Spruce and pine utilization of phosphorus in soil amended with 33P-labelled hydroxylapatite

Mined rock phosphate is expected to become a scarce resource within the next few decades as global phosphorus (P) deposits are declining. As a result, mineral P fertilizer will be less available and more expensive. Therefore, improved knowledge is needed on other P resources, for example, apatite fertilizers derived from the by-products of iron mining. Forestry is a potential future consumer of apatite-rich products with the aim of obtaining more wood per hectare. The actual P availability in apatite to plants has so far been barely quantified. We therefore examined tree P uptake using ³³P apatite under chamber-grown and outdoor conditions. We examined the P uptake for the two main conifer species spruce (Picea abies) and pine (Pinus sylvestris) used in Fenno-Scandinavian forestry. We synthesized ³³P-enriched apatite and applied it to mesocosms with growing seedlings of spruce and pine. The P uptake from ³³P-labelled hydroxylapatite was subsequently traced by (bio)imaging of radioactivity in the plants and by liquid scintillation counting (LSC) upon destructive harvest in all plant fractions (leaves, stem and roots) and rhizosphere soil. Two climatic conditions were compared, one at natural outdoor conditions and one set as 5°C warmer than the climate record from the previous years. Plant P uptake from ³³P-labelled hydroxylapatite was enhanced in chamber-grown compared with outdoor seedlings for both tree species. This uptake was manifested in the clear radioactive images obtained over ca. 1 month after soil apatite application. Furthermore, all aboveground plant fractions of both spruce and pine seedlings showed a higher P uptake in warmer than colder daytime environments. The observed quantities and rates of P uptake from ³³P-labelled hydroxylapatite by spruce (18 Bq g⁻¹ hour⁻¹) and pine (83 Bq g⁻¹ hour⁻¹; averages in chamber condition) are as to our knowledge unique observations. Natural forest soils in Sweden are often P-poor. Our research suggests that apatite-based P fertilization of spruce and pine forests can increase wood production by overcoming any existing P limitation.