Analysis of phosphorus in two humic acid fractions of intensively cropped lowland rice soils by P-31-NMR

The organic forms of phosphorus in the soil appear to be changing as rice g rowing intensifies and the soil is flooded for longer in tropical Asia. To examine these changes, we extracted the labile mobile humic acid (MHA) and more recalcitrant calcium humate (CaHA) fractions from soils supporting lon g-term field trials in the Philippines and analysed them by solution P-31-n uclear magnetic resonance (NMR) spectroscopy. Diester P and sugar-diester P accumulated moderately with increasing intensity of irrigated rice croppin g, reaching a combined 42% of all MHA-P for a triple-cropped irrigated fiel d compared with 28% for fully aerated fields growing dryland crops. The mon o- to diester P ratio decreased by 43% for the MHA and CaHA from the aerate d fields to the triple-cropped field. Smaller effects on forms of P were no ted for the rates and type of N, P and K fertilizer and site effects. The e ffects of treatment and site were more noticeable in the MHA than in the Ca HA. The proportions in the NMR spectra were tightly correlated with visible light absorption, concentrations of organic free radicals and H, and N-15- NMR spectral proportions, which indicate the degree of humification. The MH A and CaHA accounted for only 0.6-8.3% and 0.9-5.7%, respectively, of total P; most of the P is inorganic.