Crop Residue Phosphorus: Speciation and Availability.

Phosphorus (P) mineralisation from crop residues is usually predicted from total P or C:P ratio. However, these measures have limited accuracy as they fail to account for the presence of various P species which control the rate of P mineralisation from residues. Detailed P speciation on NaOH-EDTA extracts of crop residues was determined using solution 31P nuclear magnetic resonance (NMR) spectroscopy. Mature above-ground biomass of eight different crops from the field was separated into stem/leaf, chaff and seed. The main forms of P detected in stem and chaff were orthophosphate (35-75%), 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 material. The most important differences between cereal and legume residues were the absence of phytate in legume chaff and the presence of RNA (10-15%) in legume seeds. The majority (65-90%) of residue P in stems was water-extractable, and most of this was detected as orthophosphate. However, this includes organic P that was hydrolysed during the water extraction. This indicates that the majority of residue P in aboveground plant residues has the potential to be delivered to soil in a readily available form.