The relevance of different soil phosphorus fractions for short-term tree nutrition: results from a mesocosm bioassay

Trees have developed mechanisms to obtain soil phosphorus from less available and recalcitrant P forms. Therefore, assessments of P availability to trees should consider not only readily plant-available fractions, as is typically done in agriculture, but also fractions of different availability, such as those generated by sequential extraction as suggested by Hedley, M.J., Stewart, J.W.B. and Chauhan, B.S. 1982 Changes in inorganic and organic soil phosphorus fractions induced by cultivation practices and by laboratory incubations. Soil Sci. Soc. Am. J. 46, 970-976. Yet our knowledge about the contribution of these different P fractions to tree nutrition is extremely limited. Here, we assessed the short-term ecological relevance of these Hedley P fractions for tree nutrition using mesocosms with fast growing poplars. Poplar cuttings were planted in P fertilized and control treatments of two forest soils and one agricultural soil. After one growing season, P uptake in plants and changes in Hedley P fractions in soil were quantified. Reductions in total soil P ranged from 2.5 per cent (10 mg kg(-1)) at one forest site to 6.5 per cent (50 mg kg(-1)) at the fertilized agricultural site. The strongest decreases in P content were found in the labile P fractions (up to 30 per cent; -30.4 mg kg(-1) in unfertilized agricultural soil) but significant reductions occurred also in moderately labile P fractions (up to 17 per cent; -2.4 mg kg(-1) in the forest soil). Our results indicate the importance of these P fractions for tree nutrition in the short term. Additionally, our study showed that the Hedley method can capture these short-term changes in soil P fractions. To advance mesocosm approaches to study the ecological relevance of soil P fractions, future experiments might extend over different seasons and several growing periods and use fast growing tree species such as birch or aspen that are better adapted to poor forest soils.