Nutrient retention during ecosystem succession: a revised conceptual model

We propose an important revision to a previously published conceptual model of nutrient retention during terrestrial ecosystem succession, which predicted that ecosystem losses of limiting nutrients such as nitrogen (N) should increase as rates of biomass accumulation slow during late stages of succession. This revision explicitly recognizes that mineral soil horizons (the layers of subsoil beneath the organic-rich surface layers) can behave as an N bank, serving as a source of N for growing forests early in secondary succession and as a sink for N later in succession as plant biomass accumulation slows. If this soil N sink were present, mature forests would continue to retain N for decades following the cessation of biomass accumulation. This conceptual model is consistent with long-term data collected from the Hubbard Brook Experimental Forest in the US state of New Hampshire, where existing N budgets that exclude the mineral soil horizon indicate both a missing source early in succession and a missing sink as the forest has matured.