Soil Quality Assessment after 25 Years of Sewage Sludge vs. Mineral Fertilization in a Calcareous Soil

Ana Simoes-Mota,Luis Orcaray,Rosa Maria Poch,Iñigo Virto,Alberto Enrique

doi:10.3390/land10070727

Abstract

The aim of this work was to identify the most sensitive soil quality indicators and assess soil quality after long-term application of sewage sludge (SS) and conventional mineral fertilization for rainfed cereal production in a sub-humid Mediterranean calcareous soil. The treatments included six combinations of SS at different doses (40 t ha−1 and 80 ha−1) and frequencies (every 1, 2 and 4 years), plus a control with mineral fertilization, and a baseline control without fertilization. Twenty-five years after the onset of the experiment, 37 pre-selected physical, chemical and biological soil parameters were measured, and a minimum data set was determined. Among these indicators, those significantly affected by treatment and depth were selected as sensitive. A principal component analysis (PCA) was then performed for each studied depth. At 0–15 cm, PCA identified three factors (F1, F2 and F3), and at 15–30 cm, two factors (F4 and F5) that explained 71.5% and 67.4% of the variation, respectively, in the soil parameters. The most sensitive indicators (those with the highest correlation within each factor) were related to nutrients (P and N), organic matter, and trace metals (F1 and F4), microporosity (F2), earthworm activity (F3), and exchangeable cations (F5). Only F3 correlated significantly (and negatively) with yield. From these results, we concluded that soil quality can be affected in opposite directions by SS application, and that a holistic approach is needed to better assess soil functioning under SS fertilization in this type of agrosystem.

Highlights

In the framework of circular economy and the European Green Deal goals, land application of sewage sludge (SS) is suggested as one of the most economical and ecological sludge disposal methods [1,2]
C stored in each treatment, nor to physical indicators related to water retention. This suggests that their activity in the studied soil would be dependent on other factors such as toxicity or compaction. In this framework, following the criterion for selecting the soil attributes with the highest sum of correlation coefficients (Table 5) as the most appropriate soil quality indicators [25,63], our results showed that Soil organic C (SOC), available P and trace metals, microporosity and water retention at low water potential (−50 kPa), and earthworms would be those selected at the 0–15 cm depth
The study showed that the overall response of soil quality to the managements tested (SS application and MF) was not linear or straightforward

Summary

Introduction

In the framework of circular economy and the European Green Deal goals, land application of sewage sludge (SS) is suggested as one of the most economical and ecological sludge disposal methods [1,2]. In general, has a high content of organic carbon, nutrients ( N and P) [6], and trace elements (S, Mg, Ca), and can promote the proliferation and activity of soil micro and mesofauna [2]. As a consequence, amending soils with SS can improve some soil properties, such as organic matter and nutrient content, soil porosity, bulk density, aggregate stability, or available water holding capacity [2,3,7,8,9]. The consequences of SS application on soil chemical properties and the accumulation of contaminants have been extensively studied [5,12,13] and different strategies to minimize the risk associated with SS application have been developed [14]

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Conclusion