Sustainable Agricultural Systems for Fruit Orchards: History
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Contributor:
Jorge Freitas
,
Pedro Silva

Awareness towards the loss of soil quality as well as consumer perception about the environmental impact of agricultural activity have stimulated research and government activity toward the implementation of a sustainable agricultural system. The European Commission, in the next funding program, established specific objectives to promote the conversion towards a more environmentally sustainable agricultural system through its Green Deal Strategy. The demand for ecologically and sustainably cultivated fruits increases every year; however, suppressing such demand is necessary to improve the production performance of orchards. The sustainable management of orchard production requires combined knowledge from different fields. The key challenge is to design orchard systems that can integrate sustainable practices, nutrient cycle knowledge and promotion of soil biodiversity.

  • sustainability
  • orchards
  • PGPB

1. Introduction

Chemical inputs have driven the productivity of the agricultural sector over the final part of the last century. Such overuse has led to a theoretical limit of agrochemical efficiency, a point from which further application does not directly correspond to further crop yield increments [1].
The pursuit for boosting agricultural yields, to supplement ever-increasing food demand from a rising human population, had progressed to an imbalanced application of agrochemicals to overcome the reduced crop production at higher costs. As a consequence, production costs increased to overcome the reduced crop yield as well as alterations of soil properties, such as its physical, chemical and biodiversity characteristics. The progress of these alterations could disrupt fundamental services provided by the soil ecosystem, such as the water cycle, nutrient cycle, or climate regulation [2]. To overcome the rising environmental and climate problems, there is an incremental effort to develop efficient solutions in all economic activities, including agriculture. Awareness of soil ecosystem importance and associated problems has increased since the beginning of the XXI century (2002) through several initiatives from the Food and Agriculture Organization (FAO) and the United Nations (UN), such as the International Initiative for the Conservation and Sustainable Use of Soil Biodiversity. The peak of such campaigns was achieved with the declaration of the United Nations Decade on Ecosystem Restoration (2021–2030) and the release of the UN–FAO report, the State of Knowledge of Soil Biodiversity [3][4].
In the case of the European Union (EU), similar concerns have been part of agricultural policies for example, the Common Agricultural Policy (CAP) and investment initiatives programs (e.g., Horizon 2020) [5]. More recently, to overcome climate challenges, as well as to protect and improve natural capital and citizens’ health, the EU has released its plan to improve sustainability in all economic sectors: the European Green Deal Strategy. At the center of the plan is the Farm to Fork strategy and Biodiversity Policy for 2030, where the need is emphasized to improve the balance between biodiversity and food systems to increase competitiveness and resilience [6]. Implementing the European Green Deal strategy will generate challenges and opportunities. Even though strategies of appropriate production practices and sustainability programs have been in place for years, large-scale adoption is still lacking. Beliefs about the high costs of sustainable approaches over the expected return in the medium and long term, as well as the lack of knowledge of some producers about the correct implementation of such approaches, could hinder the Green Deal’s implementation [6][7].

2. Sustainable Practices

Sustainable practices are a set of medium- and long-term strategies based on ecological cycles to maintain soil functions and services, as well to provide economic crop production and ecological protection. Nevertheless, the sustainability of the system is more dependent on the choice of management practices other than the farming structure. Alternative practices generally diverge from conventional operations, in the intensity and quantity of soil manipulations as well the nature of nutritional inputs (e.g., large volumes of agrochemicals nutrients vs. organic amendments) [8][9].
Several studies have evaluated the efficiency of sustainable management practices as the pros and constraints of such practices [10][11][12][13][14]. The antagonism between the conventional and sustainable approaches is centered on maximizing economic growth, protecting the environment and providing adequate metrics for evaluation to support evidence of the unsustainability of industrial agriculture and that alternative approaches can achieve adequate yields [15]. On the encouraging side, organic tactics serve higher ecological outcomes, improve soil quality, increase profitability, and present higher nutritional value. On the adverse side are higher costs and prices and lower yields [11]. Furthermore, other factors that influence the adoption of or enrollment in sustainable practices by the farmers were reviewed by Liu et al. (2018) [16]. According to Liu et al., the farmers’ knowledge is acquired as a temporally dynamic learning process, divided into four stages: (1) awareness—they become conscious about the alternative approaches and potential relevance to them. (2) Interest—collection of information about the practices. (3) Trial and evaluation—application on smaller portions of terrain, evaluation of the results and skills development. (4) Adoption and adaptation—the decision to scale up and customize practices in the fields. Among the main attributes that influence this process are farmers’ characteristics (e.g., age, experience, education, heritage, “lifestyle” and environmental consciousness), farm traits (e.g., size, soils, land tenure, type of production) and financial motivation (government subsidies, farm income and off-farm income). Other uncertain associated factors are related to peer pressure, social norms, geographic regions, policies and markets [16].

