Carbon cycle related indicators better describe soil quality compared to total organic carbon content

Soil organic carbon (SOC) content is the major indicator used for soil quality evaluation because provides several ecosystem functions. However, SOC content does not allow to understand the soil potential to deliver the key ecosystem functions because most of soil processes are linked to soil biota. This research aimed to demonstrate the importance of soil indicators related to the SOC cycle rather than SOC content for soil quality evaluation. To reach this goal, three farms characterized by diverse soil types (Fluvisol and Cambisol) were selected in the Po plain of Emilia-Romagna Region, Italy. Moreover, different agricultural practices were performed: three-year-old pear trees using conventional management for Maccanti farm (MAC), 10-year pear orchard with integrated management for Zani (ZAN) and 10-year peach orchard with organic management for Biondi (BIO). MAC is located in ancient reclamation area, where Fluvisols are enriched of peat and organic matter. In each farm, soil samples from 0–15 (hereafter called topsoil) and 15–30 cm (hereafter called subsoil) depth were collected and analysed for the contents of SOC, labile organic carbon (Clab), fulvic acids, humic acids, humin and microbial biomass–C (Cmic), and for microbial respiration (Resp). In order to evaluate the soil processes related to C cycle, the humification rate (HR), metabolic quotient (qMET) and microbial quotient (qMIC) were calculated. MAC soil showed the highest SOC content without differences between topsoil and subsoil, due to ancient reclamation and agricultural management. BIO and ZAN showed similar SOC contents and it was higher in the topsoil than in subsoil due to grassy turf. Compared to BIO and ZAN, MAC soil showed a higher amount of Clab, and SOC was composed by a lower percentage of stable organic carbon (humin). Despite the higher Clab concentration, which is an easily available C source for microbes, no differences of Resp were observed among the sites, and MAC showed the lowest Cmic content. These data would indicate the presence in MAC of stress conditions which do not allow the growth of microbial biomass. The occurrence of stress conditions is clearly showed by the lowest qMET indicating how the conventional agricultural practices in peaty Fluvisol negatively affect the carbon use efficiency of microbial biomass. As a consequence, these stress conditions do not allow the C stabilization as suggested by the lowest qMIC. Further, the low C stabilization processes are highlighted by the highest HR. Conversely, despite the lowest content of Clab, BIO soil showed the lowest qMET and the highest qMIC suggesting how organic managements tend to improve the soil quality. Hence, the present study highlighted the importance of indicators linked to soil microbiome for soil quality evaluation in order to preserve its ecosystem functions. Indeed, organic carbon rich soils as those of MAC would indicate high quality soils but, because of the highly impacting practices, they showed stress conditions when the indicators linked to soil microbiome are taken in account. Therefore, if these indicators are not considered for soil quality evaluation, several fields used for agricultural purposes could become degraded.