Introduction

Water deficit causes significant problems in arid and semi-arid countries especially with the acceleration of population and economy growth (Al-Busaidi and Ahmed 2014; Padmavathiamma et al. 2014; Ahmed et al. 2008; Prathapar et al. 2005). Therefore, the usage of treated municipal wastewater is a very useful resource for irrigation and industrial purposes (Saffari and Saffari 2013; Ahmed et al. 2005). Because of the rapid growth in Oman’s water demand, studies were conducted to improve the suitability of using treated wastewater for irrigating crops without having any adverse impact of heavy metals. On the other hand, municipal sewage sludge (the end products of treated domestic wastewater) comprises heavy metals and organic chemicals which can cause risk to the food chain and subsequently to human health (Harrison et al. 2006; Zhao et al. 2012; Baawain et al. 2014a, 2014b; Jaffar Abdul Khaliq et al. 2017). Therefore, studies are needed for land application of sewage sludge to avoid any serious risk to the environment.

Municipal sewage sludge fertilizers can improve the physiochemical properties of soil and affect crop yields and its growth (Antonkiewicz and Pelka 2014; Onwudiwe et al. 2014; Baawain et al. 2014a, b; Jaffar Abdul Khaliq et al. 2017). These fertilizers are very rich in macro and micronutrients, which can supply nutrients to plants and increase their fertility (Al-Busaidi 2014b; Zhao et al. 2012).

The aim of this study was to determine the effect of Kala compost (organic fertilizer) and NPK (inorganic fertilizer) on the quality of soil and crops (green beans Phaseolus vulgaris and white radish Raphanus sativus) by measuring heavy metals in soil and in the crops. In addition, the aim was to measure other parameters such as crop yields, TN in soil and crops, TOC in soil and crops, chlorophyll index when both plants were irrigated with groundwater (GW) and treated wastewater (TWW).

Materials and methods

This study was performed in the field of an open area in Agricultural Experimental Station (AES) at Sultan Qaboos University (SQU). There were 6 plots in 3 sites, each plot of 3 m width and 3.5 m length, these plots were divided into two rows by 1 m buffer zone and amended with either 22 kg Kala compost or 0.5 kg of NPK (Al-Busaidi and Ahmed 2016). 5 seedlings (15 days after sowing seeds) of each plant were transplanted in each of half plot as per the treatments detailed in Table 1.

Table 1 Experimental details of municipal sewage sludge on the quality of soil and crops
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The composition of NPK fertilizer was 20% total nitrogen (N): 4.6% nitrate nitrogen, 2.5% ammoniacal nitrogen, 12.9% uric nitrogen, 20% phosphorus pentoxide (P2O8) = 8.7% P and 20% K2O (16.6% K).The composition of Kala compost is given as follows:

  • pH 7.6, EC 31 mS/cm, total organic carbon (TOC) 28.04%, total nitrogen (TN%) 2.6–2.8, moisture content 24%.

  • Cd 1.03 mg/kg, Pb 63.8 mg/kg, Cu 225.4 mg/kg, Zn 519.2 mg/kg, Cr 119.5 mg/kg, Ni 87.07 mg/kg, Hg 1.524 mg/kg, Mo 6.789 mg/kg and Se 0.5096 mg/kg (Haya 2016).

Before growing, all plots were mulched to prevent direct effects of heat on the plants. Each plot was irrigated with 20 L by drip irrigation method with GW and TWW for 15 min daily. TWW was obtained from the sewage treatment plant at SQU, whereas GW was obtained from a well at the AES; the chemical analyses of GW and TWW are given in Table 2. Both plants were planted on 3 October 2015. Radish was grown before beans in a period of 18 days, whereas beans were grown within a period of 1 month.

Table 2 Chemical analysis of GW and TWW (mg/l)
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The field experimental plots were set up in a randomized block design as shown in Fig. 1 and each site had 4 treatments for two crops (four replicates for each treatment were analyzed) and the chemical analysis data were recorded for the statistical analysis. The treatment details of the experiment are illustrated in Table 3.

