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

Abstract

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 NaHCO3-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 NaHCO3-Pi and the less labile EDTA-NaOH-Po and labile NaHCO3-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 NaHCO3-Po and increased levels of other-monoesters in crop growing organic C-enriched layers may suggest microbial mobilization and crop use of this pool.