Long-Term Fertilizer Effects on Oxalate-Desorbable Phosphorus Pools in a Typic Paleaquult

Abstract

Inorganic phosphate rapidly forms strong inner sphere bonds with hydrous oxides via ligand-exchange reactions. With time, P can be absorbed into minerals or occluded by oxides in highly weathered acidic soils. Phosphate ion concentrations in the soil solution are typically very low, which limits plant growth in pine plantations of the southeastern United States. While some plants and microbes secrete low-molecular-mass organic acids such as oxalate to access sorbed P pools, intensively managed plantations are commonly fertilized to ameliorate P deficiencies. Our goal was to determine if there were fertilizer treatment effects on ligand-desorbable P pools in a loblolly pine (Pinus taeda L.) plantation 27 yr after fertilization. We quantified the fertilizer effect by measuring P (organic and inorganic) desorbed with increasing oxalate concentrations. Based on the increasing concentration of Fe, Al, and fulvic acids in the oxalate extracts, we were able to distinguish between oxalate-exchangeable P pools and oxalate-dissolvable P pools (P released with the dissolution of the mineral surface). The quantity of desorbed Fe and Al exceeded CuCl2–exchangeable concentrations when oxalic acid concentrations exceeded 20 mmol kg−1 soil. Concurrently, the P desorbed from mineral surfaces increased when oxalate exceeded 20 mmol kg−1 soil, indicating a shift from oxalate-exchangeable P to oxalate-dissolvable P pools. Oxalate-dissolvable P pools (0.06 mmol kg−1, 60%) increased significantly in response to the fertilizer treatment. These results show that the primary mechanism for the long-term retention of fertilizer P on this highly weathered, acidic soil is the sequestration of P into the oxalate (dissolvable) P pool.