Forms and Lability of Phosphorus in Humic Acid Fractions of Hord Silt Loam Soil
2011
He, Zhongqi | Olk, D. C. | Cade-Menun, Barbara J.
Phosphorus has long been known to be present in soil humic fractions, but little is known about specific P forms in humic fractions or their lability. We extracted the mobile humic acid (MHA) and recalcitrant calcium humate (CaHA) fractions from a Nebraska Hord silt loam soil (a fine-silty, mixed, superactive, mesic Cumulic Haplustoll) under continuous corn (Zea mays L.) receiving either inorganic fertilizer or animal manure. Solution ³¹P nuclear magnetic resonance spectroscopy demonstrated that P in both MHA and CaHA was predominantly present in organic forms, mostly as orthophosphate monoesters. Spiking experiments indicated no phytate present in these humic fractions, but scyllo-inositol P was identified in all samples. Potato phosphatase hydrolyzed some humic-bound P. Fungal phytase released more humic-bound P, which may come from scyllo-inositol P. No additional P was released by including nuclease. Ultraviolet (UV) irradiation increased soluble inorganic P in MHA fractions, but total hydrolyzable P in MHA fractions did not increase, suggesting that the portion of P that was UV labile was also enzymatically hydrolyzable. In contrast, UV irradiation increased soluble inorganic P and total hydrolyzable P in CaHA fractions, which suggests that UV-labile P in CaHA fractions did not overlap with enzymatically hydrolyzable P. Fertilization management did not significantly alter the lability of humic P in these humic fractions. This research has the potential to improve P management by increasing our knowledge of P lability for more efficient crop uptake.
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