To quantify the profitability, nitrogen legacy, and rotational value of vetch and lupins across multiple end-uses.

Pre-sowing, the soil had a good level of mineral nitrogen available (74 kg N/ha in the 0 – 30 cm layer) and low soon-to-be available nitrogen (PMN 3.3 kg N/ha at 0–10 cm and <2.8 kg N/ha at 10–30 cm). Despite this, both legumes relied strongly on biological fixation at peak biomass. The specific drivers of this response cannot be confirmed without in-season soil N measurements.

At early flowering, lupins accumulated more plant N largely because they produced more biomass, resulting in higher total N fixed (121 kg N/ha in lupins vs 76 kg N/ha in vetch), even though vetch had a higher proportional reliance on fixation. The difference in lupin nitrogen derived from air (NDFA) between the grain (34%) and manure (88%) sections likely reflects spatial variation in soil nitrogen availability and seasonal influences on fixation intensity, rather than the intended end-use management, as sampling occurred before biomass removal.

Nitrogen tracing by end-use indicates that after harvest/termination, the biologically active N pool (PMN) increased strongly at the surface level. Manure treatments showed the highest increase in N mineralisable potential post-harvest. This was not reflected in Total N (%), as PMN represents nitrogen that has not yet converted into plant-available forms but has the potential to supply nitrogen to the following crop.

Further nitrogen tracing will be undertaken in 2026, as the trial progresses into a cereal rotation. This next phase will evaluate the agronomic response of the following crop and enable a comprehensive economic assessment of the rotational value associated with each legume end-use.