*3.2.1. Sampling area*

The soils were sampled from the natural reserve "El Cortijo", in which the dominant vegetation is mesquite (*Prosopis laeviginata*), located in Dolores Hidalgo, Guanajuato, Mexico. The soils were collected from two sites: under the canopy and outside the canopy of mesquite tree. The average altitude of sites is between 1750 and 2000 m above sea level and the average annual rainfall is between 400 and 600 mm (mainly from June to September).

### *3.2.2. Tannery sludge*

Tannery sludge produced during leather manufacturing was sampled from a tannery in Leon (Guanajuato, Mexico). The sludge contained large quantities of hair, soluble proteins, and fatty fleshings from processing the skin to hide, and sulphide, lime, chromium-sulphate, salts, dyes, acid, and leather trimmings from processing the hide to leather. Two tannery sludge samples were collected in two different tannery industries with different Cr concentrations. A third tannery sludge was sampled from a tannery in Leon where the fleshing sludge was separated from the leather at the beginning of the process and then followed by chemical treatments. The fleshing waste contained small quantities of hair, soluble proteins and fatty fleshing from the processing of skin to hide. The three different sludge samples and fleshing waste were used in different studies with the same semi-arid soils. Chemical characterization is shown in **Table 1**. Tannery sludge was air-dried before used for the experimental aerobic incubation.


**Table 1.** Characteristics of tannery waste from Leon Guanajuato, México.

Amounts of tannery sludge 1 and 2 added to soils were in accord with the criteria of the amount of N covering the recommended dose of N for maize crop growth in the region. The amount of tannery sludge 1 which amended soils from outside and under the canopy of mesquite trees was 1.5 g of wettannery sludge to 50 g of soil. The total amount added to under and outside the canopy soils was approximately 1308 mg C kg−1, 320 mg total N kg−1, 45 mg total P kg−1,and 414 mg Cr kg−1 soils.

The amount of tannery sludge 2 added was 0.0125 g g−1 soils, an amount which covered three times the requirement for the region recommended dose of N for maize crop (i.e. 260 kg N ha −1). The following treatments with three replications were applied at a rate of 250 g g−1 soil: control (without any amendment),Cr(III), Cr(VI), tannery sludge, Cr(III)+ tannery sludge, Cr(VI) + tannery sludge, Cr(III) (Cr2O3), and Cr(VI) (K2Cr2O7). The criteria for dose selected was near to the value of the maximum upper limits of 300 mg kg−1 for acceptable utilization of waste and by-products in agriculture as established by the US Environmental Protection Agency, Part 503 [56].The jars were sealed with air-tight plastic lids and incubated at 25 °C for 180 days. After 0, 30, 60, 120, and 180 days' incubation the CO2 and inorganic N (NH4 + , NO2 − , and NO3 − ) were analyzed as described above.

The amount of tannery sludge 3 (**Table 1**) was 0.046 g g−1 soils which was equivalent to 22.08 Ton ha−1 and fleshing waste was 0.092 g g−1 soil and was equivalent to 3.7 Ton ha−1. The amount of tannery sludge was applied to give less than 300 mg Cr kg−1 that is the critical level of Cr for acceptable utilization of waste and bio-product in agriculture [48]. These amounts of tannery sludge and fleshing waste contained approximately 13.0 and 2.4% less than 170 kg N ha−1, which is the maximum dose recommended by the European nitrate directive [57] to reduce water pollution by NO<sup>3</sup> − –N from agricultural sources.

### **3.3. Results and discussion**

*3.2.2. Tannery sludge*

252 Organic Fertilizers - From Basic Concepts to Applied Outcomes

─ = no determined. Data from: Refs. [2, 49, 55].

414 mg Cr kg−1 soils.

and NO3

−

**Table 1.** Characteristics of tannery waste from Leon Guanajuato, México.

) were analyzed as described above.

Tannery sludge produced during leather manufacturing was sampled from a tannery in Leon (Guanajuato, Mexico). The sludge contained large quantities of hair, soluble proteins, and fatty fleshings from processing the skin to hide, and sulphide, lime, chromium-sulphate, salts, dyes, acid, and leather trimmings from processing the hide to leather. Two tannery sludge samples were collected in two different tannery industries with different Cr concentrations. A third tannery sludge was sampled from a tannery in Leon where the fleshing sludge was separated from the leather at the beginning of the process and then followed by chemical treatments. The fleshing waste contained small quantities of hair, soluble proteins and fatty fleshing from the processing of skin to hide. The three different sludge samples and fleshing waste were used in different studies with the same semi-arid soils. Chemical characterization is shown in **Table 1**. Tannery sludge was air-dried before used for the experimental aerobic incubation.