3. Nutritional Management

Nutrient management considers the estimation of nutrient budgets. This means integrating knowledge about the soil’s nutrient capacity and crop nutrient needs and quantifying the amount of nutrients present in inputs (e.g., manure) to avoid the application of disproportionate nutrient concentrations to the plants and soil. In the case of orchards, nutrient application should be performed cautiously before and after harvest [17]. Nutrients are biochemical elements with organic or synthetic origins that are used by plants and other organisms for their development. For fruit orchards, such nutrients also have a significant role in fruit development [18], nutritional value [19] and pest control [20].
Nutrient deficiency may result in decreased plant quality and/or productivity. It also can induce an imbalance of overall biodiversity since plants reinforce above-ground and below-ground food webs. In addition, appropriate nutrient concentration up to the tolerance levels stimulates the absorption of other nutrients (synergism). The occurrence of excess levels of a particular nutrient may inhibit the accumulation of others (antagonism). Therefore, is necessary to improve plant nutrient efficiency, which requires knowledge about how they are used by the plant considering the development stage of the plant/tree and species specificity [21].
The nutrient cycle refers to the transformation of compounds from the original bedrock and soil organic matter decomposition (SOM), into simple molecules that are assimilable by several organisms and plants. SOM is a complex element of soil because it consists of several carbon sources (e.g., plant, microbial, and animal bodies) in diverse disintegration stages and provides a mixture of heterogeneous macro and micro, organic and inorganic constituents. Therefore, it is an integrated part of the nutrients cycle, with benefic effects on soil properties and consequently on plant development [22]. Factors that influence the nutrients availability and accessibility for plants uptake are climate (e.g., temperature), soil physical properties (e.g., texture, structure, moisture), and chemical parameters (e.g., pH, SOM). Furthermore, nutrient use efficiency is influenced by cover plant chemical composition, as well as the taxonomic and functional diversity of soil biodiversity (e.g., microorganisms) [4]. The soil nutrient pool includes macronutrients as well as micronutrients. The macronutrients are constantly referred to as the most important in any crop or orchard system, due to their impact on plant growth and production. They are represented by nitrogen (N) [23][24], potassium (K) [25][26] and phosphorus (P) [27]. Others that are required but possess a secondary degree of importance are calcium [28], magnesium and sulfur [29].
Knowledge about the nutritional needs of plants is important for an efficient administration not only of the nutrients but also of the adopted management practices. The analysis of plant nutritional status can be achieved by four different approaches: (i) foliar symptoms; (ii) plant tissues analysis; (iii) soil analysis; (iv) biological tests of higher plants or microorganisms. None of the previous approaches should be taken as the optimal method, but should be considered as supplements to each other.

