Comment on bg-2022-6

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.