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.