Phosphorus uptake benefit for wheat following legume break crops in semi-arid Australian farming systems

This field study assessed phosphorus dynamics (crop-P uptake, resin-extractable P in the root-zone, P mobilisation and microbial-P) in break crop-cereal rotation sequences at four Australian semi-arid field sites differing in soil P fertility. Phosphorus mobilisation (9–30 kg P ha⁻¹) was apparent under break crops, consistently under canola and peas at three sites with low soil P fertility (i.e. pre-sowing soil resin-extractable P < 20 mg P kg⁻¹). Enhanced biological cycling of P (i.e. increased microbial-P) was limited to a low P site in the break crop phase. Phosphorus content of break crop aboveground residues following grain removal was 1–7 kg P ha⁻¹; P input was greater (12–18 kg P ha⁻¹) where legumes were green/brown manured. Varied residue P input did not result in differences in resin-extractable or microbial-P in soil prior to sowing wheat. Phosphorus uptake was greater for wheat after legume break crops compared to continuous wheat (2.0–4.7 kg P ha⁻¹) at all sites, especially where crops were green/brown-manured (3.9–5.9 kg P ha⁻¹). Greater P uptake by wheat was associated with increased grain yield at three sites but was not significantly correlated with the quantity of P input from break crop residues at all four sites or with soil mineral nitrogen pre-sowing of wheat at three sites. Break crops can directly contribute to P resource-use efficiency by mobilising residual P from soil but the agronomic significance of P supply from break crop residues to a P uptake benefit for following wheat remains to be elucidated.