Nature of soil organic phosphorus as affected by long-term fertilization under continuous corn (Zea mays L.):: A 31P NMR study

Soil organic phosphorus (Po) may have a significant role in supplying P to plants, but measurement of Po is tedious and often incomplete. Using P-31 nuclear magnetic resonance (NMR) techniques may enhance information from Po analyses. Soil samples from a Chicot sandy clay loam (Grey Brown Luvisol; fine-loamy, mixed, frigid Typic Hapludalf) and a Ste. Rosalie clay (Humic Gleysol; fine, mixed, frigid, Typic Humaquept) under continuous corn (Zea mays L.) were sequentially extracted with NaHCO3 (0.5 mol/L) and NaOH (0.1 mol/L) solutions and analyzed for Po compounds using 31P NMR. Monoester-Po accounted for 41 to 81% of Po extracted with NaHCO3 and 70 to 85% of soil Po extracted with NaOH. Small amounts of diester-Po (0 to 15%) and some unidentified Po compounds were also found. Monoester-Po was always present in the Po of any soil P fractions, but the amount varied with the amount of fertilizer P applied, soil type, and soil P fractions. In the Chicot soil, monoester-Po extracted with NaHCO3 and NaOH increased with the addition of inorganic fertilizer P at 132 kg P ha(-1). In the Ste. Rosalie soil after 8 years of cropping, the concentration of monoester-Po in NaHCO3 extracts increased when fertilizer P was added, but it decreased with zero-P addition, whereas monoester-Po in NaOH extracts decreased with zero-P addition and the effect was reduced with additions of fertilizer P. Organic P in NaHCO3 and NaOH extracts can be determined using conversion coefficients of 1.54 and 1.20, respectively, from monoester-Po.