Fig. 1
figure 1

The design of the study site

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Table 3 Treatment details for two crops in each site
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The third location was ignored as the crop growth was very poor due to operational problems.

Initial physical and chemical soil analysis

Soil samples were collected from each site in all plots from a depth of 10 cm. The samples were air dried, sieved through 2-mm sieve and kept in plastic bags. The composition of soil was determined using the hydrometer method (Klute 1986) and its texture was determined using soil triangle (Brady 1984). EC and pH were measured using pH and EC electrodes (Thermo Scientific Orion 4-star) after applying the saturated paste method (Richards 1954). Soil TOC was analyzed by the FORMACSHT TOC/TN ANALYSER model (SKALAR) and the concentrations of heavy metals were measured using ICP instrument after applying the saturated paste method.

Soil analysis after harvesting

Soil samples after harvesting were collected from a depth of 10 cm from all plots. These were kept in labeled plastic bags, ground and passed through a 2-mm sieve. EC, pH, TOC and concentrations of heavy metals were determined using the same method as described above. TN% was determined by the Kjeldahl method as mentioned by Alkhamisi (2013) and Bremner and Mulvaney (1982).

Plant analysis after harvesting

Biomass of plants and yield

The two crops were harvested two times weekly and washed to remove sand and dust. For their physical analysis weights were recorded and their growth was measured each time (leaf color, surface area and chlorophyll measurements). A measurement of chlorophyll content index (CCI), which is related to nitrogen content in the leaves, was measured by a chlorophyll meter. At the end of each harvesting day, plant samples were kept in plastic bags to be frozen for further chemical analysis.

Plant analysis

In order to determine heavy metals in the plants samples after harvesting period, leaves and roots of the radish crop were cut as well as the edible part of the beans crop and the same method of heavy metals analysis using Wet Acid Digestion method EPA 3050 (http://www.cropsoil.uga.edu/oplank/sera368.pdf) was performed, four replicates of each treatment were analyzed and the chemical analysis data were recorded for the statistical analysis, TN% was also determined using the Kjeldhal method.

Statistical analysis

All data for EC, pH, TN% and heavy metals for soil and plant samples were analyzed using the analysis of variance (ANOVA). Data were analyzed using the computer software JMP (SAS Institute Inc 2013) and 5% probability level was used to determine the significance of the test.

Results and discussion

Soil analysis before harvesting

Soil texture was determined as sandy loam of 70.2% sand, 18.8% silt and 11.0% clay. EC and pH before adding fertilizers were at 1.3 mS/cm and 8.0, respectively. The initial analysis results of heavy metals in soil samples are given in Table 4, these results represent values before adding any fertilizers.

Table 4 Initial values of heavy metal concentrations in soil samples in mg/kg
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Soil analysis after harvesting

EC and pH

Mean EC values in (mS/cm) are shown in Table 5.

Table 5 Mean values of EC in mS/cm and pH of soil samples
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Electrical conductivity values for sites which were amended with NPK fertilizer and were irrigated by GW and TWW as shown in the table above were higher than the sites which were amended by Kala compost. This revealed that, although Kala compost has a high initial EC value of 31 mS/cm, it leached much better than NPK fertilizer. The statistical results of ANOVA for both EC and pH were not significant (p > 0.05) among all treatments (see Table 12), this is in line with El-Nahhal et al. (2013) who found that both EC and pH values were not significantly different among treatments and were high at the top soil profile of a depth ranging from 0 to 30 cm compared to a depth from 90 to 120 cm. This was because the top profile of soil is more acidic than the deep profile and the highest EC values at the upper layers of soils may have been affected due to the decomposition of organic compounds (Nogueirol et al. 2013).

TOC (total organic carbon) analysis

Figure 2 shows the analysis of total organic carbon (TOC) in soil samples.

