Characterization of Humic-Bound Phosphorus From Corn Soils by Wet Chemistry and Solution P-31 NMR Spectroscopy.

Phosphorus (P) has long been known to be present in humic substances from various sources. Only limited information is available on the form and lability of humic-bound, although such information is critical for understanding the role of humic substances in P cycling and nutrition. We extracted the mobile humic acid (MHA) and recalcitrant calcium humate (CaHA) fractions from a Hard silt loam (fine-silty, mixed, mesic Cumulic Haplustolls) from Nebraska amended with beef cattle manure, inorganic fertilizer, or neither (Control). Phosphorus in the six humic fractions was characterized by solution P-31 NMR spectroscopy, enzymatic hydrolysis and UV irradiation. All three CaHA fractions contained more P than their corresponding MHA fractions. Manure application increased P concentration in both MHA and CaHA fractions. Both NMR and wet chemistry analysis identified about 10% of P in these six humic fractions as labile inorganic P or orthophosphate. NMR analysis demonstrated that organic P was present in various forms of mono- and di-esters with a small portion of phosphonate (0-3.7% of P). However, the phytate form of P was not present in these humic fractions. Enzymatic hydrolysis demonstrated that only 18-32% of P in these humic fractions was enzymatically hydrolyzable. UV irradiation released an additional 0 to 11% of humic P as labile inorganic P. These data indicated that P in humic fractions was present in multiple forms having varying labilities.