Evaluating P Availability Influenced by Warming and N Deposition in a Subtropical Forest Soil: a Bioassay Mesocosm Experiment

Aims Phosphorus (P) is a limited nutrient in highly weathered subtropical or tropical forest soils and it is unclear how both the cycling and availability of P will respond to future global warming and nitrogen (N) deposition. This study aims to evaluate the influence of soil warming and N deposition on soil P fractions in relation to plant growth and P uptake. Methods We carried out a field mesocosm bioassay experiment with a soil warming and N addition treatment in a factorial design and measured soil Hedley P fraction and P uptake of the Chinese fir ( Cunninghamia lanceolata ) seedlings. Results Soil warming significantly increased soil easily-available and moderately-available inorganic phosphorus (Pi) but had no effect on soil easily-available and moderately-available organic P (Po), or on plant growth or plant biomass P, and decreased soil microbial biomass P (MBP). Nitrogen addition increased plant growth and leaf biomass P, but had no effect on soil Pi and MBP, and decreased soil moderately-available Po and leaf P concentration. There were no interactions between soil warming and nitrogen on Hedley P fractions. None of the Hedley P fractions had correlated with plant height and diameter growth or with plant P uptake. Conclusions Our results suggest that the Hedley P fractions do not reflect soil P availability and that the growth of the Chinese fir is not limited by soil P availability in this subtropical Ultisol, rather it is most likely limited by N and water availability based on the results from this bioassay experiment.