Biochar from Cereal Straw Can Offset Soil Carbon Losses from Increased Straw Harvest in German Croplands

Soil organic carbon (SOC) stocks in European croplands are declining, and increasing demand for biomass to substitute fossil fuel–based products may further exacerbate this trend. Cereal straw represents a substantial and still underutilized biomass resource; however, its additional removal may intensify SOC losses. This study quantifies the impact of increased straw removal on SOC stocks in German croplands and evaluates the potential of biochar (BC) produced from cereal straw to compensate for these losses.

SOC dynamics were modelled for 1,115 cropland sites using soil and management data from the first German Agricultural Soil Inventory. Seven scenarios representing different levels of straw removal and BC application were simulated using a model ensemble combining three allometric approaches for carbon input estimation with the RothC soil carbon model.

Annual straw harvest from German croplands could theoretically be tripled to 26.3 million Mg straw (dry matter). Removing all additionally harvestable straw (17.4 million Mg) reduced mean carbon inputs to cropland soils by 21% and resulted in an average SOC loss of 8.9 Mg C ha⁻¹ within 100 years. During the first 20 years, SOC stocks declined by 0.10 ± 0.20 Mg C ha⁻¹ a⁻¹, corresponding to additional emissions of 4.2 million Mg CO₂ a⁻¹ at the national scale compared to business-as-usual. Although SOC losses decreased over time, they persisted beyond 200 years and were more pronounced on clayey than on sandy soils.

In contrast, converting straw to BC and applying it to the same cropland soils led to an SOC increase of 16.7 Mg C ha⁻¹ within 100 years, equivalent to a net carbon sink of 25.6 Mg C ha⁻¹ and negative emissions of 93.9 million Mg CO₂ ha⁻¹. An average BC application rate of 4.9 Mg C a⁻¹ increased SOC by 10.8 million Mg CO₂ a⁻¹ relative to business-as-usual, corresponding to approximately 10% of Germany’s current annual agricultural greenhouse gas emissions. Accounting for SOC losses due to straw removal reduced cumulative SOC gains over 100 years by 25%.

Our results demonstrate that additional straw harvesting can undermine SOC stocks and climate mitigation efforts, whereas BC application from cereal straw has substantial potential to offset these losses. However, climate mitigation strategies relying on agricultural residues should integrate SOC losses from biomass removal to avoid overestimating the mitigation potential of BC systems.