Phosphorus (P) limitation to terrestrial productivity is presumably widespread. Thus, it is surprising that we lack a reliable assay to determine the fraction of soil P that is available for plant uptake over the growing season. Common extractions of "biologically available" P remove only a small fraction of total P, while estimates of the turnover time and availability of the remaining P are almost entirely speculative. To this end, we used a bioassay to elucidate what forms of soil P are plant-available during a month-long growth chamber experiment. We sequentially extracted soil with 0.5M sodium bicarbonate (NaHCO3) and 0.1M sodium hydroxide (NaOH). After each extraction, a subsample of soil was removed, rinsed thoroughly with deionized water, and buffered to near pretreatment soil pH with 0.03M hydrochloric acid. We then grew Arabidopsis thaliana, in each of the three soil types: untreated control, NaHCO3 extracted, and NaHCO3 and NaOH extracted. The plants were grown under one of two fertilization regimes: F (all nutrients added) or -P (all nutrients except P added). We measured survival rate, total biomass, root:shoot ratio and whole-plant P content to determine how P stress varied across treatments.
Treatment with NaHCO3 and NaOH removed ~2 and ~23% of the P in the soil, respectively. Both fertilization treatments in the control and NaHCO3 soils had 100% survival rates, and the difference in survival rate under F and -P fertilization was not significantly different in the NaOH soils (p= 0.55). Since NaHCO3 and NaOH are commonly thought to extract all labile P forms, we were surprised to find no significant difference in total plant mass (shoots and roots) between F and -P treatments across the three soil treatments (p= 0.48, 0.052, and 0.31 for control, NaHCO3, and NaOH soils, respectively). Nor did fertilizer treatment have a significant effect on root to shoot ratios across the soil treatments (p= 0.81, 0.62, and 0.40 for the three extractions). Finally, there was also no significant difference in whole-plant P concentrations between F and -P treatments (p= 0.83, 0.27, 0.13 for the three soil types). These data suggest that NaHCO3 and NaOH did not extract all of the P available to the plants over the month, and that perhaps a more sophisticated assay of "biologically available" P would help us better understand P dynamics in terrestrial ecosystems.