Soil organic carbon stocks and dynamics under contrasting organic matter management practices – Results from a 65-year-old experiment in Belgium

Long-term field experiments (LTEs) are key for evaluating the long-term effect of agricultural practices on soil organic carbon (SOC) storage. In this work, we studied the long-term effect of organic matter (OM) management practices on SOC in a 65-year LTE under arable cropping in Belgium. To evaluate SOC dynamics, a time series (1959–2021) of topsoil SOC content was analyzed for three subperiods in which N fertilization remained unchanged. Topsoil and subsoil SOC stocks were calculated by the equivalent soil mass method for one individual sampling campain in 2018. Cattle manure (CM) application led to the most significant SOC storage, increasing topsoil SOC stock and content by approximately 20 % over the long term. Other individual OM management practices – pig slurry (PS) application, cereal straw incorporation and cover crops (CC) cultivation – induced smaller, non-significant SOC increases of about 5 %. Nevertheless, their combined effects were cumulative, resulting in significant SOC accruals. In contrast to the topsoil, SOC stock in the subsoil was unaffected by the OM management practices. Annual humified C inputs calculated from the amount and quality of livestock manures and (cover) crops residues correlated strongly with SOC content in the topsoil (r = 0.8), which underlines that the amount and quality of C inputs are key drivers of SOC storage. Changes in N fertilization over time and N fertilization adjustments among treatments significantly affected SOC dynamics. This highlights the complexity of interpreting time series of data from LTEs, for which the evolution of management practices over time may act as confounding factors. • Cattle manure significantly increased SOC storage in the topsoil (0 – 25 cm). • Incorporation of cereal straw, cover crops and pig slurry prevented SOC loss. • Subsoil SOC stocks remained unchanged after 65 years of contrasting management. • Changes in N fertilization over time significantly affected SOC storage. • Humified C inputs are the main drivers of SOC storage.