whether it is better to let tree grows on a parcel, or cut grass regularly and let it compost, adding biomass to it.
Of those two options, trees. Compost does not sequester carbon in and of itself. That's a wishful thinking myth that is based on a lack of understanding of soil. Compost is organic matter, in various stages of life, death and decay. We want soil to have more organic matter. So people assumed adding compost to dirt would accomplish this. It does not. The organic matter is decomposed into CO2 and returns to the atmosphere. We need to increase STABLE organic matter in the soil, and compost does not do that simply by adding it to dirt. The only thing that creates stable organic matter is microbes living in the rhizosphere in symbiosis with plants. In order to increase soil SOM, we need more root mass. The way compost can help is to plant a forest or meadow and keep soaking it in compost tea so that the roots get deeper and deeper, expanding the rhizosphere and the depth of the soil. Normal roots will only grow as deep as there are symbiotic microbes, and yet those microbes will only live where there are roots, so there's a catch-22. We can use the microbes in compost, dissolved into water as compost tea, to get microbes deeper than the roots, so the roots go deeper, and then keep doing that. There's people with grass that has 20 foot deep roots from doing this.