P-NMR spectra of extracts from different materials and concentration of various P species (mg kg
). As re-dissolved in (I) 1 M or 10 M NaOH and (II) 1 M NaOH with and without HCl addition to solution. (A) Farmland soil. (B) Farmland soil added with compost. (C) Forest soil. (E) Compost. Spectra were acquired for 15 h. MDP, Metylendiphosphonic acid-P, P′-disodium salt as internal standard; PL, phospholipids.