Organic carbon storage potential in deep agricultural soil layers: Evidence from long-term experiments in northeast Italy

The significant contribution to the global carbon budget made by soil organic carbon (SOC) inspired the “4 per 1000” initiative. It promotes agricultural practices aimed at raising SOC stocks 0.4 % annually over 20 years. However, the response of topsoil and deep soil profiles to agroecosystem changes is largely unknown at present. Our work aims to quantify deep SOC accumulations in stabilized croplands and grasslands of northeast Italy and identify the best management practices to increase those stores. Soil profiles were collected to 70 and 90 cm depths from three well-established long-term experiments. A total of 1242 soil samples were analyzed for SOC concentrations as a function of soil type, soil management practice, and cropping system. SOC stocks were quantified using the equivalent soil mass method. Results show that SOC stocks averaged 23.2 Mg ha−1 in sandy Arenosol, 59.3 Mg ha−1 in silty loam Cambisol, 111.6 Mg ha−1 in clay loam Gleysol, and 383.5 Mg ha−1 in peaty Histosol. Substantial SOC stocks were found in the subsoil beneath the tilled layer, ranging between 59 % and 74 % in sandy and clay loam soils. Among the considered managements, the SOC accumulation rate of permanent meadow topsoil was higher than croplands (0.299 Mg ha−1 yr−1), which fell to 0.256 Mg ha−1 yr−1 when estimated along the full soil profile. In contrast, organic carbon added through manures and residues, coupled with minimum tillage practices, led to increased average SOC stock rates in the topsoil (0.205 Mg ha−1 yr−1) and throughout the full soil profile (0.386 Mg ha−1 yr−1), suggesting that some translocation dynamics occurred. The long-term adoption of permanent meadow, along with manure or residue addition under minimum tillage, made it possible to achieve “4 per 1000” goals to great depths in naturally poor-SOC sandy and silty loam soils. In SOC-rich soil, only in the topsoil layer achieved this.