Performance of Medicago sativa Grown in Clay Soil Favored by Compost or Farmyard Manure to Mitigate Salt Stress

: The use of saline water for the irrigation of forage crops to alleviate water scarcity has become necessary in semi-arid and arid regions and researchers have been seeking ways to o ﬀ set the harmful results of soil salinity. Soil amendments with compost, manure and other organic material provide a valuable source of plant nutrients and appear to speed up soil recovery. The aim of this study was to compare the beneﬁts of farmyard manure and a municipal solid waste (MSW) compost (40 mg ha − 1 ) for raising alfalfa ( Medicago sativa , cv. Gab è s) under salt-water irrigation. Both compost and manure improved plant mineral uptake and growth of alfalfa cultivated in clay soil. Using compost in clay soil increased the content of copper (Cu), cadmium (Cd), and zinc (Zn) in plant tissues compared to manure, while the bio-accumulation factor (BAF) of Cu, Pb and Zn was higher in plants grown with manure compared to MSW compost with salt stress. Compost addition could enhance alfalfa growth under salt stress, which depends on salt doses and can greatly improve the recovery e ﬀ ects in a cost-e ﬀ ective way, although additional amendment type should receive special attention in order to be used as a tool for sustainable agriculture.

evaluated the environmental hazard of compost-soil heavy metal using agriculture clay soil under a salt condition by assessing the heavy metals bioaccumulation in alfalfa.

Experimental Setup
The collection of clayey, loamy soil was performed in the Mornag region in the North East of Tunisia (36 • 40 45.52" N, 10 • 17 31.02" E) (0-20 cm deep). The soil was collected just before cultivation and after harvesting and sieved at 2 mm. The MSW compost was obtained using a mixture of separated and shredded organic material from garden waste and household rubbish by aerobic fermentation. The pilot composting station was located in Beja, approximately 100 km west of Tunisia. The cow manure used in the experiment originated from the research agriculture station in the region of Mornag, Tunisia. Table 1 lists the characteristics of compost (Co), manure (M), and clay soil (C). We also checked the inorganic contaminants in the plants as a result of different management, which is a main risk when you add organic amendments. Two weeks after sowing, the stepwise addition of 50-100 mM NaCl to the essential nutrient solution started. The following treatments were performed in six replicated pots: (1) clay soil irrigated with distilled water (C); (2) clay soil supplied with 50 mM NaCl solution (C + S1); (3) clay soil irrigated with 100 mM NaCl solution (C + S2); (4) clay soil amended with MSW compost and supplied with distilled water (Co); (5) clay soil amended with MSW compost and supplied with 50 mM NaCl solution (Co + S1); (6) clay soil amended with MSW compost and supplied with 100 mM NaCl solution (Co + S2); (7) clay soil amended with farmyard manure and irrigated with distilled water (M); (8) clay soil amended with manure and supplied with 50 mM NaCl (M + S1); and (9) clay soil amended with manure and irrigated with 100 mM NaCl solution (M + S2).

Physicochemical Properties of Soil, Compost and Manure
The organic matter, N content, and concentration of several ions in soil and compost extracts were determined. The pH and EC (electrical conductivity) of the soil and compost suspension were measured. Sodium (Na) and potassium (K) were extracted, and the proportioning of the ions was carried out on the filtrate by flame spectrophotometry with a standard Corning photometer. Total nitrogen (N) was determined by the Kjeldahl method. Cu, Pb, Cd and Zn (see Table 1) were extracted using fluorhydric and nitric acids and the concentrations were assessed by atomic absorption spectrophotometry (Perkin-Elmer 3110) [28,29].

