Root biomass in cereals, catch crops and weeds can be reliably estimated without considering aboveground biomass

Reliable information on belowground plant biomassis essential to estimate belowground carbon inputs to soils. Estimations of belowground plant biomassare often based on a fixed allometric relationship of plant biomassbetween aboveground and belowground parts. However, environmental and management factors may affect this allometric relationship making such estimates uncertain and biased. Therefore, we aimed to explore how root biomassfor typical cereal crops, catch cropsand weeds could most reliably be estimated. Published and unpublished data on aboveground and root biomass(corrected to 0–25 cm depth) of cereal crops (wheat and barley), catch cropsand weeds were collected from studies in Denmark. Leave one out cross validation was used to determine the model that could best estimate root biomass. Root biomassvaried with year, farming system (organic versus conventional) and cereal species. Shoot and root biomassof catch cropswere higher than for weeds (sampled in late autumn), and farming system significantly affected root biomassof catch cropsand weeds. The use of fixed root biomassbased on the most influential factors (farming system and species) provided the lowest error of prediction for estimation of root biomass, compared with the use of fixed allometric relations, such as root/shoot ratio. For cereal crops, the average root dry matter in organic farmingsystems was 218 g m−2 (243 and 193 g m−2 for wheat and barley, respectively), but in conventional systems only 139 g m−2 (142 and 129 g m−2 for wheat and barley, respectively). For catch cropsand weeds, the root dry matter in organic farmingsystems were around 127 and 35 g m−2, and in conventional farming systems 75 and 28 g m−2, respectively. In conclusion, the present analysis indicates that root biomassin cereals, catch cropsand weeds can be reliably estimated without considering aboveground biomass, and it may be better estimated using fixed values based on species and farming systems than using fixed allometric ratios.