AT A MOMENT in the year when gift-giving is in the public mind, one of the national organizations that knows something about the art is the Nature Conservancy. For the past quarter-century, the nonprofit group - its national office is in Arlington - has been involved in preserving more than a million acres of wetlands, praries, forests, deserts, mountains and islands in 47 states. If performs an invaluable service, considering the disregard for the fragility of the land that has marked much of the nation's estate development, highway construction, mining and urbanization.

We mention the conservancy at this particular moment because it recently announced a move to acquire 6,500 acres of undisturbed land on the outer banks of North Carolina. The coastal properties lie about 20 miles south of the North Carolina - Virginia border and include 3 1/2 miles of oceanfront. Marshes, several small islands and acres of shoal water are also part of the acquisition. The marshlands alone, according to conservancy officials, are crucial to the survival of several species of waterfowl, from the common mallards to the majestic Canada geese. Researchers estimate that about 15 per cent of the waterfowl on the Atlantic flyway use this particular marshland as a habitat.

The conservancy, which secures funds from its own membership of 43,000 citizens, as well as from foundations, corporations and other philanthropies, was able to acquire this latest property through a $4 million grant from the Richard King Mellon Foundation of Pittsburgh; the sun is said to be the largest grant ever given to a private conservation group.

That the sum is large is obvious. It is obvious, too, as Will Rogers put it, that land is the one thing they aren't making any more of. That the Nature Conservancy is beginning to be noticed now by the large foundations suggests that more and more citizens are becoming aware of that fact.