Integrating leys, cover crops, and animal manures constitute promising avenues to reach annual soil organic carbon changes (ΔSOC) >0.4% in forage and grain-based crop rotations, rates required to offset the increasing C emissions from fossil fuels (“4 per mille” initiative). How these practices and rotations perform in reaching this aim was object of analysis in this paper. Five cropping systems (CS), including three three-year forage and grain-based crop rotations containing annual grass-clover leys (FR and MR) or cover crops (GR), and two contrasting controls (continuous silage maize (CM), and permanent grassland (PG)) were compared for their impact on SOC stocks over eight years (2010–2018). The CS were unfertilized (N0) or fertilized using cattle slurry (N1) at a rate of 240 kg N ha−1 yr−1 applied in the non-leguminous crops. The ΔSOC of the top 30 cm soil layer and the annual carbon inputs (Cin) from slurry applications and plant residues were estimated, their relationship established, and the slurry-induced C retention coefficient was determined. The FR and MR SOC stocks remained stable at N1, while the GR and CM SOC decreased over time by tendency even at N1. Only the PG reached ΔSOC >0.4%. Differences in ΔSOC between CS and N rates were highly associated with the system-specific increase in belowground Cin, induced by slurry applications. Slurry-induced C retention coefficients differed strongly between CS: CM (3%) followed by GR (12%), and by FR and MR (20−15%), and lastly by PG (24%). Promoting belowground carbon inputs was identified as an efficient way to reach significant increases in ΔSOC. We conclude that a ley in only one out of three years is not sufficient to significantly increase SOC stocks in arable crop rotations of the study region.

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.

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Abstract. Land-use change (LUC) and conventional tillage (CT) result in the reduction of the high soil organic carbon (SOC) stocks stored in grassland ecosystems during their conversion and renovation, contributing to global warming. While plenty studies show the use of no-tillage (NT) as a promising option to increase the topsoil SOC stocks of arable lands, its potential to conserve SOC during grassland conversion and renovation events has been poorly investigated. Further, the effects of LUC and tillage methods on the SOC dynamics have been limited to the topsoil by most studies, thus overlooking their impact on the subsoil where significant amounts of SOC are stored, and changes in vegetation and residue distribution can negatively affect these. In this study, a 10-year-old grassland was converted to continuous silage maize (CM) using NT (NT-CM) and CT (CT-CM), and renovated using NT (NT-GR), while some part remained undisturbed as a control (GC). The systems were either non-fertilized (N0) or fertilized according to a demand of 180 and 380 kg N ha−1 yr−1 (N1) in the silage maize and grassland systems, respectively. SOC stocks were measured annually and annual SOC changes (ΔSOC, in Mg C ha−1 yr−1) were calculated for different soil layers (0–30, 30–60 and 60–90 cm) and across the whole profile (0–90 cm) over a 6-year period (2014–2020). Annual soil carbon inputs (Ci) via plant residues were quantified and related to ΔSOC. Results showed that cropping systems significantly affected SOC dynamics over time. At 0–30 cm, SOC stocks were significantly reduced after conversion using both tillage methods, however, 44 % lower annual losses were obtained in NT-CM (−1.5 Mg C ha−1 yr−1) compared to CT-CM (−2.7 Mg C ha−1 yr−1). Conversely, SOC stocks remained unchanged after NT-GR same as the GC. In the subsoil, SOC stocks increased under GC (1.1 Mg C ha−1 yr−1) and remained unchanged in the other systems. Across the whole profile, SOC stocks increased in GC, remained stable in NT-GR, and decreased in NT-CM and CT-CM with mean annual change rates of 1.3, −0.1, −1.9 and −3.4 Mg C ha−1 yr−1, respectively. The differences in ΔSOC between the unploughed systems (NT-GR, NT-CM and GC) were strongly related to the annual soil Ci from plant residues in the topsoil. Our findings highlight the great potential of NT to slow down the annual SOC losses after grassland conversion or renovation, and that C sequestration can occur in the subsoil of permanent grasslands when the topsoil C is already saturated. This strengthens the need to consider the SOC changes occurring in the whole profile after a LUC event.