Usable forms of nitrogen in the soil can be intentionally increased by allowing organic matter to decompose. When organic matter breaks down, it releases nutrients, including nitrogen, in forms that can be utilized by plants.
Planting more trees and planting fewer beans may not significantly affect nitrogen levels in the soil. In fact, legumes (like beans) have the ability to fix atmospheric nitrogen through a symbiotic relationship with nitrogen-fixing bacteria, which can increase nitrogen availability in the soil. Lighting, on the other hand, does not directly contribute to increasing nitrogen content in the soil.