Crop mobilization of retained phosphorus and changes in phosphorus compounds in soils amended with compost

Background: Microbiota can favour crop access to phosphorus (P) pools retained in soil. Microbial activity and P mobilization can be enhanced by soil carbon and root exudates. Aims: We aimed to study the changes in organic and inorganic P (Po; Pi) forms and compounds in organically fertilised soils growing crops for two years. Methods: Crops were grown in pots with compost added at the soil surface. We measured changes in microbial carbon, soluble carbon, total Po and Pi forms, the less labile EDTA-NaOH-Pi (-Po), and labile NaHCO 3 -Pi (-Po) in surface and subsurface soils from crop-free pots amended with compost and unamended, and from amended pots growing crops. P compounds in the EDTA-NaOH extract were measured by 31P-NMR. Results: Compost addition increased the levels of Pi and although it had no effect on Po it increased inositol hexakisphosphate and other-monoesters. After the application of compost, the amount of total organic carbon, soluble carbon and P forms increased in surface soil, while in the subsurface soil there was a reduction in organic carbon and an increase in soluble carbon, total Pi, EDTA-NaOH-Pi and NaHCO 3 -Pi and the less labile EDTA-NaOH-Po and labile NaHCO 3 -Po. Growing crops reduced all measured Pi forms and had no observable impact on total Po in either organic C-enriched or organic C-depleted soils. Conclusions: Crops mostly used Pi from the low P availability C-depleted layer. Large reductions in NaHCO 3 -Po and increased levels of other-monoesters in crop growing organic C-enriched layers may suggest microbial mobilization and crop use of this pool.