The land spreading of olive mill wastewater (OMW) derived from olive oil production can represent a suitable option to enrich and maintain agriculture soils under south Mediterranean climates. Therefore, OMW spreading field may represent a low cost contribution to crop fertilization and soil amendment. The main objective of this study was to investigate the long-term effects of raw OMW application on soil macronutrients and phenolic compounds dynamics. The results showed that regular application of three doses: 50, 100 and 200 m 3 ·ha -1 of OMW for nine successive years increased the soil electrical conductivity significantly (p ≤ 0.05%) with the increase of OMW rates at the depth 0 - 20 cm. The pH variations were not detected after ten months of the spreading date. Furthermore, soil sodium adsorption ratio (SAR) and exchangeable sodium percentage (ESP) values were substantially affected by OMW salinity. The soil organic matter (SOM) increased from 0.068% observed for the control sample to 0.2%, 0.34% and 0.48%, respectively, with the increase of OMW rate in the top layer (0 - 20 cm). The potassium, phosphorus and nitrogen increased gradually with the OMW application dose. The Ca2+ contents on soil decreased with the spreading of OMW rate, as referred to control. In addition, the phenolic compounds variations were not proportional to doses applied and its levels remained high as compared with the control essentially on top layers (0 - 40 cm). This practice should be beneficial to organic farming and is an alternative solution to direct spreading of raw OMW on soil.
Mediterranean countries are the major world producers of olive oil with 2,205,300 t produced by EU member states in 2011 [
OMW is known as recalcitrant effluent which characterized by an acidic pH (4 - 5.5), a very complex redox system (conductivity: 6000 - 16,000 µS), a high buffer capacity, tension activity and stability [
To reduce OMW environmental impact, different remediation methods have been developed such as evaporation in storage ponds, physico-chemical and biological treatments [
The present study involves the effect of annual application of three OMW doses (50, 100 and 200 m3∙ha−1); at different successive layers on sandy soil. We investigate somephysico-chemical soil properties such as sodium adsorption ratio (SAR), exchangeable sodium percentage (ESP) and phenolic compounds dynamics. This study was carried over nine years period in olive field located at experimental station “Chaâl” (Sfax, Tunisia).
The soil samples were taken from an agriculture area. The field “Chaâl” is located in experimental station, at 60 Km South-West in Sfax region (Tunisia, latitude North 34˚3', longitude East 10˚20'). The climate of the region is typical Mediterranean with a mean rainfall of 210 mm∙year−1, the average temperature around 27.8˚C in summer and 11.1˚C in winter. The olive-trees field for experimental purposes was divided into four plots (T0, T50, T100 and T200). The latter three were regularly spread with the same annual dose of raw OMW on each January for nine successive years. The experimental plots T50, T100 and T200 have been respectively irrigated with 50, 100, and 200 m3∙ha−1 of untreated OMW [
Ten months after every OMW amendment, soil samples were collected at different layers 0 - 20, 20 - 40, 40 - 60 and 60 - 80 cm for each plot. The field-moist soil samples were sieved (<2 mm), delivered and then stored at 4˚C prior to analysis.
OMW was taken from evaporation ponds at the extraction factory plant in Chaâl, stored at −20˚C and then characterised accordingly before application.
The OMW characteristics depend on the olive variety, climate and the oil extraction method [
Soil analysis for pH, EC, Na, K, Ca, Mg and organic matter (SOM) was performed in triplicate at four differents depths: 0 - 20; 20 - 40; 40 - 60 and 60 - 80 cm. Soil texture analysis was determined using the standard pipette method [
The exchangeable Na+ percentage (ESP) was calculated from exchangeable cations (1) and Sodium Adsorption Ratio (SAR) was determined from Na+, Ca2+ plus Mg2+ (2) in the soil solution [
The calcium/magnesium (Ca/Mg) ratio was found by dividing the quantity of calcium (cmol+・Kg−1) by the quantity of magnesium (cmol+・Kg−1).
