Similar root and stubble biomass carbon in grass–clover leys irrespective of yield, species composition, sward age, and fertilization

Background Crop‐ and site‐specific quantification of non‐harvestable aboveground residues and root biomass is essential for predicting management‐induced changes in soil C storage. Aims The aim of this study was to quantify stubble and root biomass C from productive grass–clover leys used for cutting as affected by fertilization and sward age. Methods Based on an organic long‐term dairy crop rotations experiment with 4 years of grass–clover in a six‐course rotation, we examined the effects of fertilization (unfertilized and 300 kg total‐N ha−1 in cattle slurry) and sward age (1–4‐year‐old) on herbage yield and composition, stubble biomass, and composition and root biomass of grass–clover ley. Results Ley duration and fertilization altered plant community composition and aboveground productivity but did not affect stubble and root biomass C. Conclusions The results question the use of yield‐dependent allometric functions for grass–clover ley used in simulation models and life cycle assessments for C accounting in agricultural systems. For predictions of soil C changes, we recommend the use of a fixed stubble‐derived C input from grass–clover ley regardless of sward age and fertilization‐induced differences in species composition, and herbage yield. Likewise, a fixed root‐derived C input for 1‐year‐old grass–clover, irrespective of fertilization, may be implemented. However, the contribution of continuous rhizodeposition and fine root turnover to root‐derived C input need to be accounted for.