Soil phosphorus fractions after seven decades of fertilizer application in the Rengen Grassland Experiment

Declining global P reserves require a better understanding of P cycling in soil and related plant uptake. On managed grasslands, application of lime and fertilizer affects not only soil nutrient status, but also plant‐species composition of the sward. We examined the P fractionation in the Rengen Grassland Experiment (RGE) on a naturally acid Stagnic Cambisol in the Eifel Mts. (Germany) 69 y after the setup of the experiment. A modified sequential Hedley fractionation was carried out for samples from 30 plots at 0–10 cm depth. Application of inorganic phosphorus fertilizer had diverse effects on inorganic (Pi) and organic P (Po) fractions. Resin‐Pi, NaHCO3‐Pi, NaHCO3‐Po, NaOH‐Pi, HCldil‐Pi, HClconc‐Pi, and HClconc‐Po contents increased, while NaOH‐Po significantly decreased and residual‐P remained unaffected. Strongest enrichment occurred in the HCldil‐Pi fraction, probably due to the chemical nature of the basic Thomas slag applied as P fertilizer. Without P fertilization, all fractions except residual‐P were more or less depleted. Strong P limitation of the vegetation in the limed treatments without P led to lowered contents also for NaOH‐Pi and NaOH‐Po. However, NaOH‐Po was largest in the Control and even exceeded the respective content in the treatments with P. It remained unclear why species adapted to a low soil P status did not access this P fraction though being P‐limited. Published theory on the availability of Hedley P fractions does neither match P exploitation nor P nutritional status of the vegetation in the RGE. Regarding NaOH‐Po as stable and HCldil‐Pi as moderately labile led to a more realistic evaluation of plant P uptake. Evaluation of P availability on the basis of chemical extractions alone is questionable for conditions like in the RGE. On long‐term grassland, plant‐species composition has to be taken into account to estimate access of plants to soil P.