Phosphorus form and bioavailability in the pine rotation following fertilization: P fertilization influences P form and potential bioavailability to pine in the subsequent rotation

Abstract

Soils in some parts of the world are known to be severely phosphorus (P) deficient. As little as 20 kg P ha −1 have produced large growth responses in forest stands. If increased tree growth and augmented P cycling are caused by a one-time P application, a valid question is whether the effect of the initial fertilization will be evident in the regenerating forest, thereby reducing the need for P fertilization early in the second rotation. The objective of this study was to evaluate the P status of the forest floor and inorganic P status of mineral surface soil of two long-term fertilizer trials in order to determine if there were increases in soil P amount and bioavailability that benefits the next rotation. Long-term fertilizer trials in Georgia, USA and New Zealand (29 and 22 years after fertilization) were the study sites. In Georgia, forest fertilization increased the mass of the F horizon by 239% and its P content by 318%. Anion exchange membranes measured up to a 47% increase in bioavailable P in the mineral soil in a fertilized treatment. While bioassays did not show significant residual P in the mineral soil under the fertilized plots, soil from an adjacent operationally fertilized stand, which had the forest floor bedded into its planting rows, showed a 100% increase in seedling P content. At the New Zealand site, the mineral soil contained 142% more P in the anion exchange membrane form than in the unfertilized soils. Bioassays indicated a 224% increase in seedling P content when grown in fertilized soil at the highest fertilization rate. Operational levels of fertilization did not yield a significant bioassay result. The data show that there can be a residual effect of fertilizer P in the rotation following fertilization. The bioassays, anion membrane exchangeable P and forest floor P from Georgia suggest that there is enough P in the soil system to support the needs of a regenerating stand of loblolly pine, while residual bioavailable P in the mineral soil from operational levels of fertilization at New Zealand site would require residual P in the forest floor or immediate fertilization to meet the demands of newly planted seedlings. Conservation of the forest floor is an important part of P management in these P deficient sites.