Characteristics | Average value |
---|---|
pH | 4.63 ± 0.48 |
EC (dS∙cm−1) | 14.53 ± 0.20 |
TOC (g∙L−1) | 24 ± 0.93 |
COD (g∙L−1) | 87 ± 1.09 |
Minerals matters (g∙L−1) | 13.2 ± 0.28 |
Nitrogen (g∙L−1) | 0.34 ± 0.25 |
Carbon/Nitrogen | 70.5 |
P (g∙L−1) | 0.19 ± 0.049 |
Na+ (ppm) | 1400 ± 0.05 |
K+ (ppm) | 4300 ± 0.05 |
Ca2+ (ppm) | 380 ± 0.05 |
Mg2+ (ppm) | 320 ± 0.05 |
Total phenols (p.p.m) | 4200 ± 0.49 |
Polyphenolic compounds from the soil samples were extracted with ethyl-acetate. The soil polyphenols were monitored as follows: the samples were extracted with ethyl-acetate using a ratio of 5:1 (v/w). The collected organic fraction was dried at 40˚C in a rotary evaporator. The dry residue was then re-dissolved in methanol and the level of total polyphenol was determined by Folin-Ciocalteau colorimetric method using gallic acid as a standard [
On the toplayer (0 - 20 cm), the soils textures are a sandy (sand 94.3%, clay 4.7%, silt 1.3%) with CaCO3 content about 4.3% accordingly a moderately alkaline pH (7.88). This layer presented a low content of organic matter (0.068%) and cation exchange capacity about 6236 ppm as described in
A minimum of three replicates were used for each analysis and field test. Statistical analysis was performed by using the SPSS 16.0. The treatments means were compared by using the Spearman’s test at 5% significance level.
In order to establish the correlation of the different parameters after OMW amendment, principal component analysis (PCA) was adopted. The PCA is a correlation method transforming the experimental variables data to a set of compound axes, noted principal component (PC). This analysis concerned on ten variables per treatment. For all parameters the Spearman value inferior to 0.05 (p < 0.05) corresponded to correlation coefficient. The correlation matrix between parameters showed a significant interrelation and could be classified into groups.
As a vital component of soil fertility, the soil chemical property reflects its potential ability to provide nutrients for plants [
A regular application of three doses 50, 100 and 200 m3∙ha−1 of OMW were amended in châal site. The evolution of pH and EC were determined at different successive layers as shown in
Properties | Soil layer (cm) | |
---|---|---|
0 - 20 | 20 - 80 | |
Sand (%) | 94.3 | 95.4 |
Silt (%) | 1.3 | 3.5 |
Clay (%) | 4.7 | 1.1 |
CaCO3 (%) CEC (ppm) SOM (%) pH EC (µS∙cm−1) | 4.3 6236 0.068 7.88 595 | 8.3 5979.4 0.068 8.12 437 |
Layers (cm) | Doses (m3∙ha−1∙year−1) | ||||
---|---|---|---|---|---|
0 | 50 | 100 | 200 | ||
pH | 0 - 20 | 7.88 | 7.76 | 7.88 | 7.83 |
20 - 40 | 8.05 | 8.12 | 8.12 | 8.13 | |
40 - 60 | 8.13 | 8.13 | 8.13 | 8.16 | |
60 - 80 | 8.16 | 8.16 | 8.19 | 8.17 | |
EC (µS/cm) | 0 - 20 | 595 | 2560 | 3220 | 5610 |
20 - 40 | 472 | 601 | 736 | 1654 | |
40 - 60 | 422 | 515 | 651 | 1560 | |
60 - 80 | 416 | 445 | 506 | 1211 |
significant difference in soil pH after 3 years of successive OMW spreading. This result could be explained by the buffering capacity of the soil which counterbalance the negative effect of OMW [
The analysis of soil organic matter (SOM) in the experimental plots T0, T50, T100 and T200 which have been respectively irrigated with 0, 50, 100, and 200 m3∙ha−1 of OMW was studied as shown in
The soil phosphorus (P) on toplayer increased slightly from 52.5 ppm to 64.5, 69 and 77 ppm with the application of gradual doses 50, 100 and 200 m3∙ha−1, respectively (
layers even with a higher dose (200 m3∙ha−1). This result could be explained by Carreira et al. [
The evolution of nitrogen (N) on soil after long-term repeated OMW spreading doses showed an increase in the top layer (0 - 40 cm) with increasing of gradual doses (
Altough potassium (K+) is not considered as polluant, it is present in OMW with a high concentration about 4300 ppm.
