**2. Research methods**

#### **2.1. Sampling areas**

using other organic wastes as organic fertilizer, farmyard manure has been used as a reference when sewage sludge or other organic materials have to be judged [3]. The sewage sludge and sludge compost amendment were ceased when it was found that soluble heavy metals in the soil and the total heavy metals in the crops were increasing when these wastes were applied [4]. However, sewage sludge and compost are the most widely used organic amendments, mainly by their high organic matter, N and P content, which are important nutrients for plant growth, their contribution to soil structure, and resistance to soils erosion [5–8]. On the other hand, application of sewage sludge as agricultural fertilizer is associated to numerous environmental and health problems such as those implicated by toxic metals, organic com‐

The gradual increase in industrialization and urbanization in the last decades has created an enormous increase in volumes of wastes produced all over the world. Usually, these wastes are discharged to the environment [13], especially in countries which regulatory control is not

The objective of the case study described in this chapter, together with methodology and literature review is to provide different scenarios of the role of organic waste in semi-arid soils poor in organic carbon, the effect of heavy metals on soil biogeochemical processes, their

Tannery sludge derives from a complex combination process where organic and inorganic materials become chemically bound to the protein of the hides and preserve it from deterio‐ ration. A significant number of operations within the tannery industry involve large amounts of water, chemicals, and energy leaving as waste large amounts of polluted water. These industrial effluents contain several types of chemicals such as dyes, levelling agents, acids, alkalis, phenols, carbonates, alcohols, cyanide, and heavy metals, among others [14].Byproducts generated during leather manufacturing are usually rich in proteic matter and organic substances, thus it is a potential resource that can be used as fertilizer in agriculture production. This leather processing waste is an attractive disposal for soil amendment as it has proven to improve the physical properties of soil and supply organic matter and plant nutrients

The use of these waste in semi-arid soils as organic fertilizer of plants could be an alternative disposal method and simultaneously it will resolve the environmental risks presented when

Under natural environmental conditions, chromium is present in either the trivalent Cr(III) or the hexavalent Cr(VI) [20, 21]. The effect of Cr on health have been widely studied [22]. Cr(VI) is about 300 times more toxic than Cr(III).Health effects of Cr have been reported in lung cancer, and birth defects [23, 24].Cr(III) have relatively low toxicity and are easily precipitated and immobilized; however, Cr(VI) is toxic, water soluble, and highly mobile, and can then be transported into the surrounding surface soil and ground water [25, 26].Tannery sludge contains both trivalent (CrIII) and hexavalent (CrVI) chromium. There is little information

pounds, and possible other health problems related to pathogens [9–12].

dispersion and mobility in soils, and the availability of heavy metals to plants.

**1.1. Semi-arid soils and tannery sludge**

244 Organic Fertilizers - From Basic Concepts to Applied Outcomes

they are abandoned to open sky.

strict.

[15–19].

The soil was sampled from three sites: Two around mesquite trees (*P. laevigata*), the dominant vegetation (Dolores Hidalgo, Guanajuato, Mexico), under the canopy and outside the canopy of mesquites (**Figure 1**), and the third one from a site cultivated with maize (*Zea mays*) for 20 years.

**Figure 1.** Under the canopy tree of mesquite (U), Outside the canopy of mesquite (O).

Soil was collected from a 0- to 5-cm layer, where the highest organic contents can be found. The first sampling was taken from under the canopy of four isolated mesquite trees, 1–2 m from the stem in four perpendicular directions randomly selected. The second one at a distance of 6–8 m from the stem, outside the canopy in the same perpendicular directions (**Figure 1**). The soil was bulked; all the stones, visible roots, and fauna were removed, it was sieved to less than 2 mm and stored at 5°C to use latter.

#### **2.2. Tannery sludge**

Tannery sludge, produced during leather manufacturing, when processing skin or hide to leather, was sampled from a tannery in Leon (Guanajuato, Mexico). It contained large quantities of hair, fatty fleshings, and soluble proteins, as well as sulphide, lime, chromiumsulphate, salts, dyes, acids, and leather trimmings.

### **2.3. Incubation experiments**

A pre-incubation process of soil samples was necessary to allow the soil microbial activity to stabilize after the sampling and sieve management. Soils were pre-incubated for 1 week prior to starting the experiment at conditions similar to the experiment, i.e. at 20°C in the dark, in a temperature and humidity controlled room. Three replicates were destructively harvested at days 0–90 or 120 and stored at −20°C for N mineralization and soil microbial activities analysis.

#### **2.4. Soil microbial activities and nitrification**

Maintaining soil fertility depends on biomass and activity of soil microorganisms vital in the biological cycles of most major plant nutrients [32]. Microorganisms are also involved in forming soil structure [33]. Several microbiological parameters have been suggested to measure soil environmental quality [34]. For instance, soil respiration and enzyme activities such as dehydrogenase activity and nitrification, can inform about the presence of viable microorganisms, and on the intensity, kind and time length of the effects of pollutants on the metabolic activity of soils.

Sub-samples of 40 g of soils were placed in 110-ml glass bottles, which were then put into 1-l jars containing 10 ml H2O and a vessel with 20 ml 1 M NaOH solution. The jars were air-tight sealed with plastic lids and incubated at 25 °C for 7 days. After incubation, vessels with 20 ml 1 M NaOH solution were removed, resealed, and stored for future CO2 analysis. At the mentioned dates, soil was removed for analysis of NO3 − –N, NO2−N, and NH4 + −N, done by shaking for 30 min with 100 ml 0.5 M K2SO4 solution and filtered through Whatman No. 42 paper. Similarly, control fresh samples were extracted. Extractants were stored at −20°C until analysis. Concentration of NO3 − –N and NO2 − –N in the extracts was determined by colorimetric method [35] and NH4 + –N by Indophenol blue [36].

Dehydrogenase activity in soils has been used as measure for overall microbial activity [37]. The method is based on the estimation of triphenyltetrazolium chloride (TTC) reduction rate to triphenyl formazan (TPF) in soils after incubation at 30°C for 24 h. Soil dehydrogenase activity was measured using a modified form of the method used by Casida [37]. Five grams of fresh soil were incubated at 37°C for 24 h in test tubes containing 1 ml 3% 2,3,5- triphenyl‐ tetrazolium chloride (TPF), 67 mg CaCO3, and 2.5 ml distilled water. The accumulation of the end-product triphenyl formazan (TPF)was determined in acetone extracts (50 ml) using a PerkinElmer Lamda 3A Spectrophotometer at 520 nm.
