The grain N uptake response of an opportunity cropping regime comprising summer and winter cereal and legume crops to fertiliser nitrogen (N) and phosphorus (P) applications was studied in 2 long-term experiments with contrasting durations of cultivation. At the longer cultivation duration Colonsay site (>44 years at commencement), grain N uptake increased with fertiliser N application in 15 of 17 harvested crops from 1985 to 2003. Grain sorghum on short-fallow consistently responded to applied fertiliser N at higher rates (≥80 kg N/ha) than crops grown on long-fallow where either fertiliser at nil or 40 kg N/ha maximised grain N uptake. Winter cereal response to applied N was influenced by fallow length, generally smaller responses in long fallow years, although in-crop rainfall affected this. Short-fallow crops responded up to 40 or 80 kg applied N/ha, while seasonal growing-season rainfall affected the responses of the double-crop winter cereals the most. Responses to applied fertiliser N at the shorter duration cultivation Myling site (9 years at commencement) generally occurred only under high-intensity cropping periods, or in those crops sown following periods of slower potential N mineralisation. Phosphorus fertiliser application influenced grain N uptake at both locations in some years, with winter cereals, legumes, and sorghum sown following long-fallow generally significant.

Cumulative grain N uptakes in both experiments were independently influenced by fertiliser N and P treatments, P having an additive effect, increasing grain yield and grain N removed. Recovery efficiency of fertiliser N in grain, derived from cumulative N fertiliser application and grain N uptake, in general declined as amount of fertiliser N applied increased; however, as N supplies became less limiting to yield, P fertiliser generated higher fertiliser N recovery in grain. At Colonsay, RE_NG from cumulative uptake and removal was ≥0.48 with fertiliser P application for cumulative fertiliser N input ≤1340 kg N/ha (≈80 kg fertiliser N/ha.crop).