Nutrient retention and loss during ecosystem succession: revisiting a classic model

In 1975, Vitousek and Reiners proposed a conceptual model relating the net retention of a limiting nutrient to the net biomass accumulation in terrestrial ecosystems, whereby terrestrial systems should be highly conservative of nutrients during ecosystem succession when plants are actively accumulating biomass, but should be relatively leakier in older stands, when net plant biomass accumulation nears zero. The model was based on measurements in the White Mountains of New Hampshire. However, recent data showing that nitrate output in streams is declining across this region even as forests are aging seem to be inconsistent with this theory. Because the more recent data do not match the predictions of the Vitousek and Reiners model, either new hypotheses, or modifications of the original hypothesis, need to be considered. I suggest that the original model can be amended by accounting for increased woody debris; an accumulation of both above and belowground high C:N coarse woody debris from tree mortality in these regenerating forests can lead to high microbial immobilization of N and can explain the recent patterns of declining stream nitrate. Few studies or models have attempted to calculate the impacts of coarse woody debris (CWD) decomposition products to the retention of C and N in forested ecosystems and their receiving streams, but evidence suggests that CWD can significantly affect stream N exports and should be considered in future models of ecosystem biogeochemical cycles.