**Tannery sludge 1 Tannery sludge 2 Tannery sludge 3 Fleshing waste (F)**

Amounts of tannery sludge 1 and 2 added to soils were in accord with the criteria of the amount of N covering the recommended dose of N for maize crop growth in the region. The amount of tannery sludge 1 which amended soils from outside and under the canopy of mesquite trees was 1.5 g of wettannery sludge to 50 g of soil. The total amount added to under and outside the canopy soils was approximately 1308 mg C kg−1, 320 mg total N kg−1, 45 mg total P kg−1,and

The amount of tannery sludge 2 added was 0.0125 g g−1 soils, an amount which covered three times the requirement for the region recommended dose of N for maize crop (i.e. 260 kg N ha −1). The following treatments with three replications were applied at a rate of 250 g g−1 soil: control (without any amendment),Cr(III), Cr(VI), tannery sludge, Cr(III)+ tannery sludge, Cr(VI) + tannery sludge, Cr(III) (Cr2O3), and Cr(VI) (K2Cr2O7). The criteria for dose selected was near to the value of the maximum upper limits of 300 mg kg−1 for acceptable utilization of waste and by-products in agriculture as established by the US Environmental Protection Agency, Part 503 [56].The jars were sealed with air-tight plastic lids and incubated at 25 °C for

180 days. After 0, 30, 60, 120, and 180 days' incubation the CO2 and inorganic N (NH4

+ , NO2 − ,

Total C (g kg−1) 281 257.8 7.65 5.68 Total N (g kg−1) 53.4 18.7 0.77 1.5 pH 8.34 8.09 8.65 7.66 Cr (mg kg-1) 6690 1663 6516 136 Na (mg kg-1) ─ ─ 1174.6 119.95 Ca (mg kg-1) ─ ─ 10.94 188.7

Addition of tannery sludge 1 to outside and under the canopy of mesquite soils had no inhibitory effect on N mineralization and increased CO2 production and inorganic N concen‐ trations, but did not increase available P concentrations. These results suggest that tannery sludge could provide valuable nutrients to mesquite tree, the dominant vegetation in Dolores Hidalgo [2].

### *3.3.1. N and C mineralization*

The amount of tannery sludge 2 used in another experiment [58]shows that inhibition of ni‐ trification in outside the canopy soils increased when adding tannery sludge plus Cr6 + from 30 to 180 days. Soils under the canopy, amended with the same treatment, did not show a constant effect on nitrification throughout the incubation time (**Table 2**).

Results from this study showed that nitrification is sensitive to Cr(VI) added alone in outside the canopy soils from 30 to 120 days' incubation (**Table 2**) and to Cr(VI)plus tannery sludge from 30 to 180 days in soils outside the canopy (data not shown).Cr(III)added alone or Cr(III) plus tannery sludge added to the two soils had no specific effect on the microbial activities (CO2 production or dehydrogenase activity) or N-mineralization [58].



All the values are significant at *p* ≤ 0.05, ─ not significant. U = under the canopy, and O = outside the canopy soils, VI = Cr(VI), III = Cr(III), T = tannery sludge 2. Source [55].

**Table 2.** Inhibition of CO2 production rate, dehydrogenase activity, and NO<sup>3</sup> concentration in under- and outside-thecanopy soils from Dolores Hidalgo, Mexico, incubated at 25°C for 30 and 120 days.

There is a conflicting effect on using mineralization of organic C in metal contaminated soils because of the stimulation and inhibition on respiration [59, 60]. Results of under the canopy soils show that Cr(VI)could have effects on complex soil organic matter and render it less available by reducing Cr(VI) to Cr(III) [61, 62]. Thus, under the canopy soil which had more organic C, there was less inhibition of CO2 production rate than outside canopy soils.

In soils outside the canopy with low organic matter, Cr(VI) may have effects on soil organic matter available which increases with the dying of cells. Thus, the sum of CO2 produced by death and surviving microorganisms will reflect a small or not inhibition of CO2 production by Cr(VI) (**Table 2**). Adding tannery sludge plus Cr(VI)may reduce inhibition of CO2 produc‐ tion rate by complexing the Cr(VI) with organic matter from tannery sludge [62].

#### *3.3.2. Cr fractionation*

Results from Cr fractionation in under and outside the canopy soils amended with Cr(VI), Cr(III), Cr(VI) plus tannery sludge 2, Cr(III)plus tannery sludge 2, or tannery sludge 2 alone was that the level of total Cr increased in the more resistant fraction (fraction VI which is the least soluble form) and increased further over time. The opposite trend occurred with nonresidual fraction (sum of fraction I, II, III, IV, and V) which tended to decrease with time in the two soils (**Table 3**).



