According to a ruling on Genju of the Fields, its activated ability can be activated multiple times to enhance its lifelink effects:
8/1/2005: When activated multiple times, it will have the triggered ability multiple times. When the animated Plains deals damage, the ability will trigger once for each copy of the ability. If, for example, you activate the ability three times, the Plains will deal two damage, and you'll gain six life.
I would like to know why because I interpret it differently.
I think that the ruling was made with the thought that the creature and ability were separate, such that even though the land becomes a creature multiple times with no effect, the ability accumulates.
This confuses me because I've always had the notion that when a land becomes a creature with its abilities listed out, it replaces the other types. This (misconception?) is reinforced by the fact that when lands become creatures, they inherently stop being lands unless they say otherwise.
Can somebody point me to the comprehensive guide ruling that causes this behavior?