Enzymatically and Ultraviolet-Labile Phosphorus in Humic Acid Fractions From Rice Soils

Humic acid (HA) is an important soil component that call improve nutrient availability and impact other important chemical, biological, and physical properties of soils. We investigated the lability of phosphorus (P) in the mobile HA and calcium humate fractions of four rice soils as measured by orthophosphate-releasing enzymatic hydrolysis and UV irradiation. Enzymatic hydrolysis releases hydrolyzable organic P, and UV irradiation abiotically releases P by breaking down phosphate-HA complexes. Less than 25% of the P in these fractions was detected in the soluble orthophosphate form (detectable soluble P(i)). Enzymatic incubation increased detectable Soluble P(i) to 60% of total P. Treatment by UV irradiation alone released an additional 5% to 20% of humic-bound P compared with the untreated fractions. However, treatment by both UV irradiation and enzymatic hydrolysis released 0% to 14% of humic-bound P more than the amount of organic P that was enzymatically released from nonirradiated samples. The,;matter amounts of P that were released from some humic fractions by both UV irradiation and enzymatic hydrolysis than by UV irradiation alone indicated some overlap between UV-degradable organic P and enzymatically labile organic P. Generally, about one half to two thirds of humic P in these rice soils was labile. These data demonstrated that either assumption that no P in humic fractions was labile P or all P in humic fractions was labile is not true. The lability of humic-bound P should be experimentally evaluated as reported in this work.