Upper limits to sustainable organic wheat yields

Area of Science:

• Agricultural Science
• Agronomy
• Soil Science

Background:

• Current synthetic nitrogen (N) fertilizer use is unsustainable due to high fossil energy demands and biosphere N enrichment.
• Biological nitrogen fixation (BNF) via legumes is a key renewable N source, particularly in organic farming systems.

Purpose of the Study:

• To assess the potential of BNF to sustainably replace mineral N fertilizers.
• To evaluate if BNF can close the yield gap between organic and conventional farming (O:C yield gap).
• To determine BNF's contribution to food security.

Main Methods:

• An agronomic modeling approach was employed.
• Simulations considered high-yielding areas with a realistic proportion of fodder legumes (33%) in crop rotation.
• Optimistic BNF levels (282 kg N ha⁻¹) were analyzed.

Main Results:

• Farming systems solely reliant on BNF are unlikely to achieve the yield levels of mineral N-based systems in high-yielding regions.
• Even with optimistic BNF, the O:C yield ratio was significantly below parity (0.62) for a reference wheat yield of 7.5 t ha⁻¹.
• Constraints such as limited arable land for legumes and variable on-farm BNF performance restrict its agricultural application.

Conclusions:

• Significant increases in BNF performance and minimized N losses are necessary to reduce the O:C yield gap.
• Inevitable productivity limits exist for legume-based farming systems, even with enhanced BNF.
• Sole reliance on BNF may not sustainably meet the demands of high-yielding agricultural systems or fully ensure food security.