No-Till Mitigates SOC Losses after Grassland Renovation and Conversion to Silage Maize

Many studies recommend no-till (NT) to increase soil organic carbon (SOC) in the topsoil (<30 cm) of arable land to counterbalance greenhouse gas emissions. Its potential use to mitigate SOC losses during conversion and renovation of grassland ecosystems in the top meter soil is yet to be determined. The SOC dynamics of a 10-year-old grassland converted to silage maize (CM) and renovated and seeded (GR) using either conventional tillage (CT) or NT were compared to an undisturbed grassland control (GC) for 7 years, across three fixed soil depth increments (0–30, 30–60, 60–90 cm). The annual C inputs (Cinput) from crop residues were further analyzed. The systems were either non-fertilized (N0) or fertilized with mineral N (N1) according to a demand of 180 and 380 kg N ha−1 year−1 in the silage maize and grassland systems, respectively. For the 7-year period, the renovated grassland using NT ensured maintenance of the initial SOC in the topsoil, while a conversion toward arable cropping resulted in SOC losses, regardless of the tillage method. The use of NT during conversion significantly reduced these losses from 2.5 Mg ha−1 year−1 to 1.8 Mg ha−1 year−1, for a 28% reduction compared to CT. In the subsoil (30–90 cm), SOC remained stable and was not affected by the cropping systems nor by the tillage method. Reduced annual Cinput was found as the main factor affecting SOC losses after grassland removal, regardless of the tillage method. Our findings highlight the potential of NT to mitigate annual SOC losses after grassland conversion if annual Cinput remains high.