H₂S gas produced in rice field has . . . . . . . . effect on availability of P.

H₂S gas produced in rice fields has synergistic effect on the availability of P.
The synergistic effect here refers to the yield due to the combined application of two nutrients being more than the yield expected based on the effects from the individual applications of the nutrients.
Thus the production of hydrogen sulphide gas in rice fields is good for the yield.