Soil organic matter quality in an olive orchard differently managed for 21 years: Insights into its distribution through soil aggregates and depth

Among the current global challenges, the research of new practices aimed at mitigating soil impoverishment, exacerbated by the pressing climate changes, is the most urgent. Studying soil organic matter (SOM) ecological dynamics and comparing the conventional intensive farming practices with the emerging alternative sustainable ones can represent a key indicator in soil health investigation, helping to find new guidelines for conservative agrosystems management. In this study, the soil from a Mediterranean olive orchard, with both sustainable (Smng) and conventional (Cmng) land use for 21 years, was investigated for its physicochemical properties, with a particular attention to the soil organic matter from aggregates (SOM-A) and its interaction and distribution at different soil depths. Significantly higher amounts of total carbon (+50.7 %) and nitrogen (+74.9 %), as well as of SOM-A aromatic component (+76.0 %), were detected in the topsoil layer (0–5 cm) of the Smng, compared to the Cmng, a sign that the organic matter from surface deeply seeps slowly. This evidence was highlighted especially in micro-aggregates (< 0.063 mm) of the Smng, compared to the Cmng (C = +59.3 %; N = +86.7 %; SOM-A aromatic component = +87.7 % in the Smng). This trend was also reflected in an increase in the bacterial abundance and in a different accumulation of organic compounds deriving from microbial fermentation processes in Smng soil, as highlighted by the SOM-A qualitative characterization by metabolomics. The soil mineralogical analysis showed that minerals maintained a higher crystallinity in the Smng than in the Cmng, where soil tillage promoted their alteration. Moreover, Fourier-transform infrared (FTIR) spectroscopy analysis highlighted that soil disturbance in the Cmng can affect SOM distribution, creating different spatial distributions in the particle aggregates and soil depths. Distinguishing SOM quantity, quality, and interaction with mineral components can help to understand its degradability and dynamics, both essential for mitigating the effects of climate change and promoting land protection.