Plant Material, Growing Conditions and Sampling
The study was realized as a pot experiment in a greenhouse at an experimental station in Mornag, Tunisia. Seeds of alfalfa (Medicago sativa L.), cultivar Gabès, were sowed in containment pots with 2 kg of clayey loamy soil with or without the addition of manure or compost with the doses corresponding to 40 mg ha −1 and supplied with water to reach 66% of the field capacity along with the addition of 0, 50 and 100 Mm NaCl. The amendments were applied before sowing. The aboveground biomass was cut every 40 days. The soil water status was assessed by the gravimetric method. The weight of pots was controlled every two days and the pots were watered manually to 66% of water holding capacity (field water capacity). Alfalfa shoots and roots were collected 40 days after owing. Plant samples were dried at 60 • C, crushed, and demineralized with nitric acid 0.5% to determine the P, K and Na content. The determination of ion content was performed by the flame, spectrophotometry analysing the standard photometer corning analysing the filtrate of the product of the extraction. The proportioning of P was carried out by colorimetry, on the vegetable extracts decolorized by activated charcoal according to the method of Fleury and Lederc [30].
The determination of concentration of heavy metal content (Zinc (Zn), Lead (Pb), Copper (Cu), and Cadmium (Cd)) was performed, as well. The dried samples of plant shoots and roots were ground into a powder. From each sample, 500 mg was added into the mix containing 2 mL of concentrated HCl (30%) and 6 mL of concentrated HNO 3 (65%). The digestion of the samples was performed in an autoclave for 66 min at 132 • C and the product was determined by Inductively Coupled Plasma Optical Emission Spectrometry (ICPE-9000, Shimadzu, Japan).
The bioaccumulation factors (BAF of shoots) of Zn, Cu, Cd and Pb were calculated as: BAF = Cshoot/Csoil [31]. Csoil and Cshoot represented the metal concentrations in the soil (mg kg −1 ) and the plant shoots (mg kg −1 ), respectively.

Statistical Analysis
Statistical analyses were carried out using SPSS software, v.22. Data were statistically analysed with a one-way ANOVA test, and a comparison of means was performed by Duncan's multiple range test (DMRT) at p < 0.05 level indicating significance.

Results
The present study demonstrated that MSW compost could be used as a natural solution to intensify plant production in food and fodder plant agriculture and improve soil characteristics [32].

Plant Biomass Production and Mineral Status
Alfalfa is widely cultivated as a forage crop due to its excellent forage quality characteristics, forage yield potential, and high adaptability. In Tunisia, alfalfa, especially cultivar Gabès, is the most commonly grown legume crop for forage production. Our results revealed the interaction effects of compost amendment and the salinity of irrigation water on plant yield. MSW Compost and farmyard manure amendments increased alfalfa productivity at a similar rate of 40% compared to the control treatment. In the clay soil, we observed a two to three times increase in the alfalfa plant biomass from the first cut to the third cut ( Figure 1A). However, saltwater irrigation decreased the yield in this soil. Salt stress was more pronounced at 100 mM NaCl. The percent of DW decrease compared to the control irrigated with distilled water was 55% for the clay soil in variant S2. The irrigation with salty water negatively affected plants due to a reduction in osmosis and increase in ion toxicity [33]. Salt stress and nutritional imbalance affect physiological status and are responsible for harmful effects on plant biomass production [34]. MSW compost increased plant biomass of alfalfa by 40% in soils without salt stress. Reported yields of plant dry weight were always higher than the control, indicating that growth was enhanced in the presence of organic amendments ( Figure 1A  Meena et al. [24] found that amendment with organic compost mitigated ion toxicity from saline stress by capturing sodium ions, which were preferentially adsorbed onto clay-humic complexes, while releasing potassium, calcium and magnesium ions from organic matter into the soil solution. Under saltwater provision, either manure and compost amendments led to an increase in the biomass of alfalfa in the second and third cut, but without significant effect in the first harvest ( Figure  1A). In the first year after application, only a fraction of nutrients from manure or compost become available to plants [35]. Kusmiyati et al. [36] found that plant growth was higher with applications of manure to saline soil, and the effect was evident at the second cutting.