Residual = fraction VI; Non-residual = sum of fraction I, II, III, IV, and V. Values followed by the same letter in the same column are not significantly different at *p* ≤ 0.05, according to the Duncan's test.

**Table 3.** Fractionation of chromium (mg kg*<sup>−</sup>*<sup>1</sup> ) in semi-arid soils from Dolores Hidalgo, México, amended with Cr(III), Cr(VI) and/or tannery sludge 2, incubated at 25°C for 30 and 120 days.

The greater percentage of Cr in the residual fraction at 120 days of incubation (**Table 3**) (40– 65%) probably reflects the greater tendency of Cr to become unavailable once it is in the soil [63]. The residual and non-residual Cr suggest that the metal bioavailability does not only depend on its concentration but is also affected by the characteristics of the tannery sludge and soil components (such as Fe, Mn, oxides, or the quality of organic matter) into which it is sorbed [61].This will have an impact in the interaction between Cr and the biota [64].

Tannery sludge 3 and fleshing waste were added to the same semi-arid soils incubated for 6 months and subsequently subject to simulated rainfall. In this study we evaluated the Cr loss that occurs due to runoff and infiltration, as well as Cr fractionation, Cr speciation, soil pH, and soil microbial activities before and after the simulated rainfall event.

### *3.3.3. pH*

**CO2 production rate**

NO3-concentration

*3.3.2. Cr fractionation*

two soils (**Table 3**).

OTVI 49.61 ─

UVI 52.82 ─ OVI 69.64 83.81 OTVI 62.40 95.36 OTIII 43.38 27.73

**Table 2.** Inhibition of CO2 production rate, dehydrogenase activity, and NO<sup>3</sup>

canopy soils from Dolores Hidalgo, Mexico, incubated at 25°C for 30 and 120 days.

Cr(VI), III = Cr(III), T = tannery sludge 2. Source [55].

254 Organic Fertilizers - From Basic Concepts to Applied Outcomes

All the values are significant at *p* ≤ 0.05, ─ not significant. U = under the canopy, and O = outside the canopy soils, VI =

There is a conflicting effect on using mineralization of organic C in metal contaminated soils because of the stimulation and inhibition on respiration [59, 60]. Results of under the canopy soils show that Cr(VI)could have effects on complex soil organic matter and render it less available by reducing Cr(VI) to Cr(III) [61, 62]. Thus, under the canopy soil which had more

In soils outside the canopy with low organic matter, Cr(VI) may have effects on soil organic matter available which increases with the dying of cells. Thus, the sum of CO2 produced by death and surviving microorganisms will reflect a small or not inhibition of CO2 production by Cr(VI) (**Table 2**). Adding tannery sludge plus Cr(VI)may reduce inhibition of CO2 produc‐

Results from Cr fractionation in under and outside the canopy soils amended with Cr(VI), Cr(III), Cr(VI) plus tannery sludge 2, Cr(III)plus tannery sludge 2, or tannery sludge 2 alone was that the level of total Cr increased in the more resistant fraction (fraction VI which is the least soluble form) and increased further over time. The opposite trend occurred with nonresidual fraction (sum of fraction I, II, III, IV, and V) which tended to decrease with time in the

organic C, there was less inhibition of CO2 production rate than outside canopy soils.

tion rate by complexing the Cr(VI) with organic matter from tannery sludge [62].

**Time incubation 30 days 30 days**

Treatments Residual Non-residual Residual Non-residual

Cr(III) 5.4f 4.1g 41.4c 0.90 h Cr(VI) 12.7e 31.2f 27.7g 16.8 f Tannery sludge 26.0d 53.6c 50.1b 25.7 d Tannery sludge + Cr(III) 31.2c 49.1d 66.4a 33.0 c


Under the canopy

concentration in under- and outside-the-

The highest pH was in soils amended with fleshing waste (OTF > UTF) (average 8.49, 8.12, respectively), followed by OT and UT at 3 months' incubation after rainfall (**Figure 3**). Similar results were observed in soil before simulated system [49].

The increment in soil pH also increased the solubility of organic molecules, which is especially important in semi-arid soils(Wolf, 1994).The pH decreases from 3 to 6 months agree with those found in a study conducted by Apple [65] who demonstrated that cycles of drying and rewetting might cause disruption of soil aggregation, resulting in a rapid mineralization by soil micro-organisms due to the release of accessible substrate.

