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Analysis of phosphorus in two humic acid fractions of intensively cropped lowland rice soils by 31 P‐NMR
Author(s) -
Mahieu N.,
Olk D. C.,
Randall E. W.
Publication year - 2000
Publication title -
european journal of soil science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.244
H-Index - 111
eISSN - 1365-2389
pISSN - 1351-0754
DOI - 10.1111/j.1365-2389.2000.00329.x
Subject(s) - chemistry , humic acid , humus , phosphorus , soil water , fertilizer , aeration , agronomy , environmental chemistry , environmental science , biology , soil science , organic chemistry
Summary 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 31 P‐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 15 N‐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.