1. Introduction
Organic fertilization is an alternative approach to supplying the critical quantities of nutrients for crops in the framework of organic and sustainable agriculture without negatively influencing crop yields or the environment [
1]. In many cases, the overuse of inorganic fertilizers has led to the following: (i) soil quality deterioration; (ii) surface and groundwater pollution; (iii) increased greenhouse gas emissions [
2]. Thus, as a result of the recent energy crisis, which has increased the prices of inorganic fertilizers and the cost of production for farmers, it is expected that organic fertilization will play a crucial role in the near future in decreasing inorganic fertilization inputs and improving farmer incomes. There is a strong interest in organic fertilization [
3]. Apart from the beneficial impact of organic fertilizers at reducing production costs, other beneficial roles of organic fertilizers are as follows: (i) increasing soil organic C and matter [
4,
5,
6]; (ii) enhancing soil microbial activities and improving soil quality [
7,
8]; (iii) boosting yields [
8,
9,
10]; (iv) ameliorating fruit quality [
8,
11]; (v) increasing root production [
12]. Different kinds of organic materials and agricultural/food industry bio-products (e.g., olive mill waste composts and pomace, other kinds of composts, municipal wastes, and recycled pruning materials) are used as soil amendments and bio-fertilizers for crops [
13,
14,
15,
16,
17,
18]. In other cases, in order to support organic farming and ecosystem services, decrease fertilizer inputs, and enhance the sustainable management of agricultural ecosystems, the integration of laying hens with orchards and the use of cover crops were suggested [
15,
19].
Animal manures are included among the commonest, cheapest, and easily used organic fertilizers in tree orchards. Manures from different animal origins (e.g., cow, goat, sheep, horse, poultry, and pig) may be used as soil amendments to improve organic C, enhance nutrient uptake, and support plant growth [
5]. According to Therios (1996) [
20], animal manures differ in nutrient content. These differences in nutrient content may be owed either to the kind of animal, animal feeding, or manure preservation [
20]. Chatzistathis et al. (2020) [
5] realized a comparative study among cow and goat manure supplies and the inorganic fertilization of olive plants and showed the beneficial role of both manures to improve organic C, soil fertility, and influence plants’ nutrition and physiology (including WUEi and translocated sugars). These beneficial effects of manure applications depend not only on the kind of animal but also on the rate at which the manure is applied. According to El Gammal and Salama (2016) [
21], both the rate of sheep manure application and the application method had a significant impact on the growth, fruit setting, and fruit qualitative characteristics of guava (cv. “Balady”) trees. According to our knowledge, no further investigations were realized on the effects of sheep manure on the nutrition and physiological performance of olive trees, while no studies at all were realized on the influence of well-decomposed litter from indigenous field vegetation of evergreen broadleaf species (such as that of
Quercus sp.) on the soil’s nutrient availability and nutrient absorption by crops. Based on the above published information, our study was based on the premise that SM and EBLS could influence the nutrient uptake, photosynthetic rate, and WUEi of olive plants.
The aim of our study was to investigate the influence of the following: (i) sheep manure and (ii) well-decomposed litter from indigenous field Quercus sp. vegetation (as well as their combinational application) on the growth, nutrition, photosynthetic rate, and leaf sugar concentrations in potted olives (Olea europaea L., cv. “Koroneiki”). “Koroneiki” was selected for the investigation since it is the most well-known Greek olive cultivar producing high-quality oil (rich in oleuropein).
4. Discussion
The SM was a better source of macronutrients (especially N, P, and K) compared to the EBLS (
Table 1), which is why higher Kjeldahl N, Olsen P, and exchangeable K concentrations were recorded in the SM-treated soil compared to the control and to the EBLS-treated soil (
Table 2). In contrast, higher micronutrient concentrations were determined in the EBLS compared to the SM (
Table 1), and this also influenced the soil DTPA-extractable concentrations of Fe, Mn, and Zn, which were higher in the EBLS-treated soil compared to the SM-treated soil (
Table 2). According to Therios (1996) [
20], the different organic byproducts of vegetal origin (e.g., leaves of different annual or perennial plant species) significantly differ in their N, P, and K contents. It was found that the N content varied from 0.25% (in the leaves of
Lactuca sativa L.) to 2.45% (in the leaves of
Medicago sativa L.), while the P content fluctuated from 0.05% (in the leaves of
Pisum sativum var. saccharatum L.) to 0.50% (in the leaves of
Medicago sativa L.) and the K content varied from 0.30% (in the leaves of
Avena sativa L.) to 2.10% (in the leaves of
Medicago sativa L.) [
20]. Based on these ranges of nutrient concentrations from materials from other plant species, EBLS is a relatively moderate source of N (containing 1.31% of N) for crops, a poor source of P (containing 0.08% of P), and a very poor source of K (containing only 0.31% of K) (
Table 1). Especially for fruit crops (such as
Olea europaea L.), in which high K demands exist (particularly in “on-years” with high crop loads, where high quantities of K are removed after fruit collections), EBLS as an organic fertilizer seems to be of limited importance to satisfying the necessary macronutrient needs (especially for K and, afterwards, for P and N). In contrast, SM seems to be a good source of macronutrients (especially of K and N, containing 2.99% and 2.63%, respectively) and a moderate source of P (containing 0.16%) (
Table 1). Therios (1996) [
20] supports the idea that the different kinds of animal manures (from chickens, cows, horses, sheep, pigs, and goats) differ significantly in their nutrient content. It was found that the highest N content was determined in goat manure (2.77%) and the lowest in pig and cow manures (approximately 0.50–0.55%). Chatzistathis et al. (2020) [
5] found that goat manure’s N, P, and K contents were 2.80%, 0.13%, and 2.54%, respectively, i.e., they were similar to those determined in our study for SM (2.63% for N, 0.16% for P, and 2.99% for K). Thus, it can be concluded that SM is of approximately equal importance for macronutrient supply as goat manure. In contrast to our data, it was supported that SM is of lower importance as an N, P, and K supplier compared to goat manure [
20]. The difference between our data and those of Therios (1996) for SM may be attributed to different feed rations between sheep and/or to different conditions of manure fermentation and preservation [
20].
