It is known that more plant mass is formed in forests than in steppes, but forest soils are poorer than chernozems. Why?

Despite the fact that plants in forests exceed steppe in their mass, the soils there contain much less organic matter. The reason lies in the fact that in the steppes the plants, for the most part, are annual, and every autumn, both aboveground parts of the grass (leaves, branches) and underground (roots) die. The forest is formed by perennial trees that shed only leaves, and therefore less dead organic matter gets into the soil here.