Salt stress increased the concentration of N in the aerial parts ( Figure 2A), due to factors of dilution of plant production. Compost increased N concentration at high rates of NaCl. Plant shoots showed no significant differences in N for plants grown in the presence of manure with different doses of salt ( Figure 2A). The phosphorus (P) content was increased by more than 50% in conditions of salinity and compost amendment, and the increase in P by more than 80% was observed when these two factors were combined ( Figure 2B). Application of compost to the saline soil improved soil fertility, which, in turn, enhanced the yield of alfalfa cultivar Gabès and its nutritional status. Available N, P, K, micronutrients, and organic C are the best indicators of soil fertility. Amendment with biological nutrient resources, organic matter management, and slow-release fertilizer are key functions of organic amendments that might be the result of a high level of micro-and macronutrients [37]. The type of compost and the rate of compost application are essential factors that affect biomass production [38]. . Co-soil amended with compost (40 mg ha −1 ). +S1, +S2-variants irrigated with 50 mM NaCl and 100 mM NaCl solution, respectively. C1, C2 and C3 represent three subsequent harvests of above ground biomass done at approximately 40-day intervals. The first harvest (C1) was performed 45 days after sowing. Data are means of six repeats ± SE. The different letters ( abcd ) represent a significant difference between treated samples. p < 0.05 by Duncan's multiple range test.
Meena et al. [24] found that amendment with organic compost mitigated ion toxicity from saline stress by capturing sodium ions, which were preferentially adsorbed onto clay-humic complexes, while releasing potassium, calcium and magnesium ions from organic matter into the soil solution.
Under saltwater provision, either manure and compost amendments led to an increase in the biomass of alfalfa in the second and third cut, but without significant effect in the first harvest ( Figure 1A). In the first year after application, only a fraction of nutrients from manure or compost become available to plants [35]. Kusmiyati et al. [36] found that plant growth was higher with applications of manure to saline soil, and the effect was evident at the second cutting.
Salt stress increased the concentration of N in the aerial parts (Figure 2A), due to factors of dilution of plant production. Compost increased N concentration at high rates of NaCl. Plant shoots showed no significant differences in N for plants grown in the presence of manure with different doses of salt ( Figure 2A). The phosphorus (P) content was increased by more than 50% in conditions of salinity and compost amendment, and the increase in P by more than 80% was observed when these two factors were combined ( Figure 2B). Application of compost to the saline soil improved soil fertility, which, in turn, enhanced the yield of alfalfa cultivar Gabès and its nutritional status. Available N, P, K, micronutrients, and organic C are the best indicators of soil fertility. Amendment with biological nutrient resources, organic matter management, and slow-release fertilizer are key functions of organic amendments that might be the result of a high level of micro-and macro-nutrients [37]. The type of compost and the rate of compost application are essential factors that affect biomass production [38].