The micronutrient (Fe, Mn, Zn, and Cu) content of SM (3142, 283, 81, and 18 mg kg
−1, respectively) (
Table 1) was of similar importance to that determined for goat manure in another study [
5]. Therios (1996) [
20] states that the different kinds of animal manures contain Fe, Mn, Zn, and Cu in amounts of 40–460, 5–90, 15–90, and 5–15 mg kg
−1, respectively. Based on these values, it is clear that the SM used in our experiment contained approximately seven times higher Fe concentrations and three times higher Mn concentrations compared to the relevant maximum values quoted by Therios (1996). These differences in the Fe and Zn concentrations between our data and the values quoted by Therios (1996) could be attributed to the different animal origins, as well as to the different feed rations and/or to different conditions of manure fermentation and preservation [
20]. The zinc and copper concentrations in SM (
Table 1) were, however, very close to the maximum values quoted by Therios (1996). In a previous study with cow and goat manure applications in olive plants, it was found that the Fe, Mn, Zn, and Cu concentrations were approximately 2000–3000, 300–370, 78–80, and 15–17 mg kg
−1, respectively [
5], i.e., they were all close to the micronutrient concentrations determined in the present study for SM.
With regard to the influence of SM and EBLS applications on other soil properties, the clear influence of organic materials on organic matter (OM) boosting and the increase in Kjeldahl N should be distinguished. The most beneficial treatment was the combinational application of SM with EBLS, where the OM content was 5.12% (
Table 2). Other researchers also found that manure applications boosted soil organic matter contents [
33,
34]. Apart from the positive influence of organic amendments on soil organic matter, SM and EBLS applications also positively influenced Kjeldahl N, Olsen P, and exchangeable K (
Table 2). These data are in agreement with those found by other researchers [
35,
36,
37].
The main shoot length, total plant biomass, and shoot/root values were not significantly influenced by the treatments (
Table 3). These data are partially in agreement with a previous comparative study between the impact of inorganic and organic fertilizations on potted olive plants, showing that among the different organic treatments, insignificant differences in plant growth were recorded. Significantly higher shoot/root and total plant biomass values were found when the olive plants were supplied with an inorganic controlled-release fertilizer [
5]. In contrast, Mazeh et al. (2021) [
38], who also realized a comparative study between an organic and an inorganic fertilizer application on the growth of young potted and field-grown olive trees, found that the organic fertilizer promoted the growth of both potted and field olive trees (an increment of 22–29% of the trunk cross-sectional area), showing its biostimulant action. In the potted trees, a rapid elongation of the stem (an increment of +30% of the tree height compared to the inorganically fertilized trees) and a higher number of leaves occurred [
38]. In the study of Perez-Murcia et al. (2021) [
37], where agri-food composts were used in organically managed almond trees, it was found that sheep manure applications showed similar results with agri-food composts on yields. Finally, it was found that a significantly higher tree height was recorded in apple trees fertilized with cow manure and compost compared to cases when they were grown on other organic mulches (barley straw, chipped pine bark, forest humus, commercial peat moss, and commercial mycorrhizal substrate) [
10]. Although no direct comparison of our data with those of Kiczorowski et al. (2018) [
10] can be performed due to the different experimental conditions (a pot experiment vs. a field study, respectively), it cannot be neglected that these results are partially in disagreement with our data.
