Summary
This two-year organic field trial establishes a strong positive correlation between cover crop biomass and nitrogen fertiliser replacement value available to subsequent cereal crops, with legume-containing mixtures (particularly red clover and ryegrass + plantain + red clover) achieving the highest NFRV (up to 100 kg N ha⁻¹). The research identifies a critical biomass threshold of 1 Mg DM ha⁻¹ below which cover crops provide negative residual nitrogen effects, and demonstrates that autumn biomass can serve as a practical estimator to guide nitrogen fertilisation reduction in organic arable systems.
UK applicability
These findings are directly relevant to UK organic farming, particularly in northern regions with similar climatic conditions to Denmark. The identified cover crop species (ryegrass, plantain, red clover) and their mixtures are widely available and established in UK practice, making the biomass thresholds and NFRV values actionable for UK organic farmers seeking to reduce purchased nitrogen inputs.
Key measures
Cover crop aboveground biomass (Mg DM ha⁻¹), biomass nitrogen yield (kg N ha⁻¹), nitrogen fertiliser replacement value (kg N ha⁻¹ equivalent), Pearson correlation coefficients (r) between autumn and spring biomass and NFRV
Outcomes reported
The study measured nitrogen fertiliser replacement value (NFRV) of different cover crop types and their biomass accumulation in autumn and spring. It quantified the residual nitrogen contribution of cover crops to a subsequent spring barley test crop under organic management.
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