Na content was more accumulated in plant subjected to salt stress depending on the dose of NaCl applied ( Figure 2B). The application of compost reduced Na accumulation in plant shoots in contrast to manure treatment at a high dose of NaCl ( Figure 2B). Plants exposed to salt stress showed a high accumulation of Na that inhibited the nutrient uptake, particularly N, P, and K [39]. MSW compost amendment significantly enhanced plant growth under non-saline conditions ( Figure 1A,B). Mbarki et al. [23] drew attention to the ameliorative effect of urban waste compost fertilizer on the growth of alfalfa plants under salt-water irrigation. As reported by several studies [40] plant development was positively linked with nutrient uptake. K + concentration in shoots was decreased in Agronomy 2020, 10, 94 6 of 14 variants exposed to salt stress with the addition of either compost or manure treatment ( Figure 2B). Massa et al. [41] mentioned that reduced potassium absorption by ornamental plants was foremost due to reduced plant biomass production under long-term saline stress. Moreover, enhanced concentration of potassium in the soil may help to counteract passive Na resulting in enhanced potassium ion flow into the symplast [42]. Na content was more accumulated in plant subjected to salt stress depending on the dose of NaCl applied ( Figure 2B). The application of compost reduced Na accumulation in plant shoots in contrast to manure treatment at a high dose of NaCl ( Figure 2B). Plants exposed to salt stress showed a high accumulation of Na that inhibited the nutrient uptake, particularly N, P, and K [39]. MSW compost amendment significantly enhanced plant growth under non-saline conditions ( Figure 1A,B). Mbarki et al. [23] drew attention to the ameliorative effect of urban waste compost fertilizer on the growth of alfalfa plants under salt-water irrigation. As reported by several studies [40] plant development was positively linked with nutrient uptake. K + concentration in shoots was decreased in variants exposed to salt stress with the addition of either compost or manure treatment ( Figure  2B). Massa et al. [41] mentioned that reduced potassium absorption by ornamental plants was foremost due to reduced plant biomass production under long-term saline stress. Moreover, enhanced concentration of potassium in the soil may help to counteract passive Na resulting in enhanced potassium ion flow into the symplast [42].
The compost contains also some other basic ions (Ca, Mg) in the form of bases, such as carbonates, hydroxides and oxides. Thus, the compost addition may prevent soil acidification, improving nutrient availability for plants [43]. As reported by Walker and Bernal [44], more Ca 2+ in the exchange complex can be especially beneficial in saline-sodic soils, eliminating the contribution and irrigated with 0 mM, 50 mM (S1), 100 mM(S2) NaCl water. Data are means of six repeats ± SE. The different letters ( abcd ) represent a significant difference between treated samples. p < 0.05 by Duncan's multiple range test; (B) the concentration of potassium (K) and sodium (Na) in shoots of alfalfa grown for six months on clay soils amended with or without MSW Compost (Co) or farmyard manure (M) and irrigated water containing 0 mM, 50 mM (S1), 100 mM (S2) NaCl. Data are means of six repeats ± SE. The different letters ( abcd ) represent a significant difference between treated samples. p < 0.05 by Duncan's multiple range test.
The compost contains also some other basic ions (Ca, Mg) in the form of bases, such as carbonates, hydroxides and oxides. Thus, the compost addition may prevent soil acidification, improving nutrient availability for plants [43]. As reported by Walker and Bernal [44], more Ca 2+ in the exchange complex can be especially beneficial in saline-sodic soils, eliminating the contribution of Na + in the exchange complex, with positive effects on the soil nutrient properties. Plant growth (dry weight of shoots and roots) increased in the presence of compost in tandem with salt water irrigation for both salt doses (S1 and S2). Analogous results were obtained for manure; moreover, a similar biomass production to the control was obtained. The highest biomass was obtained in variant S1 ( Figure 1A,B).
Under salt stress, compost can raise the mineral nutrient status and growth of plants under salt stress by furnishing greater amounts of N and P [44,45]. However, plant K + status did not improve by the addition of compost ( Figure 2B), probably as a result of the already high K + concentration of the soil used in this experiment (Table 1). Gradual mineralization followed the organic amendment providing alfalfa plants a long-term supply of macronutrients overcoming salt stress that decreases compost decomposition [46]. The result is that organic amendments can be a significant input of trace elements. It leads also to the increase of Ca 2+ in the rhizosphere, replacing Na + on the sites of adsorption. Thus, compost amendment under salt water irrigation is a good strategy for the remediation of sodic and saline-sodic soils.
Chowdhury et al. [47] suggested that soil amendments with farmyard and poultry manure significantly increased the growth, grain and straw yields, K + /Na + ratio and nutrient uptake of the cultivars of rice under saline conditions leading to the improvement of plant salt tolerance.
However, the potency of each organic amendment was different, partly due to their chemical composition, type, origin, and duration of mineralization [48].
