Elevated atmospheric CO2 increased soil plant available and soil organic phosphorus in a mature temperate oak (Quercus robur L.) forest

Enhanced productivity of forest ecosystems in response to rising levels of anthropogenically generated atmospheric carbon dioxide (CO₂) has the potential to mitigate against climate change by sequestering carbon in woody biomass and soils. However, the physiological response of trees to elevated atmospheric CO₂ may be constrained by the availability of soil nutrients, predominantly nitrogen and phosphorus (P). Here, we assess the impact of elevated atmospheric CO₂ on P cycling in a temperate 180-year-old oak (Quercus robur L.) forest exposed to free-air CO₂ enrichment (ambient + 150 ppm) for six years. Soil cores were collected to a depth of 1 m in July 2023 and separated into three horizons and three layers (O, A, B, 30-50, 50-70, 70-100 cm) before analysis using the Hedley¹ sequential P fractionation and the DeLuca²biological based P extraction techniques. Plant available P in soil pore water and total organic P from the O horizon increased by 84 and 128%, respectively, whilst organic P extracted with phosphatase increased by 62% under elevated CO₂. Total organic P in soil horizons beyond the B horizon (> 15 cm) decreased under elevated CO₂ in comparison with ambient CO₂. As soil organic P is derived from the turnover of both vegetation and microbial biomass, increased soil organic P in the O horizon may be due to the faster turnover of organic matter or an increase in the net primary productivity of the forest. Soil P cycling in this forest ecosystem appears to be predominantly influenced by biological rather than chemical processes, since elevated CO₂ only affected the organic P and not inorganic P fractions. Forest productivity may be constrained by P limitation in future elevated CO₂ environments, if there is faster organic matter turnover which is probably the case in our study.

¹Hedley, M. J., Stewart, J. W. B., & Chauhan, B. (1982). Changes in inorganic and organic soil phosphorus fractions induced by cultivation practices and by laboratory incubations. Soil Science Society of America Journal, 46(5), 970-976.

²DeLuca, T. H., Glanville, H. C., Harris, M., Emmett, B. A., Pingree, M. R., de Sosa, L. L., Cerdá-Moreno, C. & Jones, D. L. (2015). A novel biologically-based approach to evaluating soil phosphorus availability across complex landscapes. Soil Biology and Biochemistry, 88, 110-119.