Nutrient limitation and soil development: Experimental test of a biogeochemical theory

Walker and Syers (1976) proposed a conceptual model that describes the pattern and regulation of soil nutrient pools and availability during long-term soil and ecosystem development. Their model implies that plant production generally should be limited by N on young soils and by P on old soils; N and P supply should more or less equilibrate on intermediate-aged soils. We tested the application of this model to nutrient limitation, using a well characterized substrate age sequence in Hawaiian montane rain forest. Earlier experiments had evaluated nutrient limitation in forests on a young (300 y) and an old (4,100,000 y) substrate on the same developmental sequence; N alone limited tree growth on the young substrate, while P alone did so on the old one. An additional fertilizer experiment based on replicated treatments with N, P, and all other nutrients combined, applied in individually and in all factorial combinations, was established in an intermediate-aged site in the Laupahoehoe Forest Reserve, Hawaii. Here, diameter increments of the dominant tree Metrosideros polymorpha increased slightly with N additions, and nearly doubled when N and P were added together. Additions of elements other than N and P had no significant effect on growth. These results show that N and P had equilibrated (relative to plant requirements) in the intermediate aged site. Together with the earlier experiments, these results suggest that the Walker and Syers model provides a useful starting point for explaining the nature and distribution of nutrient limitation in forest ecosystems.