#### *3.3.4. Hexavalent chromium*

The concentration of Cr(VI) in soils after application of the simulated rainfall was significantly higher in the soils under the canopy treated with tannery sludge and fleshing waste, or tannery sludge only than in soils outside the canopy with the same treatments (**Figure 4**), even though there were no significant differences between soils treated with tannery sludge or tannery sludge mixed with fleshing waste (data not shown).Values of Cr(VI) remained constant for 1– 3 months (1.18–1.74 for outside soils and 2.30–2.80 mg Cr g−1 soil for under the canopy soils) under alkaline conditions (pH > 8.0).

**Figure 4.** Chromium hexavalent after a simulated rainfall in semi-arid soils amended with fleshing waste and or tan‐ nery sludge. Bars indicate standard deviation. Source: Ref. [49].

#### *3.3.5. Total Cr loss by runoff and infiltration*

Total Cr loss in mg was observed in runoff compared to infiltration. The highest values of total Cr released was observed in runoff for UTF treatment, mainly at 1 and 3 months' incubation. The lowest value of total Cr was in infiltration in OTF treatment followed by OT (**Table 4**) [66].

The high concentration of total Cr observed in runoff from 1 to 3 months corresponds with the highest increase in pH (8.01–8.46). Cr measured between these pH values must be all Cr(VI) where anionic Cr(VI) formations are favoured [67]. Thus, the high concentration of Cr loss in runoff from 1 to 3 months suggests that it might be Cr(VI) which was available to the soil solution. Cr values at 1 and 3 months of incubation for under the canopy soils treated with tannery sludge alone or mixed with fleshing waste were higher than the upper limits of Cr (VI) established by the Mexican Norm [68].



U: soil sampled under the canopy of mesquite tree; O: soil sampled outside the canopy of mesquite tree; T: tannery sludge; F: fleshing waste. Values in parentheses indicate standard deviation. Source [49].

**Table 4.** Total Cr (mg) releases in runoff and infiltrations in semiarid soils.

#### *3.3.5.1. Chromium fractionation*

sludge mixed with fleshing waste (data not shown).Values of Cr(VI) remained constant for 1– 3 months (1.18–1.74 for outside soils and 2.30–2.80 mg Cr g−1 soil for under the canopy soils)

**Figure 4.** Chromium hexavalent after a simulated rainfall in semi-arid soils amended with fleshing waste and or tan‐

Total Cr loss in mg was observed in runoff compared to infiltration. The highest values of total Cr released was observed in runoff for UTF treatment, mainly at 1 and 3 months' incubation. The lowest value of total Cr was in infiltration in OTF treatment followed by OT (**Table 4**) [66].

The high concentration of total Cr observed in runoff from 1 to 3 months corresponds with the highest increase in pH (8.01–8.46). Cr measured between these pH values must be all Cr(VI) where anionic Cr(VI) formations are favoured [67]. Thus, the high concentration of Cr loss in runoff from 1 to 3 months suggests that it might be Cr(VI) which was available to the soil solution. Cr values at 1 and 3 months of incubation for under the canopy soils treated with tannery sludge alone or mixed with fleshing waste were higher than the upper limits of Cr

**Time (months) OTF UTF UT OT**

Total 1.25 4.73 5.58 1.59

 0.42 (0.161) 0.87 (0.074) 1.29 (0.123) 0.50 (0.074) 0.09 (0.042) 1.68 (0.134) 0.97 (0.141) 0.29 (0.233) 0.40 (0.124) 1.02 (0.128) 2.53 (0.133) 0.30 (0.132) 0.34 (0.127) 1.16 (0.187) 0.79 (0.147) 0.50 (0.169)

under alkaline conditions (pH > 8.0).

256 Organic Fertilizers - From Basic Concepts to Applied Outcomes

nery sludge. Bars indicate standard deviation. Source: Ref. [49].

*3.3.5. Total Cr loss by runoff and infiltration*

(VI) established by the Mexican Norm [68].

Infiltration

Dominant Cr fractionation in semi-arid soils was bound to carbonates (Fraction II) at 0, 3, and 6 months' incubation after rainfall (**Table 5**). Dominant Cr fractionation at 1 month incubation was bound to reducible (Fraction III bound to manganese oxide) followed by Fraction IV (bound to Fe oxides). These results demonstrate that Cr binding was influenced by pH and might also have been influenced by ammonium.


6 Months


Source: Ref.[49].

Fractions: I = exchangeable, II = bound to carbonates, III = bound to Mn oxides, IV = bound to Fe oxides, V = bound to organic matter and VI = residues.

