**2. Macronutrients and environmental pollution**

The application of organic amendments to the soil is a very common practice, especially in areas with low organic matter content [39]. The application of MSW as a source of organic matter and nutrients has been described for agriculture, mining restoration, and gardening [39, 40]. But it carries the associated risk of possible pollution, focused mainly in the nitrate contamination of surface and groundwater, since the mineralization of this organic matter can release large amounts of ammonium that will oxidize to nitrate [41, 42]. However, there are also other risks derived from the composition of MSW (hazardous materials) and the presence of plant nutrients as phosphorus [43], chloride, and sulfur [44].

A common composition of a composted MSW is indicated in **Table 1**. This composition shows important amounts of plant nutrients (i.e., phosphorus) as well as the presence of environmental pollutants like nickel and cadmium [39].

 The urban wastes can differ in origin and changes due to the different style of life, conditioning the composition, and the total amount of wastes. The


#### **Table 1.**

*Composition of a composted MSW [39].* 

composition of waste in landfills could differ due to the joint storage of industrial and domestic waste containing toxic elements [45]. Moreover, the composition of MSW can be different considering the seasons of the year and seasonal impacts should be taken into consideration when dealing with MSW [46].

#### **2.1 Nitrogen and organic matter**

 Nitrogen is one of the major nutrients for plants, and soil is the main source in terrestrial ecosystems. Nitrate is the preferable chemical form for the absorption of most of the plants. However, this is a very mobile chemical form [42]. In order to minimize the risk of groundwater contamination, Jorge-Mardomingo et al. [41] recommend the use of stable organic amendments (with a more stabilized organic matter), which could produce a lower content of leachable nitrogen forms. Risk is also minimized by planting rainfed crops and particularly by choosing crops with a high demand for nitrogen such as wheat or maize [47].

Applications of MSW (composted or not) should be planned to avoid the coincidence of peaks of soluble nitrogen forms with rainfall periods in order to prevent their transport to groundwater and increase their residence time in the root zone. Diffuse nitrogen losses from agricultural fields are the major cause of excessive nitrate concentrations in ground- and surface waters [48].

MSW compost contains large amounts of organic matter and both organic nitrogen and inorganic nitrogen [49]. The organic matter plays a key role in improving soil properties such water retention capacity or soil structure, among others [50]. The use of composted organic wastes produces changes in soil physical, chemical, and biological properties and can enhance plant growth after its application [51].

#### *The Use of Composted Municipal Solid Waste under the Concept of Circular Economy… DOI: http://dx.doi.org/10.5772/intechopen.83386*

Moreover, the organic matter added with MSW can be the main source of nitrogen in impoverished soils with low organic matter content.

 The amounts of plant-available nitrogen and phosphorus from MSW are closely related to the degree of compost maturity, the addition of mineral fertilizers, soil characteristics, and environmental parameters [49]. All of them can affect the availability of nutrients. For instance, the addition of inorganic fertilizers can increase the plant and microbial activity of soils and may induce an increment of the mineralization of the organic matter of MSW, favoring the inorganic nitrogen forms.

 Not only the plant nutrition is directly affected by using MSW, but also improving soil properties, the plant can response positively. Civeira [51] studied the response of an urban-degraded soil to different MSW compost application rates, as an alternative to MSW disposal and soil recovery. As indicators from soil response, physical (bulk density, soil moisture, and water infiltration) and chemical (pH, electrical conductivity, organic C, total N, and extractable P) parameters were evaluated. Compost application positively affected total N content in soils, improving soil physical properties in a similar way to chemicals, after MSW compost addition.

After the application of MSW compost to the soil, nitrogen is transformed into mobile forms, which can be accumulated in the soil, absorbed by plants, or released into the atmosphere or water system. The amount of nitrogen released into the soil solution determines the form of nitrogen availability to the plant and, consequently, the yield. Nevertheless, the environmental risks are well known. The amendment of the soil with organic fertilizers containing easily decomposable organic carbon compounds can trigger denitrification processes [50].

 If MSW is poor in nitrogen or the rate C/N is inadequate for the mineralization of the organic matter, additional sources of nitrogen are needed. Mkhabela and Warman [52] found that the low availability of compost-N means that supplementary nitrogen in the form of inorganic fertilizer may have to be added together with compost in order to enhance N availability to crops. They observed that inorganic fertilizer (NPK) and a mixture of MSW compost and inorganic fertilizer produce higher yields than MSW compost alone.

## **2.2 Phosphorus and other macronutrients**

Some authors observed that MSW compost effectively supplies phosphorus to soil with its concentration increased when increasing application rates. MSW composts provided equivalent amounts of phosphorus to soil as mineral fertilizers [52, 53].

In an experiment in plots in a quarry restoration, where 3 kg/m3 of MSW were applied to a substrate composed by limestone outcrop from the rejection of the quarry, an increment of nutrients associated to the composition of the composted MSW was obtained. In the plots in which MSW was applied, an important increase in the soil content of N-Kjeldahl, available P, and the rest of macro- and micronutrients was found, favoring the plant growth [54]. The results reflected the contribution of MSW to the plant nutrition and reinforced the idea of the positive use of the organic fraction of MSW in mining and landfill restoration (**Figure 3**).

 Baldi et al. [55] studied the effect of applying 5 and 10 t dw/ha·year of composted MSW to a nectarine crop for 11 years. They found that the content of N, P, macro-, and micronutrients increased with respect to the control, both in the plant and in the fruit. The authors concluded that in their experiment the slow release of nutrients in the soil from compost mineralization seemed to match with plant demand, supporting the hypothesis that compost can be used effectively in fruit

#### **Figure 3.**

*Composted MSW for soil restoration in a landfill area and a quarry with compost derived from MSW. The use of compost of MSW for seed germination (Photos from J. Navarro Pedreño).* 

tree nutrient management, since it promotes an increase of tree growth and yield by maintaining an optimal nutritional status of plants.

Calleja-Cervantes et al. [56] studied the effect that 13 years of applying three different composted organic amendments have had on soil quality, GHG emissions, and the dynamics of its microbial communities 15 days after the annual application. They found that total nitrogen increased with respect to the control by amending with organic fraction of municipal solid waste. Organic amendment application resulted in higher levels of phosphorus and potassium in the soil. They concluded that significantly higher organic matter contents, total N, P, and K contents, in the soil when compared to the control validate the fact that organic waste-based fertilizers contribute to enhanced soil fertility.

 The balance between the addition of nutrients that can be available for plant nutrition and the possible pollution, especially of waters with N-forms, needs to study previously the type of soil and, in general, the environmental conditions where MSW is going to be applied. The criteria established to control the addition of MSW as amendment to the soil might be improved including new criteria based on environmental conditions.
