Biomass Carbon in Harvest Residues of Winter Oilseed Rape Across Fertilizer N Rates: Implications for Carbon Input Estimations in Soil Carbon Models

ABSTRACT Background : Reliable estimation of carbon (C) inputs from above‐ and belowground residues at different yield levels is crucial, as these inputs drive soil organic carbon (SOC) models for C accounting. Aims : This study quantifies C inputs from different components of winter oilseed rape (WOSR) and compares them with estimates from various allometric functions. Methods : The effect of six mineral nitrogen (N) fertilization rates (40, 90, 140, 190, 240, 290 kg N ha −1 ) was examined on the rapeseed yield and its components (straw, stubble, chaff, and root), assessed at crop maturity, in a Danish field trial on a sandy loam soil. The biological optimum N rate was identified to evaluate rapeseed yield response to fertilization. Additionally, C‐TOOL model was utilized to simulate SOC stock changes over time, using fixed C inputs and allometric‐based inputs. Results : The biological optimum N rate for rapeseed yield was 208 kg N ha −1 . Stubble, chaff, and fine root biomass remained stable across N rates, supporting fixed C inputs for these components. Straw biomass correlated with rapeseed yield, but the amounts differed from allometric estimations, suggesting refinement in the straw‐to‐rapeseed yield ratio. SOC model simulations indicated that standardized allometric functions produced higher SOC stock changes with increasing yield (2017–2024). When fixed C input for stubble and chaff (for all years) was combined with the refined straw C input (for WOSR), model outputs more closely matched the SOC measurements. Conclusions : WOSR showed component‐specific responses to varying fertilization rates, highlighting the need for crop‐specific residue dynamics in SOC modeling.