Application of both compost and manure to the salinized soil in this study resulted in improved soil fertility, which, in turn, enhanced crop growth and yield and nutritional status. According to Meena et al. [49], the increased N, P and K content promotes various photochemical reactions in the surface affecting the number of the leaves, which leads to enhanced photosynthetic activity. MSW compost alleviated the harmful salinity effects on the growth of plants and the mineral status of soils was improved both in the presence or absence of salt.
An adequate amount of N and its bioavailability in the soil are different ways of promoting biomass allocation in plants. On account of Yadav et al. [50], a high level of nitrogen promotes shoot growth while phosphorus stimulates root growth. Organic amendments to the soil not only enhance the availability of micronutrients in the form of slow-release but also maximize dry shoot biomass. The root-to-shoot biomass ratio depends on the photosynthetic translocation between the above and below-ground parts and is directly proportional to soil fertility [51]. The high biomass of shoots compared to roots ( Figure 1A,B) found in organic amended soil is explained by its potential to increase soil fertility. The ameliorating effect of compost at the same scale of farmyard manure in the presence of salt at low doses (50 Mm NaCl) may be attributed to the significant impact of increased organic carbon and decreased EPS after the application of a combination of farmyard manure and vermicompost in acidic soil [52]. A report by Qadir et al. [53] demonstrated that a rise in the soil Ca 2+ content resulted in the substitution of Na + by Ca 2+ at the soil particle cation exchange sites. Li [54] showed ion homeostasis altered with salt tolerance, where an increase in K + , Ca 2+ /Na + , and K + /Na + in plant tissues and a decline of Na + in the leaf for B. alternifolia was correlated with higher survival rates when compared to W. florida (Red Prince). In the presence of salt stress, compost decreased Na + uptake, while it increased K + uptake by plants ( Figure 2B).
Organic amendments increased the salt tolerance of plants and reduced EPS, EC and increased SOC resulting in a significant increase in crop yield [55]. Tejada et al. [43] demonstrated that the enrichment of soil with organic amendments favoured the flocculation of clay minerals, thereby improving the soil structure. Fertilization with MSW Compost annulled the adverse effects of salinity caused by 50 mM NaCl and to some extent, at 100 mM NaCl. The foremost role of compost in salinity tolerance through promoting an efficient antioxidant system and key C, N and S assimilatory enzymes was suggested by Tartoura et al. [56].

Heavy Metal Bioaccumulation
Boosting nutrient levels with soil amendment is undoubtedly a beneficial practice. MSW composts are considered as the paramount source of metals' input in agricultural soils and are an environmental concern due to their toxicity to plants [57].
The presence of contaminants in MSW composts leads to the accumulation of heavy metal in soil and plant tissues, with negative effects on the environment and human health due to contamination of the ground water and the food chain [58]. The soil type, quality of the MSW compost and plant species determine the concentration of heavy metals that are introduced to the environment by the addition of compost [59].
Accumulation and distribution of heavy metals and its distribution is variable also within individual species. Therefore, selection of appropriate plant genotypes accumulating lower concentrations of heavy metals in consumed parts of plants produced in contaminated soil (Pb and Cd-safe cultivars) can be an efficient alternative strategy to decrease the contamination of the food chain [60].
In our experiment, the heavy metal concentration was significantly elevated in shoots under salt stress ( Figure 3A,B).
environmental concern due to their toxicity to plants [57].
The presence of contaminants in MSW composts leads to the accumulation of heavy metal in soil and plant tissues, with negative effects on the environment and human health due to contamination of the ground water and the food chain [58]. The soil type, quality of the MSW compost and plant species determine the concentration of heavy metals that are introduced to the environment by the addition of compost [59].
Accumulation and distribution of heavy metals and its distribution is variable also within individual species. Therefore, selection of appropriate plant genotypes accumulating lower concentrations of heavy metals in consumed parts of plants produced in contaminated soil (Pb and Cd-safe cultivars) can be an efficient alternative strategy to decrease the contamination of the food chain [60].