4. Plant Growth Promoting Bacteria (PGPB)

The biological diversity of soil is important to regulate the nutrients cycle and physical properties of the soil, which also influence the provided ecosystem services (e.g., nutrient cycle, water-holding, CO2 sequestration) [30]. The soil is a complex and heterogeneous system, comprising organo-mineral aggregates of different sizes and organic components that create habitats for soil biodiversity across multiple spatial scales; the diversity in habitat composition with pores of different sizes filled with air and/or water allows an incredible number of taxa of different sizes and ecology to inhabit it. Soils are one of the main reservoirs of biodiversity, arranged in a complex heterogeneous system. They can be characterized by size fraction and functional importance [4].
Microbes with a size range of 20 nm to 10 μm (e.g., virus, bacteria, Archaea, fungi) and Microfauna 10 μm to 0.1 mm (e.g., soil protozoa and nematodes) inhabit soil. Their main functional activity encompasses the decomposition of soil organic matter into several macronutrients and micronutrients [31]. Mesofauna range in size between 0.1–2 mm and encompass microarthropods (e.g., mites). They boost the soil’s active biochemical interactions, participating in litter transformation/fragmentation, creating new surfaces for microbial attack [4]. Macrofauna, from 2–20 mm, include large invertebrates (e.g., earthworms). They actively participate in litter transformation and predation, while some are plant herbivores or modify soil structure, improving the energy and nutrient flux [32]. Megafauna (>20 mm) are vertebrates (e.g., Mammalia, reptilians and amphibia). They generate soil spatial heterogeneity as alterations in its profile through movement [4]. 
Among the described taxa, microorganisms have been gathering increasing interest and efforts in scientific works as biofertilizers. They have been recognized as an important influence on nutrient accessibility, uptake efficiency, and the ability to recover soil health and status. Biofertilizers are agricultural supplements that contain live or dormant microorganisms that assist the overall plant growth and yield increments in an eco-friendly way. The main constituent of biofertilizers is root-colonizing bacteria thriving in the plant rhizosphere and bulk soil. They are frequently denominated as Plant growth-promoting bacteria (PGPB). They are common facilitators of plant accessibility to nutrients, and endurance facing biotic and abiotic stresses [33][34].
Most of the PGPB are found in the plant rhizosphere, which is a constricted zone of soil contiguous to the plant root system. This zone, which displays essential ecological functions is colonized by prokaryotes (e.g., archaea, and viruses) and eukaryotes (e.g., fungi, algae). All of the present taxa are potential biofertilizers or important constituents of plant biostimulants [35]. The rhizosphere-specific ecosystem is supported by the synergetic effect of root exudates (e.g., carbohydrates lipids, or amino acids) and soil properties (e.g., pH, bulk density, aeration or water-holding), which directly or indirectly assist in growth promotion and stress management.
The most common bacterial strains used and studied as biofertilizers or soil amendments are: Bacillus sp.; Agrobacterium sp.; Pseudomonas sp.; Arthrobacter sp.; Streptomyces sp.; Sinorhizobium sp.; Serratia sp.; Azospirillum sp.; etc. The interactions between PGPB and plants rhizomes are commonly divided into two mechanisms, direct and indirect processes, that have been the target of several reviews [20][34][36][37][38][39]. Direct mechanisms encompass the processes that have a direct influence on plant performance, among which are: Biological nitrogen fixation; Mineral solubilization/mobilization (e.g., K, P, Zn); and plant growth regulators (e.g., auxin or gibberellin). Indirect mechanisms are related to antagonist activity against pests and pathogens. It also comprises the formation of volatile organic compounds, antibiotics, or biosurfactants, induced systemic resistance, and stress tolerance [20][34][36][37][38][39].
In the case of apples according to Kuzin et al., 2020, fruit yield (kg tr−1), was improved when PGPB was applied (12–13%), in comparison with the control (11%) [40]. Thokchom et al., 2014, applied PGPB to citrus plant and evaluated the growth, registering an increase on plant height (40–55 cm) when compared with the control (31 cm) [41]. Another area of investigation is disease tolerance, as studied by Ali et al., 2022. The application of PGPB reduced necrosis symptoms caused by PSA (Pseudomonas syringae pv. actinidiae) in 92% of kiwi leaves after 10 days [42]. Gani et al., 2021, studied PGPB’s effect on pesticide stress tolerance on peach, detecting the degradation of different concentrations of chlorpyrifos within 30 days, accompanied by the increased production of antioxidants and exopolysaccharides [43]. These examples demonstrate the capability of PGPB when applied in fruit orchards, with promising results.
However, the development of new bio inoculants, their large-scale production and field application have to address specific PGPB characteristics and overcome several operational constraints to improve its efficiency and effectivity. The mere use of primary screening strategies to obtain culture isolates for PGPB traits could result in isolates that perform well in the laboratorial environment but may not be efficient under field application. On the other hand, the discarded colonies might possess different strategies, more suitable to a specific environment due to different mechanisms of action, and be rejected. This might occur because they are not recognized by the standard screening conditions, which might not be suitable to recognize such different approaches. In the case of operational constraints, they consist mostly of required investment, the equipment needed and essential know-how to achieve the mandatory product quality and performance [2][44].

This entry is adapted from the peer-reviewed paper 10.3390/su142113952

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