chicago, IL, USA) and were subjected to analysis of variance (ANOVA). Mean values were compared using Tukey’s post hoc test for p ≤ 0.05.

3. Results and Discussion

Figure 3 represents the dry biomass of spontaneous weeds in the CON plot and seeded cover crops in the COVER and ALL plots. Significant differences were observed in dry matter content in seeded cover crops (F: 22.247, p < 0.05). The highest (8436.78 kg ha⁻¹ year⁻¹) values of dry matter were observed in ALL (dry biomass was also high in COVER, 8318.38 kg ha⁻¹ year⁻¹) and lowest in CON (2417.20 kg ha⁻¹ year⁻¹). Additionally, it has been recorded that spontaneous plants produce less biomass than seeded cover crops [30]. The dry biomass generated by the cover crop not only plays an important role in C sequestration [31], but also in soil characteristics and nutrient dynamics [32].

Plant analysis of the aboveground and belowground cover crops in ALL, COVER, and CON plots was carried out to compare and evaluate the nutrition status of the cover crops in the two sustainable management systems and the natural vegetation in the control. Figure 4 shows that the N content was significantly higher in the cover crop of ALL (2.70%) (N was also higher in COVER (2.50%) with no significant differences) compared to CON (2.34%) (F: 4.972, p < 0.05).

The C content was significantly higher in COVER (2337.91 kg ha⁻¹ year⁻¹) and ALL (2303.42 kg ha⁻¹ year⁻¹) compared to CON (1155.90 kg ha⁻¹ year⁻¹) (F: 6.603, p < 0.05).

The C content of cover crops from our study has a pattern similar to that obtained by Rodríguez-Lizana et al. [33]. Moreover, the presence of cover crops in olive groves leads to an increase in soil organic carbon (SOC), which is supported by the study results of Repullo-Ruibérriz de Torres et al. [34].

However, the Ca (F: 45.215, p < 0.05) and Mg content (F:33.792, p < 0.05) was significantly higher in CON (1.77% Ca, 1.33% Mg) compared to ALL (1.14% Ca, 0.23% Mg) and COVER (1.06% Ca, 0.24% Mg). The Fe (F: 4.895, p < 0.05) was significantly higher in ALL (437.65 mg kg⁻¹) compared to CON (312.11 mg kg⁻¹), whereas Zn (F: 16.145, p < 0.05) was significantly higher in COVER (58.37 mg kg⁻¹) followed by ALL (55.25 mg kg⁻¹) and compared to CON (37.93 mg kg⁻¹) (Figure 5).

The observed increases in cover crop N, Fe, and Zn content in the ALL treatment could be attributed to the synergic action and beneficial effects of combined sustainable management practices by cover crops, compost, and pruning residues. The higher C, P, and Zn content in the COVER treatment compared to the CON treatment could be due to the interactions between cover crops or a combination of sustainable practices and wild vegetation.

A significantly higher concentration of Cu (F: 4.659, p < 0.05) in COVER (15.55 mg kg⁻¹) compared to ALL (10.48 mg kg⁻¹) was observed. The B (F: 116.663, p < 0.05) and Mn (F: 26.662, p < 0.05) levels were significantly higher in CON (112.49 mg kg⁻¹, 411.34 mg kg⁻¹, respectively) compared to ALL (28.34 mg kg⁻¹, 65.57 mg kg⁻¹) and COVER (30.09 mg kg⁻¹, 94.66 mg kg⁻¹).

Boron is an essential nutrient in olive groves. Its fundamental function in the olive tree is improving flower fertility and fruit set, and a deficiency leads to yield loss. The higher B content in natural vegetation compared to seeded cover crops may lead to B deficiency in the olive field [35]. On the other hand, the soil of the experimental site is nonalkaline; thus, it is more vulnerable to B leaching. Moreover, since the soil is nonalkaline, the significant Ca, Mg, and Mn content in natural vegetation may also lead to a deficiency of these elements in the soil.

Long-term application of cover crop mix for three years successively enriched soil nutrition and improved soil biological properties [36]. Moreover, a recent study [37] reported that C, N, and P had lower release values from residues of natural plants compared to seeded cover crops due to the spontaneous plants yielding less biomass than the seeded cover crops.

4. Conclusions

Under realistic field conditions, the seeded mix of cover crops in this experiment seems to be more promising than natural plants for enriching the soil fertility by providing higher biomass and C. The C content was significantly higher in COVER (2337.91 kg ha^{−1 −1}) and ALL (2303.42 kg ha⁻¹ year⁻¹) compared to CON (1155.90 kg ha⁻¹ year⁻¹). These findings can help develop a better understanding of conservation agriculture and estimated nutrient releases in olive groves; they can be recommended to farmers and other recipients of the study.

We conclude that this study proves that cover crops in olive orchards are a potential strategy to contribute to climate change mitigation. Long-term monitoring of cover crops is ongoing to better understand the potential role of cover crops in nutrient retention. These results can provide and boost agroecosystem services in olive groves.