Organic and inorganic phosphorus forms in soil as affected by long-term application of organic amendments

Organic amendments contribute significantly to the phosphorus (P) supply in agroecosystems. However, their long-term effects on specific P forms in soils are not completely understood. The objective of this study was to investigate the concentration of organic P forms and inorganic P pools in soil and the activity of enzymes involved in the P turnover in a long-term field experiment running since 1998 in Northern Germany as affected by P amendments. The following treatments with different P supplies were sampled in 2012, 14 years after the establishment of the experiment: control (no P), cattle manure (manure), biowaste compost (compost), and biowaste compost in combination with triple-superphosphate (compost + TSP). The classification of organic P forms by using enzyme additions to NaOH–EDTA soil extracts showed non-hydrolyzable organic P as the dominant form in soil followed by inositol hexakisphosphate (Ins6P)-like P. Non-hydrolyzable and total organic P concentrations in soil were highest in the combined compost + TSP treatment, which received the highest amount of inorganic P. The values of the bioavailable P pools (water-extractable P and double lactate-extractable P) were in accordance with the P balance (P addition with the amendments minus P removal with harvested crops) independently of the type of amendment. The results of this research suggest that the distribution of soil P forms is more reliant on the turnover processes in the soil than on the forms of P added.