Impacts of grassland afforestation with coniferous trees on soil phosphorus dynamics and associated microbial processes: A review

Grassland afforestation with coniferous trees, occurring in many parts of the world, can greatly affect the nature and transformation of soil organic matter and associated nutrients, including phosphorus (P). This review critically examines the shifts in soil P availability and chemical nature, microbial properties and soil quality, and possible mechanisms involved in the changes in P nature and transformation as a result of grassland afforestation, based on the published and unpublished data from investigations carried out in recent years. Grassland afforestation with coniferous trees is found to enhance mineralization of organic matter and associated P and consequently improve P availability in topsoil. This is attributed to a combination of factors including the greater P demand and uptake by trees, the improved solubility of organic P by root and microbial exudates (e.g. low molecular weight organic acids), greater tree root phosphatase activity associated with ectomycorrhizae, and favourable soil moisture and temperature conditions. In addition, grassland afforestation also modifies the chemical nature of soil organic P, with a decrease of inositol phosphates (including myo- and scyllo-inositol hexakisphosphate) in soil under forest compared with grassland. On the other hand, grassland afforestation leads to lower soil microbial biomass carbon (C) and P, soil respiration and phosphatase activity, indicating a decrease in soil biological fertility. This may be associated with lower and less labile organic inputs into the mineral soil under coniferous forest compared with grassland and other chemical changes (e.g. lower soil pH) due to grassland afforestation. Future studies should focus on quantifying root and leaf litter inputs and turnover, characterization of the source, chemical composition and role of organic acids in the solubilization and hydrolysis of organic P using isotopic tracing and nuclear magnetic resonance (NMR) techniques, assessment of shifts in the composition, activity and function of the soil microbial biomass in relation to grassland afforestation using appropriate molecular techniques, and comparative study on particular roles of different mycorrhizae-root associations on the utilization of organic P in the soil environment. Crown Copyright (c) 2007 Published by Elsevier B.V. All rights reserved.