**Table 5.** Cr Fractionation (%) in semi-arid soils after rainfall amended with fleshing waste (F) and or tannery sludge 3 (T).

In addition, it has been shown that Cr associated with carbonate becomes susceptible to changes in pH, which results in its becoming soluble [42, 69].

#### *3.3.6. Runoff, soil loss, and infiltration*

Water infiltration into soil is one of the most important processes in the hydrological cycle and is crucial in agriculture. Amount of cumulative soil loss and runoff was significantly higher (P*<*0.01) in soils from outside the canopy than those from under the canopy tree. The opposite occurred in infiltration, being higher under the canopy than outside the canopy tree [66]. Amendment of tannery sludge alone to outside the canopy soils reduced the cumulative soil loss and runoff followed by the tannery sludge plus fleshing waste. However, addition of tannery sludge reduced also cumulative soil loss and runoff under the canopy, but the mixture of tannery sludge plus fleshing waste was higher than those with tannery sludge alone. There were no significant differences in soil infiltration outside the canopy soils with or without amendments. However, infiltration under the canopy soils amended with tannery sludge or tannery sludge plus fleshing waste were higher than without amendment. Thus, the addition of tannery sludge plus fleshing waste to under the canopy soil increased the cumulative runoff [66].

Results reported by Barajas-Aceves et al. [66] suggest that high sodium and salt concentrations of tannery sludge plus fleshing waste (1174.68 mg Na kg−1 tannery sludge and 119.95 mg Na kg−1 fleshing waste and 188.70 mg Ca kg−1 fleshing waste) and soil organic C (0.46 g kg−1 under the canopy soil) affected soil aggregates of under and outside the canopy soils [70] in different ways, thereby allowing reduction of runoff and loss of solids [66, 71]. While elevated electrolyte concentration may enhance flocculation, sodium has the opposite effect in soils, causing dispersion. The contrary behavior occurred with the application of organic C, thus enhancing water infiltration, delaying runoff, and reducing erosion [72]. Thus, organic matter is known to stabilize soil aggregates. Stronger dispersive conditions (such as higher solidicity, lower salinity, and higher energy or impact of water irrigation) should be needed to disperse a stable aggregate [73].

Results of this study could indicate that the degree of dispersion and flocculation and in the treated soils is not only due to salts and sodium concentrations added in the treatments through tannery sludge and fleshing waste, but also to the clay mineralogy of soil [70, 74]. Thus, differences in CEC values for soils under and outside the canopy (51.3 and 22 meq 100 g−1 soil) might reflect the nature of the predominant clay minerals of the soils [75]. Oster et al. [76] studied the flocculation values of montmorillonite and illite suspensions saturated with mixtures of Na and Ca ions in the exchange phase. They suggest that soils with illitic clays are more sensitive to dispersion and clay movement than soils with montmorillonitic clays.

Taken together, these findings suggest that fleshing waste contains sodium at concentrations (data cited above) that adversely impact the infiltration rate of soil collected under the canopy. Previous studies [74, 77] indicate that sodium may cause dispersion and plug soil pores. In this study, repeated wetting and drying of soil could have caused sodium dispersion from sodium coming in the treatments applied to the soil. This, in turn, could have negatively affected the soil structure, reducing infiltration and surface crusting [66, 71, 77].

This would also explain the higher runoff observed in soil collected from under the canopy amended with tannery sludge plus fleshing waste when compared to soil collected from under the canopy and amended with tannery sludge alone. These results suggest that great care must be taken when using waste as an organic fertilizer in semi-arid soil if the wastes contain salts and sodium, to avoid damaging the soil, especially if the goal is to preserve or improve fertile lands. Agassi et al. [74] postulated that the combination of these ESP values and rainwater low salinity caused crust formation and thus high runoff.

#### *3.3.7. Nitrogen loss*

**0 Months Cr in fraction (%)**

258 Organic Fertilizers - From Basic Concepts to Applied Outcomes

changes in pH, which results in its becoming soluble [42, 69].

Source: Ref.[49].

(T).

[66].

aggregate [73].

organic matter and VI = residues.

