Carbon in above- and belowground harvest residues of silage maize under contrasting nutrient regimes

Models simulating management-induced changes in soil organic carbon (SOC) stocks in agricultural soils typically rely on crop-specific above- and belowground C inputs based on harvest yields and allometric functions. However, experimental data supporting these functions over a range of yield levels induced by different nutrient regimes are scarce. This study addresses this gap by quantifying above- and belowground harvest residues of silage maize grown in the Askov long-term experiment and subject to mineral fertilizers and animal manure applied at different levels (deficient, suboptimal, optimal, and over-optimal). Two methodological approaches (coring and excavation) were combined to quantify macro-root biomass (≥ 425 µm) in the 0–30 cm soil layer. Nutrient regime had a significant effect on maize harvest yield, but no significant effect on stubble and root biomass C was detected. The measured total above- and belowground C in harvest residues for silage maize grown with suboptimal to over-optimal nutrient supply averaged 1.67 Mg C ha −1 without including rhizodeposition. Based on a single site and one experimental year, our results suggest that soil C models may better rely on fixed C inputs from maize stubble and roots, independent of nutrient regime and harvest yield, rather than on yield-dependent allometrics. Measurements of above- and belowground harvest residues in silage maize are rare, and further studies are needed to verify the estimation of C inputs for soil C modelling. • Stubble and root biomass C was not related to maize harvest yield. • Using the allometric function introduced systematic errors in estimating C inputs. • Refining allometric functions for brace roots would enhance model estimates.