The leaf N and K concentrations were significantly higher either in the SM or in the SM + EBLS treatments compared to the control (
Table 4), which shows the beneficial effect of these organic amendments on enhancing the N and K uptakes by olive plants. Perez-Murcia et al. (2021) [
37] found that the N content of kernels in almond trees did not significantly differ among the organic treatments (composts elaborated from different kinds of waste, such as sheep manure) and the control, while the K content was significantly lower in the control compared to three out of the four composts used as biofertilizers. This partially agrees with our data for the K uptake, although the experimental conditions were different (a field experiment in the study of Perez-Murcia et al. (2021) and a pot experiment in our case). Similarly, for Zn, the SM and EBLS applications positively influenced the Zn uptake by the olive plants, as indicated by the data shown in
Table 5, which also agrees with the results of Perez-Murcia et al. (2021) for the Zn kernel content among the organic treatments and the control trees. In the study of Kiczorowski et al. (2018) [
10], it was found that cow manure, compost, and forest humus applications had similar beneficial effects on the foliar nutrient contents of apple trees [
10]. With regard to the total plant nutrient contents, significant differences among the treatments were recorded only in the cases for the P, Mn, Zn, and Cu contents (
Figure 1 and
Figure 2). These differences may be explained as follows: (i) differences in the dry weight of the plant tissues (
Table 3); (ii) differences in the nutrient uptake (
Table 4 and
Table 5) among the treatments; or (iii) differences in both of them.
Although non-significant differences in the stomata opening were determined among the treatments, the photosynthetic rate was significantly lower in the SM compared to the SM + EBLS (
Figure 3A,B). This could probably be attributed to the significantly lower intercellular CO
2 concentration determined in the SM (
Figure 3C). Despite the fact that the differences in the stomatal conductance were not significant among the four treatments (
Figure 3B), the similar tendency in the stomata opening and intercellular CO
2 concentration (i.e., in the SM and the control, slightly lower stomatal conductance values were determined) (
Figure 3B,C) may explain the differences recorded in the intercellular CO
2 between the following treatments: (i) SM and control and (ii) EBLS and SM + EBLS (
Figure 3C). Cow manure supplies significantly influenced the stomata opening, intercellular CO
2 concentration, and photosynthetic rates of apple trees compared to the control [
39], which partially confirms our data (only for CO
2 concentration, but not for stomata opening or photosynthetic rate, where the differences were insignificant). In our case, the most beneficial strategy to boost the stomata opening, intercellular CO
2 concentration, and photosynthetic rate of olive plants was the combinational supply of SM and EBLS (
Figure 3A–C). Finally, a study on cow manure (CM) and goat manure (GM) applications found that the optimum strategy to boost the photosynthetic rate of olive plants was GM application, while the most beneficial effect for the increase in stomata opening derived from the combinational supply of CM and GM [
5], which means that CM failed to increase stomatal conductance. These data partially confirm our results since, in our case, the CO
2 concentration and photosynthetic rate in the SM treatment were significantly lower and the stomata opening was slightly lower compared to the combinational application of SM + EBLS (
Figure 3A–C).
WUEi provides information on how efficiently water is used by plants (high internal utilization in terms of CO
2 assimilation and low losses by transpiration since it is defined as the ratio between photosynthetic rate and stomatal conductance). Our data showed that a significantly higher WUEi value was found in the SM + EBLS and the control compared to the SM- and EBLS-treated plants (
Figure 3D). Chatzistathis et al. (2020) [
5] found that the type of organic fertilization (cow or goat manure applications) influenced the WUEi in olive plants. Other researchers quoted that WUEi was affected by the K supply [
40,
41], N fertilization, and AMF inoculation [
42]. Thus, in our study, it seems possible that the differences in the nutrient availability and uptake by plants among the treatments (
Table 2,
Table 4 and
Table 5) could be responsible for the differences in the photosynthetic rates of the plants, which afterwards influenced the WUEi. The differences in the K uptake among the treatments (
Table 4) did not significantly influence the stomata opening (
Figure 3B), as it happened in other studies [
5,
41]. Thus, the significant differences in the WUEi values should be mainly attributed to the differences in the photosynthetic rates (
Figure 3A).
The main leaf translocated sugars were sucrose and mannitol, while the main non-translocated sugars were fructose and glucose. Overall, the translocated sugars were significantly lower in the control compared to the SM + EBLS-treated plants, while the opposite tendency was observed for the non-translocated sugars, which were significantly lower in the SM + EBLS treatment compared to the control (
Table 6). Diekmann and Fischbeck (2005) [
43] concluded that soluble carbohydrates in wheat plants varied in response to the N supply, while in potato plants, low rates of N fertilization led to a lower accumulation of total soluble sugars compared to those that were properly fertilized [
44]. In another study [
45], the effect of P on carbohydrates was investigated, and it was found that the P nutrition of pepper plants clearly influenced carbohydrate production, while in other cases, it was found that the foliar K nutrition influenced the accumulation of translocated sugars and the ratio of translocated/non-translocated sugars in olive leaves [
5]. In our study, significantly lower foliar N levels were determined in the control and EBLS-treated plants compared to those treated with SM + EBLS (
Table 4). In addition, the lowest leaf K concentrations, both for the basal and apical leaves, were observed in the control plants (
Table 4). These differences in the N and K uptakes by olive plants might have shown their impact on the differences determined as follows: (i) in the accumulation of translocated sugars among the treatments (
Table 6) and (ii) in the ratio of translocated/non-translocated sugars, as also explained by other researchers. However, further research on this topic will be needed under field conditions to better clarify the impact of nutrient uptake on the accumulation of translocated and non-translocated sugars.