*3.3.6. Runoff, soil loss, and infiltration*

OT 2.91 84.20 5.23 4.11 3.30 0.25 OTF 2.64 84.38 5.57 4.37 2.88 0.16 UT 3.36 83.45 4.90 4.20 4.09 0.01 UTF 2.61 86.41 3.96 3.24 3.78 0.00

Fractions: I = exchangeable, II = bound to carbonates, III = bound to Mn oxides, IV = bound to Fe oxides, V = bound to

**Table 5.** Cr Fractionation (%) in semi-arid soils after rainfall amended with fleshing waste (F) and or tannery sludge 3

In addition, it has been shown that Cr associated with carbonate becomes susceptible to

Water infiltration into soil is one of the most important processes in the hydrological cycle and is crucial in agriculture. Amount of cumulative soil loss and runoff was significantly higher (P*<*0.01) in soils from outside the canopy than those from under the canopy tree. The opposite occurred in infiltration, being higher under the canopy than outside the canopy tree [66]. Amendment of tannery sludge alone to outside the canopy soils reduced the cumulative soil loss and runoff followed by the tannery sludge plus fleshing waste. However, addition of tannery sludge reduced also cumulative soil loss and runoff under the canopy, but the mixture of tannery sludge plus fleshing waste was higher than those with tannery sludge alone. There were no significant differences in soil infiltration outside the canopy soils with or without amendments. However, infiltration under the canopy soils amended with tannery sludge or tannery sludge plus fleshing waste were higher than without amendment. Thus, the addition of tannery sludge plus fleshing waste to under the canopy soil increased the cumulative runoff

Results reported by Barajas-Aceves et al. [66] suggest that high sodium and salt concentrations of tannery sludge plus fleshing waste (1174.68 mg Na kg−1 tannery sludge and 119.95 mg Na kg−1 fleshing waste and 188.70 mg Ca kg−1 fleshing waste) and soil organic C (0.46 g kg−1 under the canopy soil) affected soil aggregates of under and outside the canopy soils [70] in different ways, thereby allowing reduction of runoff and loss of solids [66, 71]. While elevated electrolyte concentration may enhance flocculation, sodium has the opposite effect in soils, causing dispersion. The contrary behavior occurred with the application of organic C, thus enhancing water infiltration, delaying runoff, and reducing erosion [72]. Thus, organic matter is known to stabilize soil aggregates. Stronger dispersive conditions (such as higher solidicity, lower salinity, and higher energy or impact of water irrigation) should be needed to disperse a stable The highest values of NO3 − –N and NH4 + –N concentration losses after simulated rainfall were found in the solution infiltration [66].

Outside the canopy of mesquite soils treated with tannery sludge only or tannery sludge plus fleshing waste showed the highest concentrations of NO3 − –N (341 and 560 mg l−1 respectively) and a lower concentration of NH4 + –N (283 and 158 mg l−1 respectively) in the infiltration solution together with the highest reduction of soil loss and infiltration [71] (71 and 55%, respectively) [66] suggesting that no changes regarding nitrification took place in those soils. However, the opposite occurred with soils under the canopy treated with tannery sludge plus fleshing waste, a higher concentration of NH4 + –N (452 and 582 mg l−1, respectively) and a lower concentration of NO3 − –N (445 and 316 mg l−1 respectively) in the infiltration solution were observed. These results, together with no reduction of runoff and low reduction on soil loss (−7 and 43%, respectively) [66], suggest that in areas where treated soil drains poorly, the held water impedes O2 diffusion, creating anoxic areas in the soil at a redox potential appropriate for denitrification. This suggestion is supported by results of high pH (more than 8) in treatments of OTF and UTF at 1 and 3 months of incubation after the simulated rainfall event (**Figure 3**) [49]. Furthermore, the high values of runoff and low infiltration in UTF amendment [66] suggest that clay mineralogy of under-the-canopy soils and Na concentration, plus the tannery waste added disrupted soil aggregates, leaching the NH4 + –N accumulated during the dry intervals prior to the rainfall event [78, 79].

Taken all this information together, it seems that the increasing pH values might have caused competition between chromium oxyanions and OH−, thus decreasing Cr(VI) sorption [80, 81].

Results of the tree different tannery sludge study suggest that tannery sludge characteristics are so important that they define the Cr fractionation in the soil [82]. Thus, the retaining metal in the solid phase depends on metal concentration and abundance of solid phase [83]. Solubility of the metal is mainly controlled by pH, concentration, state of mineral compounds, and type of ligands. Furthermore, these findings indicate that the incubation system used may influence microbial processes in the soil and that the presence of organic compounds plays an important role in determining which fraction of Cr dominates, enhancing metal adsorption to soil phases.

The relative importance of any solid phase for retaining a metal depends on the identity, concentration of metal, and abundance of the solid phase [83, 84]. Solubility of metal is mainly controlled by pH, concentration, type of ligands, chelating agent, oxidation state of mineral component, and the redox potential of the system [85], with TOC enhancing metal adsorption to sediment phases.

