Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/2/2008
Publication Date: 2/16/2009
Citation: He, Z., Olk, D.C., Honeycutt, C.W., Fortuna, A. 2009. Enzymatically- and Ultraviolet-labile Phosphorus in Humic Acid Fractions From Rice Soils. Soil Science. 174(2):81-87. Interpretive Summary: Phosphorus (P) is present in many humic substances. However, information on the forms and bioavailability (lability) of humic-bound P is limited. To determine the role of humic fractions in P cycling and nutrition, two types of humic substances, mobile humic acid (MHA) and recalcitrant calcium humate (CaHA) were extracted from four soils under rice culture at the International Rice Research Institute in Los Baños, Laguna, Philippines. The lability of P in these humic fractions was measured. Our data indicate that not all P associated with humic fractions is easily bioavailable. About 20% of humic P in rice soils had the highest availability. Another 40% of P was moderately available. This information has potential utility for managing P availability, resulting in more efficient crop utilization of that P.
Technical Abstract: Humic acid is an important soil component which can improve nutrient availability and impact other important chemical, biological, and physical properties of soils. We investigated the lability of phosphorus (P) in the mobile humic acid (MHA) and calcium humate (CaHA) fractions of four rice soils as measured by orthophosphate-releasing enzymatic hydrolysis and ultraviolet (UV) irradiation. Enzymatic hydrolysis releases hydrolyzable organic P, and UV irradiation abiotically releases P by breaking down phosphate-humic acid complexes. Less than 25% of the P in these fractions was detected in the soluble orthophosphate form (labile Pi). Enzymatic incubation increased labile Pi to 60% of total P. Treatment by UV irradiation alone released an additional 5-20% of humic-bound P, compared to the untreated fractions. However, treatment by both UV irradiation and enzymatic hydrolysis released 0 to 14% of humic-bound P, more than the labile Pi that was enzymatically released from non-irradiated samples. The smaller 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. This information has potential utility for managing P availability for more efficient crop utilization of that P.