Soil organic matter influences and regulates the cycle of the nutrients in soil–plant systems, contributing significantly to the maintenance of global soil fertility (chemical, physical, and biological). It is involved, in fact, both in plant and micro-organism growth and, as a substrate for the mineralization processes, in the promotion of the soil’s nutrient availability. Therefore, the quantity, quality and turnover of the organic matter (OM) are correlated to potential fertility in natural and agricultural soils.
Mediterranean agricultural areas are characterized by low soil organic carbon (SOC) contents and are often prone to soil degradation and SOC depletion due to the changes in extensive land use in recent decades and they are highly vulnerable to environmental changes [1
Continuously high temperatures during the summer in the Mediterranean lead to a rapid decline in the organic matter (OM) content in cultivated soil. The loss of the soil’s organic matter from agricultural soil is increasing very rapidly because of the climatic characteristics and the intensive land use which affect the processes of humification–mineralization of native soil organic matter.
A greater effort has to be given to conservation and enrichment of the organic matter in arable lands. Nowadays, growing concern focuses on the suitability of bio-wastes such as sewage sludge, municipal solid waste (MSW), and animal slurries to be used in agriculture. The promotion of waste recycling and recovery techniques is an urgent priority. The necessity of linking agricultural practices to sustainable environment management leads to the identification of farm procedures with beneficial effects on the ecosystem. Fertilization practices with organic waste could have a positive influence on the soil’s fertility and the soil’s nutrient supply [4
]. Residual biomasses—such as sewage sludge, organic waste, and food industry residue—can enrich soil with macronutrients such as nitrogen, phosphorus, potassium, sulphur, calcium, magnesium, and micronutrients [6
] which meet the requirements of cultivated crops and enhance the restoration of soil fertility. Approximately 50% of the solid fraction of sewage sludge is organic matter, which has a significant effect on the physical, chemical, and biological properties of the soil as a result of its application. Organic matter improves soil porosity and increases water retention and movement. Some components of organic matter play an important role in soil aggregation.
Sewage sludge is a potential source of nutrients for crops, but its heavy metal content is a hazard to the environment. While few heavy metals are essential for living organisms in trace quantities, most are hazardous in high concentrations. Nowadays, the environmental directives in many countries regulate the use of sewage sludge in agriculture by limiting the total heavy metal concentration in sludge and soil; in Italy, the use of sludge in agriculture is regulated by Decree-law 99/92 [9
] in implementing Directive 86/278/EC. In order to assess the potential for the re-use of sewage sludge in agriculture, its contribution to soil organic processes should be verified [10
]. In addition, the effects on the physical and mechanical characteristics of the soil [13
], on the behavior of heavy metals in the soil and in the soil–plant system and the hazard of environmental impact [16
] should be evaluated. The bioavailable heavy metal fraction, which is potentially assimilated by biota or liscivable, is of great interest in soil contamination studies because it is the environmentally most mobile [18
The aim of this research was to evaluate the effects of medium-term fertilization with diversely processed sewage sludge on the soil’s organic carbon content and humification–mineralization processes, on the physical–mechanical properties of soil and their influence on the pool of potentially bioavailable metals in order to assess their effectiveness as soil organic amendments.
After eight years of administration to the soil, the use of sewage sludge as soil organic amendment has contributed to increasing the soil’s organic carbon content. The annual addition to the soil of the organic fraction of sewage sludge has contributed to improving not only the total soil organic carbon content but also the extractable and humified soil organic fractions, depending on the processing techniques. The analysis of humification parameters and soil respiration activity showed that the amendment with composted sewage sludge contributed positively, more than the other treatments, to the humification process. This result is comparable to what was found by Fernadez et al. [22
] which showed that the organic matter of a composted sewage sludge is more stable than that of a dehydrated sludge. Moreover, regarding the present study, the easily decomposable carbon added to the soil by the composted sludge has favored the process of mineralization of the organic substance.
In amended soil, tillage is a fundamental factor in influencing soil quality, plant behavior, and the sustainability of farming systems [12
] because it can alter the soil’s physical properties and the soil’s profile depth. Many authors use the soil penetration resistance measurements as a tool for characterizing soil strength after tillage [13
]. The deformation response of a soil under different load conditions is used to predict how the soil stability changes after the application of an effective stress. Soil moisture content depends also on the organic matter content, influenced by the amendment. Soil moisture content during tillage affects the distribution of aggregates’ size and those aggregates formed at low moisture content have three to four times more resistance to crushing than those formed at greater moisture contents [14
]. Consequently, an amendment allowing the increase of the soil’s organic matter also increases the moisture content of the soil; in these conditions, the soil aggregates are characterized by greater stability and the soil stability and workability indexes improve. The results obtained by the sewage sludge administration on the physical and mechanical parameters of soil showed an improvement of the soil tillage indexes in the soil treated with composted sewage sludge in comparison with other treatments.
An increase in concentrations of total nickel and zinc was detected in the soil. For bioavailable form (DTPA-extractable) this trend was evidenced in all heavy metals analyzed, independently of the type of sludge administered. These results suggest that long-term soil amendment with sewage sludge might shift the soil’s total heavy metals forms to bioavailable forms that are potentially more mobile, labile, and available to soil organisms and plants. In fact, the soil’s inorganic and organic components may form different kinds of bonds with micronutrients and heavy metals, influencing their bioavailability [23
]. For instance, Zn2+
show a tendency to form organic complexes with amide and amino ligands thus increasing their mobility in soils with respect to Cu and Pb which tend to be sorbed by cation exchange, especially by humic and fulvic acids [26
In any of the cases, the amendment with sewage sludge diversely processed did not influence the balance between the total and bioavailable form of the heavy metals considered. In the case of copper and lead, potentially available forms are distributed in the same percentages in both untreated and treated soils, despite the yearly administration of the two metals contained in sewage sludge. In the case of zinc and nickel, the percentage of bioavailable form is considerably increased in the theses treated in respect to the control (T) but with similar values among the amended theses (L, D, C).
This study highlighted that, comparing the results obtained with diversely processed sludge, the added organic fraction to soil by the prolonged amendment with composted sewage sludge contributed to keeping the soil humification–mineralization process in equilibrium and to improving the physical and mechanical qualities of the treated soil. Therefore, the use of composted sewage sludge would be more effective with respect to liquid or dehydrated sludge mainly in agricultural areas subject to the risk of depletion of the organic carbon of the soil with a consequent decrease in the soil’s fertility. However, it would be advisable to monitor the heavy metal concentrations in soil to verify that they always remain below legal limits when the sludge amendment practices are carried out for a long time.