Cereal straw incorporation and ryegrass cover crops: The path to equilibrium in soil carbon storage is short

Reduced use of fossil energy by removing cereal straw for bioenergy potentially threatens soil organic carbon (SOC) storage. Straw incorporation plays a key role for SOC storage in cereal-based cropping systems, but the use of cover crops (CCs) may compensate for straw removal. However, assessing the SOC sequestration potential of management changes requires field experiments with long continued treatments and frequent soil sampling. Based on a field experiment initiated in 1981 on a sandy loam soil at Askov Experimental Station (Denmark), we examined the effect of annual additions of spring barley straw (0, 4, 8 and 12 Mg ha(-1)) and undersown ryegrass CC on SOC storage in the 0- to 20-cm layer. The effect of straw incorporation and CC on SOC was additive. At steady-state conditions, the SOC stock based on equivalent soil mass (SOC stock(FM)) increased 3.9, 6.7 and 9.3 Mg C ha(-1) after annual incorporation of 4, 8 and 12 Mg straw ha(-1), respectively. The ryegrass CC increased SOC stock(FM) by 3.3 Mg C ha(-1) and thus almost compensated for removal of 4 Mg straw ha(-1). An asymptotic regression model best described the temporal changes in SOC and showed that the effect of straw incorporation and ryegrass CC on SOC sequestration peaks after 10-15 years when a new equilibrium between input and output of C is reached. Clearly, reliable assessments of SOC sequestration potentials following changes in management require field experiments with frequent soil sampling until reaching steady-state conditions. Highlights SOC sequestration potentials of straw incorporation and CCs were assessed. The crucial issue of sequestration duration of experimental treatments is often neglected. The effect of straw and CC peaked after 10-15 years when a new equilibrium in SOC was reached. Reliable assessments require frequent soil sampling until reaching steady-state conditions.