Hence, the sodium (Na+) showed the same evolution recorded for K+ (
high rate was recorded in the upper layers for plot T200. In fact, Mg was retained and adsorbed by soil but considerably less strongly than calcium [
The exchangeable Na+ percentage (ESP) and the sodium adsorption ratio (SAR) were presented in
Polyphenols are the main limiting factor for spreading OMWs because of their phytotoxic and antibacterial ac- tion. Polyphenols content in OMW used in this study was at level 4200 ppm. The phenolic compound concentra- tion in a sandy soil amended with 50, 100 and 200 m3∙ha−1∙year−1 was shown in
Layers (cm) | OMW applied (m3∙ha−1) | |||
---|---|---|---|---|
0 | 50 | 100 | 200 | |
0 - 40 | 2835a | 4275b | 4362c | 4581d |
40 - 80 | 3010a | 3184b | 3446c | 3490d |
to the control. The application of raw OMW increased the phenolic compounds in the upper soil layers to values above the levels registered in the deeper layers. This result indicated mobility of phenolic compounds in the sandy soil texture. Besides, the concentration of phenolic compounds decreased substantially with depth and the value was not proportional to the gradual increase of the OMW doses. Sierra et al. [
In order to test the relationships between macro-nutrient rates on the soil layers and the gradually applied doses of OMW at 50, 100 and 200 m3∙ha−1∙year−1, a Spearman correlation coefficient was used to quantify the strength of the relationship (
The different parameters dynamics (pH, CE, SOM, P, Na, K, Mg, Ca, Ca/Mg, and SAR) were statiscally investigated using a principal component analysis (PCA) at the nine repeated OMW application. PCA is a multivariate analysis technique and it is usually applied in environmental and agricultural studies.
This analysis reveals relationships between available nutrients in the soil treated and untreated plots. Based on the components loading after the varimax rotation (
Parameters | pH | CE | SOM | P | Na | K | Ca | Mg | Ca/Mg | SAR |
---|---|---|---|---|---|---|---|---|---|---|
pH | 1 | |||||||||
CE | −0.47 | 1 | ||||||||
SOM | −0.319 | 0.902** | 1 | |||||||
P | −0.503* | 0.735** | 0.811** | 1 | ||||||
Na | −0.427 | 0.898** | 0.872** | 0.68** | 1 | |||||
K | −0.806** | 0.830* | 0.768** | 0.824** | 0.771* | 1 | ||||
Ca | −0.638** | −0.38 | −0.040 | 0.09 | −0.073 | 0.347 | 1 | |||
Mg | −0.565* | 0.298 | 0.298 | 0.495 | 0.465 | 0.542* | 0.289 | 1 | ||
Ca/Mg | 0.377 | −0.371 | −0.337 | −0.420 | −0.556* | −0.45 | −0.13 | −0.923** | 1 | |
SAR | −0.286 | 0.885* | 0.872** | 0.622* | 0.964* | 0.701** | −0.24 | 0.307 | 0.307 | 1 |
*p < 0.05, **p < 0.01.
and component 2 (PC 2) 69.72% and 14.67%, respectively. The components were considered significant and errors being included the variation and various errors in soil sampling and analysis. This analysis displays the presence of three groups.
The first group, including SOM, P, Na, K, Mg, K, SAR and ESP, showed a positive correlation, justified the mineralization of organic matter to available nutrient for plant growth by the microbial activity and confirm the improvement of soil fertility. The second group including pH and Ca/Mg ratio correlated negatively with group 1 related to the mineralisation of carbon and the subsequent production of OH ions by ligand exchange such as K+, Ca2+ and Mg2+ or to the Na+, brought by this waste which generated NaCO3 of more alkaline hydrolysis than the CaCO3. The third group including Ca2+ showed a positive correlation with group 2 related to the relationship between soil pH, Ca/Mg.
According to PCA analysis, EC, Mg, K, P, SOM, Na and SAR were positively correlated in all soil layers while a negative correlation relationship was found between pH and Ca/Mg in the studied layers.
The yearly application of three OMW doses (50, 100 and 200 m3∙ha−1) for 9 successive years improved the fertility of Tunisian sandy soil, offering the opportunity to recycle the various compounds. This paper focused on soil macronutrients, phenolic compounds level, SAR and ESP. These results may confirm that OMW applied on soil show a positive effect due to the high amounts of organic matter and macronutrients present. Therefore, OMW could be used as a low-cost soil amendment and an effective fertilizer. However, for the applied dose 200 m3∙ha−1, it should take into account the cumulative effect of soil salinisation, which would transform the soil into an unproductive one.
This research was financially supported by Ministry of Higher Education, Scientific Research and Information and Communication Technologies.