Wheat-sulla intercropping system for nitrogen reduction and soil protection

Overview

This research proposes an innovative intercropping system that pairs winter wheat with sulla coronaria, a high-biomass forage leguminous species native to the Mediterranean area. Wheat is one of the most widely cultivated crops globally, but it is highly nitrogen-demanding, with conventional production requiring 25-30 kg of nitrogen per ton of grain. Synthetic nitrogen fertilizers are costly, subject to price volatility, and often inefficient due to leaching and denitrification losses.

The proposed system addresses these challenges by leveraging sulla coronaria's biological nitrogen fixation capability. Sown together with winter wheat, sulla grows during mild autumn conditions, accumulates nitrogen-rich biomass, and is then killed by winter frost in temperate climates. As the legume tissue decomposes, it releases nitrogen directly into the soil for the wheat crop. This approach is expected to reduce synthetic nitrogen fertilizer requirements by at least 30 percent while simultaneously providing ground cover that suppresses weeds and protects soil from erosion.

Technical specifications

Key features of the proposed system:

• Sulla coronaria exhibits a low base temperature of 6-8 °C, allowing autumn growth and biomass accumulation before winter
• The species is highly sensitive to winter frost, ensuring natural termination without herbicides or mechanical intervention
• Expected pre-winter biomass represents at least 20 percent of total yield potential (5-7 t/ha dry matter)
• Estimated nitrogen release of approximately 30 kg/ha from shoot tissues alone, with additional contributions from root and nodule degradation
• Two sowing methods under investigation: same-row sowing alongside wheat versus broadcast distribution
• Sowing density of 200 seeds per square meter for sulla coronaria
• Wheat sown at recommended density of 500 seeds per square meter
• Reduced nitrogen application of 100 kg/ha compared to conventional 150 kg/ha

Trial design parameters:

• Four treatments in a completely randomized block design with four replicates
• Plots of at least 40 square meters on silty-loam soil at the University of Padova experimental farm
• Measurements include periodical biomass of wheat and sulla, nitrogen accumulation, NDVI, final grain yield, and grain quality

Technology readiness level

This research is at an early experimental stage. Prior trials by the research team on wheat-leguminous intercrops using common vetch or red clover produced unsatisfactory results due to difficulties in timing legume sowing and excessive competition against wheat. The proposed wheat-sulla intercropping approach addresses these limitations by exploiting sulla's unique combination of low base temperature and frost sensitivity for natural crop termination. Field validation trials are planned at the University of Padova experimental farm in Italy to compare four treatments under realistic agronomic conditions, with the goal of quantifying nitrogen savings, biomass production, weed suppression, and final grain yield and quality outcomes.