Fig. 1. Raman spectra of the wheat straw (a) and cotton stalk (b) biochars before and after modification
P- and PA- represent modified by H₃PO₄ and H₄P₂O₇, respectively, Same below

Fig. 2. SEM image and the corresponding P elemental mapping images of the wheat straw (a) and cotton stalk (b) biochars before and after modification

Fig. 3. FTIR spectra of the wheat straw (a) and cotton stalk (b) biochars before and after modification

Fig. 4. XPS spectra and the relative content of characteristic peaks of wheat straw (a) and cotton stalk (b) biochars before and after modification

Fig. 5. High-resolution P(2p) (a, b) and O(1s) (c, d) spectra of the wheat straw and cotton stalk biochars before and after modification

Fig. 6. Changes of phosphorus relative content of the wheat straw (a) and cotton stalk (b) biochars before and after modification