Did God promise never again to curse the ground, or does he still threaten to strike the earth with a curse?

The two verses speak of different judgments, in different words. In Genesis 8:21 God promises not to curse the ground again as he had just done in the flood, and the same verse explains what that means: “neither will I again smite any more every thing living, as I have done”. The covenant in the next chapter states it plainly: “neither shall there any more be a flood to destroy the earth” (Genesis 9:11). God promised never again to destroy all life on earth in a judgment like the flood. He did not promise never to judge any people again, and the promise itself holds “while the earth remaineth” (8:22).

Malachi 4:6 uses other words. ‘The earth’ is ha’arets, which very often means ‘the land’, as the BSB translates it here: “strike the land with a curse”. Malachi was speaking to Judah about its own land. And ‘curse’ is not the verb of Genesis 8:21 (qalal, H7043, ‘to treat lightly, curse’) but the noun cherem (H2764), the ‘ban’ or ‘utter destruction’ once laid on Canaanite cities such as Jericho (Joshua 6:17). Malachi warns that if Judah does not turn when God sends ‘Elijah’, its own land could come under that ban. The warning is conditional (‘lest’), given so that it need not happen. The New Testament applies the promise of Elijah to John the Baptist (Luke 1:17).

So God keeps his word to Noah: there has been no second flood over all the earth. Malachi warns of a judgment on a guilty nation, which the promise to Noah never ruled out.

The final judgment

Some read Malachi’s warning more widely, of the final judgment of the whole world, which the New Testament says will be by fire (2 Peter 3:7). Even on that reading there is no conflict: the promise to Noah ruled out another flood, and it was made for the time “while the earth remaineth”.