Are fertilization-induced differences in silage maize yield related to the amount of above- and belowground crop residues left after harvest?

Simulation carbon models predicting management-induced changes in soil organic carbon (SOC) stocks in agricultural soils often rely on crop-specific carbon inputs derived from harvest yields and allometric functions. However, there is limited empirical data supporting these functions over a range of yield levels induced by different nutrient regimes. The present study examines the impact of mineral fertilizers and animal manure applied at different levels (deficient, suboptimal, optimal, and over-optimal) on C-input with above- and below-ground residues of silage maize grown in 2023 in the Askov long-term field experiment. To obtain accurate root biomass estimates, we employed two methodological approaches (soil coring and excavation) and compared the field-measured estimates with those derived from allometric functions. Nutrient regime had a significant effect on maize yield but did not significantly affect stubble and root biomass C, challenging the use of yield-dependent allometric functions when modelling C-input to the soil under silage maize. The field measured silage maize C input averaged 1.67 Mg C ha-1 (without rhizodeposition) and ranged from 1.51 to 1.86 Mg C ha-1 with no significant differences between suboptimal to over-optimal nutrient supply. Hence, our findings indicate that relying on fixed C-inputs from maize stubble and roots regardless of nutrient regime and harvest yield provides a more reliable estimate for soil C models than yield-dependent allometrics.