Crop residue phosphorus: speciation and potential bio-availability

Phosphorus (P) mineralisation from crop residues is usually predicted from total P or carbon: phosphorus (C: P) ratios. However, these measures have limited accuracy as they do not take into account the presence of different P species that may be mineralised at different rates. In this study P speciation was determined using solution P-31 nuclear magnetic resonance (NMR) spectroscopy to understand the potential fate of residue P in soils. Mature above-ground biomass of eight different crops sampled from the field was portioned into stem, chaff and seed. The main forms of P detected in stem and chaff were orthophosphate (25-75 %), phospholipids (10-40 %) and RNA (5-30 %). Phytate was the dominant P species in seeds, and constituted up to 45 % of total P in chaff but was only detected in minor amounts (< 1 %) in stem residue. The majority (65-95 %) of P in stems was water-extractable, and most of this was detected as orthophosphate. However, this includes organic P that may have been hydrolysed during the water extraction. This study indicates that the majority of residue P in aboveground plant residues has the potential to be delivered to soil in a form readily available to plants and soil microorganisms.