There is of course some nutrient depletion and natural replenishment but eventually you need to restore nutrients, the number of rotations you can do on a plot is depending on the type of land used and what it was used for previously. Ex Farmland usually has a large surplus of nutrients in the soil below the agricultural crop root level. 2nd and 3rd rotation are better wood quality.
It is important to let the needles fall off after harvest as the living leaves/needles are highest in nutrients compared to wood which is high in carbon absorbed from the air.
Can you think of any waste from other agriculture processes that may need disposal and could be used in plantation forestry? Think dairy and poultry poo. It doesnt need much, in fact wood quality is improved if the nutrients are not over-done (fast growth = low density).