The use of tannery sludge with high concentration of carbonates and Cr as organic fertilizer in semi-arid soils could be potentially harmful due to chemical forms related to solubility and carbonate forms favouring heavy metal uptake by plants, and leaching.

#### **3.4. Mine tailings amended with organic wastes**

There was a reduction in the mobility of Pb when bokashi and compost of vermicompost were added to agricultural and rangeland soils mixed with mine tailings at 0 or 169 days of incubation. However, mobility of Zn was reduced only in both soils mixed with vermicompost and mine tailings. Differences in Pb and Zn mobility between bokashi and compost treatments might have occurred because the effects of organic materials added to soils on soil properties depend on degradation of such materials, which could have affected heavy metals solubility [86, 87]. The high levels of humified organic matter in vermicompost probably influenced the mobility of Pb and Zn [86]. Some published results suggest that availability of Pb and Zn to soil microflora is also influenced by the high humus content in organic matter [62, 88].

Treatments in agricultural and rangeland soils containing mine tailings plus compost showed the greatest inhibition of cumulative C mineralization followed by bokashi [88]. The highest inhibition of N mineralization in agricultural soils was in treatment amended with vermicom‐ post and in rangeland soils in treatments with compost and bokashi plus mine tailings (20.30 and 18.74% inhibition, respectively). The highest inhibition of dehydrogenase activity was observed in both soil amendments with tannery sludge (48–80% respectively) and the lowest in agricultural soil plus bokashi plus mine tailings and rangeland soils plus vermicompost plus tannery sludge (27 and 39% respectively [88]. These results suggest that the quality of the organic material together with the chemical characteristics of the soils could be important factors influencing decomposition of organic materials. Indeed, contents of nitrogen, cellulose, hemicellulose and lignin, the C/N ratio and the lignin/nitrogen ratio [89] have been reported to be some of the most important factors controlling decomposition processes of organic substrates.

Experimental values of N and C mineralization for the two soils in each treatment [88] were used by fitting to four commonly used kinetic models: Zero order, linearized power function, first order, and first order E [90–95].

Taken all this information together, it seems that the increasing pH values might have caused competition between chromium oxyanions and OH−, thus decreasing Cr(VI) sorption [80, 81].

Results of the tree different tannery sludge study suggest that tannery sludge characteristics are so important that they define the Cr fractionation in the soil [82]. Thus, the retaining metal in the solid phase depends on metal concentration and abundance of solid phase [83]. Solubility of the metal is mainly controlled by pH, concentration, state of mineral compounds, and type of ligands. Furthermore, these findings indicate that the incubation system used may influence microbial processes in the soil and that the presence of organic compounds plays an important role in determining which fraction of Cr dominates, enhancing metal adsorption to soil phases.

The relative importance of any solid phase for retaining a metal depends on the identity, concentration of metal, and abundance of the solid phase [83, 84]. Solubility of metal is mainly controlled by pH, concentration, type of ligands, chelating agent, oxidation state of mineral component, and the redox potential of the system [85], with TOC enhancing metal adsorption

The use of tannery sludge with high concentration of carbonates and Cr as organic fertilizer in semi-arid soils could be potentially harmful due to chemical forms related to solubility and

There was a reduction in the mobility of Pb when bokashi and compost of vermicompost were added to agricultural and rangeland soils mixed with mine tailings at 0 or 169 days of incubation. However, mobility of Zn was reduced only in both soils mixed with vermicompost and mine tailings. Differences in Pb and Zn mobility between bokashi and compost treatments might have occurred because the effects of organic materials added to soils on soil properties depend on degradation of such materials, which could have affected heavy metals solubility [86, 87]. The high levels of humified organic matter in vermicompost probably influenced the mobility of Pb and Zn [86]. Some published results suggest that availability of Pb and Zn to soil microflora is also influenced by the high humus content in organic matter [62, 88].

Treatments in agricultural and rangeland soils containing mine tailings plus compost showed the greatest inhibition of cumulative C mineralization followed by bokashi [88]. The highest inhibition of N mineralization in agricultural soils was in treatment amended with vermicom‐ post and in rangeland soils in treatments with compost and bokashi plus mine tailings (20.30 and 18.74% inhibition, respectively). The highest inhibition of dehydrogenase activity was observed in both soil amendments with tannery sludge (48–80% respectively) and the lowest in agricultural soil plus bokashi plus mine tailings and rangeland soils plus vermicompost plus tannery sludge (27 and 39% respectively [88]. These results suggest that the quality of the organic material together with the chemical characteristics of the soils could be important factors influencing decomposition of organic materials. Indeed, contents of nitrogen, cellulose, hemicellulose and lignin, the C/N ratio and the lignin/nitrogen ratio [89] have been reported to be some of the most important factors controlling decomposition processes of organic

carbonate forms favouring heavy metal uptake by plants, and leaching.

