Differential effects of nitrogen and phosphorus enrichment on growth of dwarf Avicennia marina mangroves

The effects of N and P enrichment were investigated on growth and physiological responses of dwarf Avicennia marina mangroves in a hypersaline (58 ± 8 psu) field site in Richards Bay, South Africa. It was hypothesized that at high salinities mangroves allocate more resources to roots than shoots, and that nutrient enrichment with N and P will shift resource allocation to shoots and enhance growth and productivity. In unvegetated areas of the dwarf zone, 1-year-old A. marina seedlings were planted in pots and enriched bimonthly with N, P, N + P, or remained unfertilized (control-C), and growth and morphology of plants were monitored for 2 years. Enrichment with N and N + P shifted resource allocation to shoots from 38% to 55%, and increased dry biomass accumulation by over 500%, compared to the control treatment. In the N and N + P treatments, plant height, number of leaves, leaf chlorophyll content and photosynthesis increased by over 50%, 330%, 30% and 30%, respectively, compared to the C and P treatments. Enrichment with N and N + P increased N concentrations in roots by over 60% (from 1.0 ± 0.1% to 1.6 ± 0.2% of dry mass) and in shoots by over 100% (from 1.3 ± 0.1% to 2.7 ± 02% of dry mass). Plants enriched with P alone were similar to those of the control. This study has demonstrated that dwarf A. marina in Richards Bay is N limited, and that N enrichment shifts resource allocation from roots to shoots and increases growth and productivity.