Long-term field experiments are important to provide information about how soil carbon (C) dynamics is affected by soil tillage systems. In this study we directly diagnose the influence of the topsoil's (0-5cm depth) C concentration on soil quality in an Entisol under Mediterranean conditions, testing a new statistical method. The objective was to estimate the impact of conservation tillage (RT) as a C management practice, versus a traditional tillage (TT) system on biophysical soil properties. We analyzed the impact of the soil C management on biological (microbial biomass C, dehydrogenase and ß-glucosidase activities), physical soil properties (aggregates stability, conductive mean pore diameter, aggregation index) and CO 2 fluxes. The concentrations for total organic carbon (TOC), the active carbon (AC) normalised by the total carbon (AC TOC -1), served as a combined proxy for the soil C management related to the tillage system. Soil C management accounted for 0-46% of the change of biophysical soil properties in RT versus TT. The RT led to a C increase (18.9%) of microbial activities, especially in the top 0-5cm depth. Related to the physical soil properties, less C in TT led to a lower aggregation index, although this tendency was not observed for other physical parameters. The impact of soil C management was better correlated with soil microbial than with the physical properties. Our analysis directly quantified for the first time that the increase in the soil's carbon concentration could only explain a small fraction of the beneficial change in biophysical soil properties due to RT. In general the RT contributed to the long-term sustainability of the agroecosystem by improving biological and physical soil characteristics under dryland semi-arid Mediterranean conditions.