In our experiment, the heavy metal concentration was significantly elevated in shoots under salt stress ( Figure 3A,B). Figure 3. (A) Zinc (Zn) and copper (Cu) concentrations (µg g −1 DW) in alfalfa shoots grown for six months on clay soils with or without MSW Compost (Co) or farmyard manure(M) application and irrigated water containing 0 mM, 50 mM (S1), 100 mM (S2) NaCl. Data are means of six repeats ± SE. The different letters ( abcd ) represent a significant difference between treated samples. p < 0.05 by Duncan's multiple range test. (B) Lead (Pb) and cadmium (Cd) contents (µg g −1 DW) in the shoots of alfalfa grown for six months on soil with or without the addition of MSW Compost (Co) or farmyard manure (M) and irrigated water with 0 mM, 50 mM (S1), 100 mM (S2) NaCl. Data are means of six repeats ± SE. The different letters ( abcd ) represent a significant difference between treated samples. p < 0.05 by Duncan's multiple range test. and copper (Cu) concentrations (µg g −1 DW) in alfalfa shoots grown for six months on clay soils with or without MSW Compost (Co) or farmyard manure(M) application and irrigated water containing 0 mM, 50 mM (S1), 100 mM (S2) NaCl. Data are means of six repeats ± SE. The different letters ( abcd ) represent a significant difference between treated samples. p < 0.05 by Duncan's multiple range test. (B) Lead (Pb) and cadmium (Cd) contents (µg g −1 DW) in the shoots of alfalfa grown for six months on soil with or without the addition of MSW Compost (Co) or farmyard manure (M) and irrigated water with 0 mM, 50 mM (S1), 100 mM (S2) NaCl. Data are means of six repeats ± SE. The different letters ( abcd ) represent a significant difference between treated samples. p < 0.05 by Duncan's multiple range test.
Mbarki et al. [40] reported that the heavy metal concentration increased in soil on application of MSW compost, resulting in higher amounts of heavy metal uptake by the plants. Although the compost increased the heavy metals accrual in the shoot parts, without showing any toxic effect, their concentrations were within the permissible limits ( Figure 3A,B).
The heavy metal bioaccumulation not only depends on the amount of metal in the soil, but also the physicochemical properties of the soil, pH and EC, the strength, by which the element is bound in soil, and the regulation of the element uptake by the plant.
The order of heavy metal uptake by alfalfa was Zn > Cu> Pb > Cd, showing the relative lability of zinc, which is ready to transfer to plant tissues. This metal is often present in the soil enriched by the compost in higher concentrations in comparison with other heavy metals assessed in this study [61]. In turn, binding of Cu in soil is much stronger and its accumulation by plants is better regulated compared to Zn [62]. On the other hand, some studies showed that the addition of MSW compost was a good approach to remediate the soils rich in labile elements as the adsorption processes eliminated the overall bioavailability of heavy metals [63].
The supply to the soil of MSW compost with the background concentration of heavy metals (Cu and Zn) reduced their extractability in comparison to an untreated control [64].
Plant availability of the lead (Pb) is very low in soil amended with the compost [64,65], and the risk of contamination to the food chain by this element is low, as it is blocked by the soil-plant barrier [66]. The order of heavy metal transfer depended on plant, soil and compost types, as Aylaj et al. [38] ascertained that regular compost use decreased heavy metal content in leaves.
Heavy metals accumulated mostly in leaves and roots and the transfer of metals to roots increased in the order: Cr (0.12-1.64) < Zn (0.87-1.21) < Mn (0.3-1.34) < Cu (0.28-2.28) < Co (0.53-4.10) < Fe (1.08-2.14) [67]. It was shown that Pb is preferentially bound in compost into stable forms, such as sulphide, having a low availability to the plants [67]. Gigliotti et al. [68] studied the mobility of Pb from calcareous soil (pH 8.3) to maize plants and found a decreased transfer of lead ions in variants amended for six years with a high dose (90 mg ha −1 ) of a mechanically-segregated MSW-compost.