Fig. 2
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TOC in soil samples in mg/kg

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Soil samples with Kala compost media in the above figure shows a higher TOC value than in the media of NPK fertilizer. This observation is consistent with Singh and Agrawal (2011) and Mi et al. (2016) that the application of organic fertilizers leads to increasing the values of organic carbon compared with NPK fertilizer. In addition, adding municipal sewage sludge compost results in increasing of TOC values (Pena et al. 2015). The TOC in Kala compost-amended soil is 39% (irrigated with GW) which is higher than TOC in soil samples under the application of NPK. The corresponding increase was 14% when soils were irrigated with TWW. The statistical analysis of variance indicated that TOC levels were strongly significant among all treatments (p < 0.05), but both the sites and the interaction of sites with treatments were not significant (see Table 12).

Heavy metal analysis

The results of heavy metals of soils are given in Table 6.

Table 6 Mean values of heavy metals in soil in mg/kg after harvesting
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As appears in Table 6, all concentrations of metals were within the standard range of the limits as shown in Tables 7 and 8. However, Fe has the highest limit in our results, which agrees with the findings of Nogueirol et al. (2013) who reported that Fe concentration was the highest in their study especially at a depth ranging from 0 to 10 and from 10 to 20 cm, showing that Fe was mostly stuck to the mineral fraction found in the soil.

Table 7 Concentration of heavy metals in soils and plants:
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Table 8 Standard threshold limit values of heavy metals in soils and fruits:
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In our experiment, most metals in soil samples after harvesting showed higher concentrations compared to their initial concentration. The increase may be due to the more active chemical processes taking place on the top layer such as the salt accumulation, decomposition of organic compounds and irrigation water quality (Al-Busaidi 2014b), and also for the high retention of heavy metals in the soil with different amendments (Singh and Agrawal 2011). Therefore, the statistical analyses of these elements were not significantly different (p > 0.05) within all treatments (see Table 12).

Plant analysis after harvesting

Biomass of crops and yield

No phytotoxicity signs were detected in this experiment for both crops. As shown in Fig. 3, the yield of both plants was the highest under the application of Kala compost compared to NPK fertilizer. This result is in accord with Al-Toubi (2015) who found that the yield of cucumbers grown was more in Kala media compared to Diwan compost and SQU agricultural compost. In addition, the same finding was found by Al-Saadi (2016) that the yield and biomass of tomato were increased by applying Kala compost. Therefore, applying sewage sludge develops soil physicochemical properties, thus increasing the yield of the plants (Antonkiewicz and Pelka 2014). In our experiment, the yield of beans increased under the application of Kala compost compared to NPK fertilizer. The increases were 22% (TWW) and 77% (GW) for beans and 33% (TWW) and 96% (GW) for radish. The statistical analysis as summarized in Table 12 points that the yield of both crops was significant among all treatments (p < 0.05); the site in radish was significant but not in beans, also the interaction of sites with treatments in both plants was not significant.

Fig. 3
figure 3

Mean yields of two crops per plot in grams

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Total nitrogen % in soil and plant samples

Soil samples showed the lowest levels of TN% compared to plant samples as shown in Fig. 4. This can be explained that nitrogen had been taken from soil and accumulated in the plants. Therefore, the statistical analysis of TN% (ANOVA) in soil samples among all treatments was not significant (p > 0.05) as summarized in Table 12.

Fig. 4
figure 4

Total nitrogen in soil and plants

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In addition, TN% in plants, which were amended with Kala compost and irrigated with either GW or TWW, showed higher values than plants which were amended by NPK. This observation agrees with Al-Toubi (2015) that TN% was higher with Kala fertilizer than the other two types of compost (Diwan compost and compost which is produced by AES at SQU) because Kala contains high amounts of organic matter.

Therefore, in our experiment, TN showed an increase between 13 and 40% for beans and an increase between 10.25 and 21% in leaves for radish under the application of Kala compost when both plants were irrigated with GW and TWW, respectively. However, TN showed no increase in the roots of radish when the plant was irrigated with GW using either NPK or Kala fertilizers, but showed an increase of 3.86% when it was irrigated with TWW under the application of Kala compost.

The statistical analyses of TN % as summarized in Table 12 for all plant samples were not significant (p > 0.05) among all treatments; however, the result was significant (p < 0.05) only for the leaves in radish crop, this may be because the nitrogen level which relates to the chlorophyll content is found in the leaf part of plants (Daughtry et al. 2000).

