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

The organic forms of phosphorus in the soil appear to be changing as rice growing 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 long-term field trials in the Philippines and analysed them by solution P-31-nuclear magnetic resonance (NMR) spectroscopy. Diester P and sugar-diester P accumulated moderately with increasing intensity of irrigated rice cropping, reaching a combined 42% of all MHA-P for a triple-cropped irrigated field compared with 28% for fully aerated fields growing dryland crops. The mono- to diester P ratio decreased by 43% for the MHA and CaHA from the aerated fields to the triple-cropped field. Smaller effects on forms of P were noted for the rates and type of N, P and K fertilizer and site effects. The effects of treatment and site were more noticeable in the MHA than in the CaHA. 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 MHA and CaHA accounted for only 0.6-8.3% and 0.9-5.7%, respectively, of total P; most of the P is inorganic.