The interaction between the salt and compost treatments were well evident, being also significantly influenced by the heavy metal concentration ( Figure 3A,B). Salt stress led to an increase in metal uptake in non-amended soil variants, whereas in compost-treated soil this trend was not observed.
The risk from heavy metal load was assessed using the bioaccumulation factor (BAF) for individual metal elements assessed in shoots.
The BAF of Zn, Cu, and Pb was lower than 1, independent of treatments ( Table 2). As noted in previous studies, salt stress augmented BAF for plants cultivated on soil without amendments (Table 2). Both the BAF and transfer factor (the ratio of concentration of heavy metal in plant related to the metal content in soil) for Cu and Zn in basil and Swiss chard cultivated in the pot experiments using sandy loam soil decreased in soil with the addition of the compost. This meant that the relative bioavailability of both Zn and Cu were lowered by the amendment of compost compared to the control without additions [64]. Table 2. Bioaccumulation Factor (BAF) calculated for metal contents (Zn, Cu, Pb and Cd) in alfalfa grown in clay soil (C) amended with or without MSW compost (Co) and Farmyard manure (M) on irrigation with water containing 0 mM, 50 mM (S1) and 100 mM (S2) NaCl.

C
C + S1 C + S2 Co Co + S1 Co + S2 M M + S1 M + S2 Our results showed that MSW compost containing some heavy metals seemed to be useful as an organic fertilizer in agriculture with a low risk of phytotoxic effects due to excessive accumulation of Zn, Pb and Cu in plants even under salt irrigation [23,69]. However, the future consequences of regular amendment of the compost on soil texture and the bioavailability of the metals and its impact on plant performance need to be considered. Application of MSW compost may lower the metal availability by getting the soil pH higher [70], as the heavy metal availability mostly drops when the soil pH increases [71]. The addition of compost did not increase the BAFs with the effect of salt stress, whereas the manure addition showed an increase in the BAFs in the presence of high doses of NaCl, which is an important issue related to crop performance and quality for plant productivity and health safety.
However, the apparent relationship between the contents of Cu, Pb, Zn and Cd in plants and in compost can also be affected by other co-factors such as salinity that may influence the growth rate of the plants and the level of dilution of the concentration of heavy metal particles in plant tissues. On the other hand, the results of the plant transfer correspond to the data on DTPA extractable Cu fraction [72]. However, many researchers questioned the use of organic manures as remediation agents in soil systems, claiming that manures could be a source of contamination as well as remediation [73]. Salt stress increased the uptake of heavy metals by plants grown without compost compared to the amended soils ( Figure 3B and Table 2), leading to the remediation effect of compost in saline soils. For example, salt concentration and toxicity are more relevant compared to metal content in compost, especially in semi-arid and arid regions [74]. The applied dose of 40 mg ha −1 of compost increased Pb, Cu, Cd and Zn levels ( Table 2), but they remained within an acceptable range in plants [59]. On the other hand, the metal accumulation did not affect the obtained yields of alfalfa.

Conclusions
Mature MSW compost can be used successfully as a fertilizer for clay soil as its favoured nutrient content may improve the plant nutrition, development and growth as well as tissue metal content without overtaking admissible limits. Moreover, the use of this organic amendment under salt stress may represent an efficient way of increasing alfalfa biomass production.
The maximum effect on alfalfa growth was achieved by the utilization of MSW compost at a rate of 40 mg ha −1 without any toxic effects on the plant. Using 40 mg ha −1 of MSW compared to 40 mg ha −1 farmyard manure with salt water irrigation can be an excellent strategy to enhance the heavy metal remediation of saline soils, and to allow the mitigation of the salt stress and heavy metal effects on alfalfa production.