The form and fate of stubble phosphorus in cropping soils

Abstract

Phosphorus (P) within stubble can be released directly to soil as soluble P or assimilated by microorganisms and subsequently released back into the soil through mineralisation. The chemical composition of P in crop stubble may play an important role in the rate of stubble P release. Crop stubble sampled after grain maturity in 2010/11 and 2011/12 contained 1-5 kg P/ha. Nuclear magnetic resonance (NMR) spectroscopy was used to determine the different forms of P present in the stubble. On average 50% of the total stubble P was orthophosphate which is water soluble and readily available to plants and microorganisms. The remaining P forms were identified as phospholipids, ribonucleic acid (RNA) and pyrophosphate. The majority (65-90%) of P in the major stubble component (stems) was water-soluble, and most of this was detected as orthophosphate. However, this includes organic P forms that may have been hydrolysed during the water extraction. Results from a field leaching experiment show between 0 and 7.5% of stubble P was released from 52 mm (collected after 11 mm and 41.4 mm) of summer rainfall. That indicated crop stubbles in the field release some P into soil solution which could be readily accessed by plant roots. However, the slower release of stubble P in the field compared to the laboratory suggests that residues potentially play a more important role as a long term P supply to subsequent crops. Subsequent experimentation will measure the contribution of surface applied and incorporated stubble P to crop plant P nutrition using 33P isotopic techniques.