Measurements of chlorophyll

Chlorophyll contents of two crops were collected two times weekly with four replicates; their mean values are shown in Figs. 5 and 6.

Fig. 5
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Average chlorophyll in beans

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Fig. 6
figure 6

Average chlorophyll in radish

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As shown from the two figures, the average chlorophyll contents in the treatment using Kala compost was higher than in the treatment using NPK fertilizer. Similar results found by Al-Toubi (2015) that chlorophyll content in Kala media had the highest value over the other two fertilizers of Diwan and SQU agricultural compost. In addition, Singh and Agrawal (2011) reported that, chlorophyll measurement was the highest in Spinach plant Spinacia oleracea with organic fertilizer using farm-yard manure. The chlorophyll contents of beans increased by 19 and 13.6% and increased for radish by 41 and 45.6% when both plants were irrigated with TWW and GW, respectively, using Kala compost instead of NPK. The statistical analysis of variance for the chlorophyll contents in both plants showed that the there was a highly significant difference among all treatments (p < 0.05) but not the sites and their interactions were significant (see Table 12).

Heavy metal analysis in plant samples after harvesting

The results of heavy metals in plant samples are given in Table 9.

Table 9 Mean values of heavy metals of beans in mg/kg
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As appears in Tables 9, 10 and 11, Fe has the highest concentrations in plant samples; this agreed with the findings of Nogueirol et al. (2013) who reported that Fe concentration was the highest when using the method of EPA 3052 among the other methods. Therefore, maybe the EPA 3050 method which was used to extract Fe in our study had a similar affect. In addition, Zn concentration also showed high values in all results, which is in line with Ji et al. (2012) and Saffari and Saffari (2013) who found that the high limit of Zn in plants might relate to the addition of organic manure fertilizers.

Table 10 Mean values of heavy metals in roots of radish plant in mg/kg
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Table 11 Mean values of heavy metals in leaves of radish plant in mg/kg
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The statistical analyses of most heavy metals in root and leaves of radish plant as summarized in Table 12 were significantly different (p < 0.05) among all treatments. This is in agreement with Al-Busaidi (2014a, b) who found that the high concentrations of some metals such as Fe, Zn, and Ni in date plant leaves, which were irrigated either by TWW or GW were significant in all locations due to the soil formation and plant growth especially when Kala compost or organic manure was applied. Also, Gupta et al. (2010) found that metal accumulation per gram dry weight of plant tissue is more in the leafy portions than in the roots, and El-Nahhal et al. (2013) found a high concentration of heavy metals in the leaves of Chinese cabbage and corn when irrigated either with treated wastewater or with freshwater in their experiment.

Table 12 Summary of statistical analyses during the experimental study
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On the other hand, the statistical results in our experiment showed that the differences in most trace elements in bean plants were not significant (p > 0.05) among all treatments (see Table 12). However, Zn, Ni Ag and Cd were significant (p < 0.05) with all treatments for this plant.

Generally, all metal concentrations in plant samples were within the standard range of the limits as given in Tables 7 and 8, which indicate no risk of heavy metals accumulation in the plant samples.

Conclusions

The application of sewage sludge to agricultural activities has been a widely accepted for better growth and practice during recent years. The production of agricultural land is promoted with use of sewage sludge, because it is considered that such application will increase productivity in agriculture and solve the problem of disposal as well. Therefore, this experiment showed that Kala compost creates good media for producing higher crop yield as compared to NPK. Further, chemical analysis of soil and the two crops did not show any risk of heavy metal accumulation. Both groundwater and treated wastewater irrigation improve soil characteristics, growth of plants and their yield, especially when Kala compost was used for growing the two crops. Treated wastewater irrigation did not show any toxicity or contamination in the soil and the two plants used. However, there is a need for more continuous long-term experiments (at least 5 years) that will improve the understanding on the effects of composted sewage sludge on soil fertility and crop yield to contribute to the development of sustainable agricultural practices in an arid environment of Oman. Recommendations and guidelines for the farmers can be formulated after 5 years of field experiments.