Agricultural soils face a major threat as consequence of decades of conventional agricultural practices (Pagliai & Vignozzi, 2002). Long-term depletion of soil organic matter (SOM) leads to physical degradation (Jensen et al., 2019;Kopittke et al., 2020), as the soil is more vulnerable to erosion and compaction and less able to stabilize SOM.Organic wastes, such as sewage sludge (SS), are a resource that can be converted to fertilizer (Metcalf &

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.

Soils represent a fundamental part of life on Earth and represent a crucial player on climate change action, for their capacity to sequester atmospheric carbon (C) through organic matter stabilization, constituting one of the largest C-reservoirs. The management of soil organic matter (SOM) seems highly relevant for maintaining or improving soil function in Mediterranean agricultural areas. Mediterranean soils face several threats that increase soil susceptibility to degradation processes, hampering soil ecosystem services. Under the framework of the European Green Deal, and the need to promote circular economy, the use of sewage sludge (SS) as a fertilizer seems a good solution. When properly managed, it can be a way to prevent environmental pollution, recycle nutrients, and decrease the need for commercial fertilizers. Long-term experiments (LTE) can provide valuable data for soil science. The main objective of this work is to make a holistic assessment to the state of the soil after 28 years of continuous fertilization with sewage sludge. To be resilient with future challenges on food security, soil needs to be addressed according to its importance on the planet wellbeing. The work carried out in this thesis provides a functional assessment of the continuous use of sewage sludge as an organic fertilizer in agricultural land in the region of Navarra. It is important to consider the multiple soil functions and de-centralize crop-production from health indicators. Soils are not good and not bad, management yes. The results provided by this thesis highlight the importance of a healthy management of the soil and the bias provided by small-set of indicators. Agricultural soils provide important services that should be valued, not only those related to food production, but also to other key functions such as the control of soil degradation and vulnerability or climate regulation. The scientific community must coordinate and collaborate with politicians, policy makers, and stakeholders to explore these services, and to guide a transition to sustainable agriculture to address the challenges of the future.