agriculture Article The Crucial Role of Soil Organic Matter in Satisfying the Phosphorus Requirements of Olive Trees (Olea europaea L.) Niki Christopoulou 1, Theocharis Chatzistathis 2,*, Efimia M. Papatheodorou 3 , Vassilis Aschonitis 2 and Nikolaos Monokrousos 1,* 1 Laboratory of Molecular Ecology, International Hellenic University, 57001 Thessaloniki, Greece; [email protected] 2 Institute of Soil and Water Resources, Hellenic Agricultural Organization-Demeter, 57001 Thessaloniki, Greece; [email protected] 3 Department of Ecology, School of Biology, Aristotle University, 54124 Thessaloniki, Greece; [email protected] * Correspondence: [email protected] (T.C.); [email protected] (N.M.); Tel.: +30-2310-473429 (T.C.); +30-2310-807572 (N.M.) Abstract: Under high organic matter content, even under low extractable soil P concentrations, sufficient or over-sufficient foliar P levels may be found. This multi-year study aimed at examining the effects of organic matter content and irrigation management on (a) soil fertility, (b) P-cycle related soil enzymes (acid and alkaline phosphatase, pyrophosphatase) and (c) foliar nutrient concentrations. Irrigated and non-irrigated groves of fully productive trees of the cultivar “Chondrolia Chalkidikis” with low organic matter (LOM < 1.5%), medium organic matter (1.5% < MOM < 2.5%) and high organic matter (HOM > 2.5%) were selected for the experimentation. It was hypothesized that olive groves receiving high inorganic fertilization and irrigation inputs (usually with medium to relatively Citation: Christopoulou, N.; low organic matter content) would show higher soil and foliar P concentrations compared to the Chatzistathis, T.; Papatheodorou, non-irrigated groves with higher organic matter content receiving manure applications. Most of the E.M.; Aschonitis, V.; Monokrousos, N. soil variables (including the three enzymes’ activities) were affected by differences in organic matter The Crucial Role of Soil Organic Matter in Satisfying the Phosphorus content. However, organic matter content did not show a significant influence on foliar nutrient Requirements of Olive Trees (Olea concentrations. Olive trees, especially those cultivated in soils with high organic matter content europaea L.). Agriculture 2021, 11, 111. (receiving organic fertilization), can over-satisfy their P nutritional needs, even though soil analyses https://doi.org/10.3390/agriculture show low soil extractable P concentrations, probably due to the high enzymatic activity of acid and 11020111 alkaline phosphatases. The practical conclusion of this study is that P fertilizer recommendations should be primarily based on foliar P rather than on extractable soil P. Academic Editor: Elizabeth Baggs Received: 24 December 2020 Keywords: acid phosphatase; alkaline phosphatase; irrigation regime; pyrophosphatase Accepted: 27 January 2021 Published: 1 February 2021 Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in In recent decades, olive tree cultivation has shifted from traditional, widely spaced and published maps and institutional affil- rain-fed orchards to intensive, irrigated orchards, with higher fertilization rates and tree iations. density supported by mechanical harvesting to maximize productivity and profitability [1]. Enhanced yields and increased nutrient demands make nutrition and fertilization a vital part of the management of olive orchards [2]. Of all the essential nutrients, Nitrogen (N), Phosphorus (P), and Potassium (K) are the most crucial for olive trees’ nutrition. Copyright: © 2021 by the authors. Of these macronutrients, P is of high importance, since it plays a major role in nu- Licensee MDPI, Basel, Switzerland. merous plant physiological processes. Insufficient P availability may significantly depress This article is an open access article the photosynthetic rate, root growth and other functions related to plant growth; thus, P distributed under the terms and deficiency may become a limiting factor to the support of crop yields [3,4]. Soil P can be conditions of the Creative Commons Attribution (CC BY) license (https:// found in various organic or inorganic dynamic pools; nevertheless, P availability for plants creativecommons.org/licenses/by/ is controlled by physical and chemical reactions, including desorption, precipitation and 4.0/). Agriculture 2021, 11, 111. https://doi.org/10.3390/agriculture11020111 https://www.mdpi.com/journal/agriculture Agriculture 2021, 11, 111 2 of 14 biological processes, such as the immobilization of P by other plants and soil microorgan- isms [5]. Despite its relative abundance, a high percentage of P becomes immobile and unavailable for plant uptake [6]. Thus, to overcome phosphate limitation in soils, extended use of inorganic