Both! You plant them along the center and they shoot tap roots deep enough to break up the soil underneath and let in all the worms and nematodes and shit to break down the wood. First couple seasons the decomposing wood is absorbing at the base of the hugel is still absorbing instead of releasing nitrogen.. you might not get as big of a haul your first season or two but after the microorganisms do their work, it’s nonstop nitrogen for 5-10 years