**3.4. Mine tailings amended with organic wastes**

260 Organic Fertilizers - From Basic Concepts to Applied Outcomes

to sediment phases.

substrates.

Nitrogen mineralization in all treatments was best fitted to the linearized power function [88].

$$N\_x = K\_x^{\*\*} a$$

The different values of *K* and *m* among treatments and soils did not follow a defined trend. K values in agricultural soils treated with mine tailing alone or with bokashi and vermicompost plus mine tailings increased to 32.1, 26.5, and 31.8% respectively. Similarly, the value of rate constant *K* in rangeland soils plus mine tailing alone increased to 76.7 % compared to the value of K in soil alone. However, in rangeland soils treated with vermicompost plus mine tailing K decreased to 39%. These results suggest that bokashi and vermicompost in agricultural soils provide a similar pool of mineralizable N and the addition of mine tailings modified the decrease of these pools. Behavior of N mineralization in rangeland soils was different as well as the decrease of N mineralization with the three organic wastes [90–92].

The best-fit model for C mineralization was first-order E model for both soils in all treatments [88].

$$C\_\iota = C\_0 \left( \mathbf{l} - \exp^{-4\iota} \right) + \mathbf{C}\_1 \mathbf{b}$$

Potential C mineralization (C0) showed the highest values for treatments with low k values [88]. Murwira et al [93] reported that Co\*k parameter can be better estimated than only one parameter. Both single parameters are interdependent.

The combination of those two parameters showed high values of Co∗k in treatments with compost, bokashi, or vermicompost alone in agricultural soils, followed by rangeland soils(11.5–18.2 and 8.1–7.6 respectively). These values are in the range of those reported in the literature for different organic materials (40.6–1.3) [95]. Differences in the values of Co∗k in all treatments in both soils might suggest that the amount of lignin-humus present in the organic compost, the amount of nitrogen content, and the organic waste quality and the type of soil might be important in the process and rate of decomposition [96, 97]. The lowest values of Co∗k were in the treatments with mine tailings alone (3.96 in both soils plus mine tailings) compared with soils alone (56 and 65% less in agricultural and rangeland soils, respectively) or the amendment of organic waste alone. These results suggest that the chemical composition of organic waste (nitrogen content, lignin, and polysaccharides) together with the chemical characteristics of the soils could be important factors influencing decomposition of organic materials and even in the presence of heavy metals [98, 99].

*Brassica juncea* accumulates more Pb and Zn in roots than in shoots [100] growth in soil plus mine tailings. Concentration of Na and Mg measured in mine tailings was 5207 and 102,917 μg g−1 soil respectively [100]. These results demonstrated that *Brassica juncea* had the ability to survive and tolerate several metals simultaneously, in the presence of high levels of Na and Mg (SAR between 2.5 and 3.7% and ESP between 18.3 to 20.7%) [100]. All metals measured from mine tailings were accumulated in the root and there was very low translocation to the shoots.

According to the literature, the typical behavior of an accumulator species such as *Brassica* is that there is higher accumulation of heavy metals in the leaves [101], which is opposite to the results of these study.

Results suggest that high levels of Na and salt in mine tailings and the physicochemical characteristic of mine tailings might influence translocation of heavy metals from roots to shoots; metals such as Pb and Zn which are the main metals extracted by *Brassica juncea*. This suggestion is according to reports showing that salt acts antagonistically, thus when plants grow in media with a high Pb concentration and high salt concentration, the amount of Pb accumulated by *Brassica juncea* decreases [102].

The mine tailings amended with 10% compost to growth two shrubs *Acacia retinodes* and *Nicotiana glauca* were able to survive at high concentrations of heavy metals in mine tailings (**Table 1**) when 10% compost was added. The dry biomass of both shrubs increase from 62 to 79% growth in mine tailings plus compost compared to mine tailing alone. *Echinochloa polystachya* was not able to grow on mine tailings, even when it was amended with compost, as was shown by the percentage inhibition data for its root and leaf biomasses. Pb and Zn concentrations in the three plants were higher in roots compared to leaves for all treatments (Pb from 514 to 861 in roots and from 14.6 to 90 (μg g−1) in leaves and Zn from 682 to 766 in roots and 541.4 to 254 (μg g−1) in leaves) [70]. The elevated contents of Pb and Zn in roots along with the low translocation factors [70] indicate that the two shrub species used in this study are appropriate for Pb and Zn phytostabilization.