fertilizers has become a common practice, especially in intensive olive groves. Nevertheless, due to the recognized lack of response of olive trees to P application, most fertilization programs overestimate the recommended rates of the applied P [7]. Ex- cessive fertilization in many European countries has led to a build-up of an unavailable P for plant uptake, accumulating soil P pools at unacceptable environmental levels due to the risk of P transfer to aquatic ecosystems [8]. Thus, with the aim of decreasing the high (often unnecessary) P fertilization rates, discussions have recently increased regarding the role and importance of soil organic matter (SOM) on the P nutritional needs of crops. High SOM content in agricultural soils is required to improve several physicochem- ical properties [9]. Additionally, increasing numbers of studies have indicated that soil fertility is significantly improved under increased SOM [10]. This effect is attributed to the positive effect that SOM exerts on soil microbial biomass and activity, as well as on soil enzyme activities, which are closely related to soil fertility [11,12]. Soil enzymes cat- alyze several biochemical reactions, such as the decomposition of soil organic matter and transition between different forms of nutrients [13]. Enzyme activities reflect the magni- tude of these biochemical processes and are considered good indicators of soil quality. Since there is a strong correlation between soil enzyme activities, fertility and nutrient uptake, many researchers have studied the effect of fertilization management on plants’ nutritional status by investigating soil enzymatic activity [7]. A significant part of soil P can be found in several organic or inorganic chemical compounds, which are comprised of polyphosphates, orthophosphates, pyrophosphates, phosphonates, orthophosphate monoesters and diesters [14]. These forms of P can be used as a source of P for plant uptake after the release of phosphate, which is promoted by enzyme activity. Phosphatases are a broad group of enzymes that are produced by bacteria, fungi and plant roots and play a crucial role in organic P cycling, since they transform bounded and unavailable forms of organic P into assimilable phosphate, available for plant uptake [15]. However, under P deficiency, soil biota can enhance the production of extracellular phosphatases, while under high P concentrations, phosphatase production tends to be inhibited [16]. Inorganic pyrophosphatase also plays an important role in soil processes, since it catalyzes the hydrolysis of pyrophosphate to orthophosphate, which enables the absorption of P by plants [17]. Pyrophosphate is abundant in soils and it is mainly originated from soil microorganisms, especially fungi [18]. However, due to the difficulties in the quantification of pyrophosphate in soil solutions, its role in plant nutrition remains poorly understood. Although the influence of P application/fertilization on olive trees’ nutrition and physiology has been studied extensively [7,19,20], no attention has been paid to the com- binational effect of soil organic matter content and irrigation on the satisfaction of the P nutritional needs of olive trees. According to our knowledge, only Chatzistathis et al. [21] indicated the beneficial influence of organic matter on the satisfaction of P nutritional needs in fully productive olive trees; however, in their study, the effect of irrigation on foliar P nutrition was not included, and different SOM levels were not included. Thus, the novelty of this study consists of the combinational investigation of three organic matter levels (low, medium and high) and irrigation on soil and foliar P in mature olive trees. We hypothesized that olive groves receiving high inorganic fertilization and irrigation inputs (with a medium to relatively low organic matter content) would show higher soil and foliar P concentrations compared to the non-irrigated groves with higher organic matter content receiving manure applications. To investigate this, we determined soil and foliar P, and we also evaluated the activities of acid and alkaline phosphatases as well as those of pyrophosphatase, since they are all strongly involved in the P cycle and are indicators of soil quality. The present study aimed at primarily investigating how soils with different organic matter contents (low, medium and high) under different fertilization (inorganic or organic Agriculture 2021, 11, 111 3 of 14 fertilization) and irrigation (rain-fed and irrigated) regimes affect both soil and foliar P concentrations. Finally, another objective of this study was to investigate whether the determination of extractable soil P is sufficient for properly evaluating the nutritional status of