Phosphorus and nitrogen nutritional status of a northern Australian mangrove forest

Three sites along a 400-m transect within a tropical mangrove forest in northern Australia were monitored at monthly intervals for 12 mo to determine mangrove response to soil ammonium or phosphate enrichment. Growth response was monitored by measurement of interpetiolar stipule fall, a litter fall component which has been found to be highly correlated with new leaf appearance. Regression slopes for the cumulative stipule fall-time data for each site (2 replicate catchers per site) were used to estimate average stipule fall rates over the treatment year. These were compared with the rates obtained for the identical catcher pairs from a previous year when no treatment was applied. Similar between-year comparisons for 3 corresponding control sites, with no treatment either year, showed no significant changes in stipule fall rates (p > 0.17 in all cases). At the first treatment site, at low elevation within the intertidal zone, no significant response (p = 0.53) to P enrichment was found. For a higher elevation site, 170 m from the nearest tidal channel edge, a significant (p = 0.017) response to P enrichment was recorded, consistent with previous findings of chronically low soil extractable P at the higher elevation sites compared to the lower elevation (edge) sites (5 vs. 14 pg P g-'). A significant response (p = 0.018) to soil ammonium enrichment was found at the third (edge, low elevation) site. As the average soil ammonium level at this site was slightly but significantly higher than for all other sites, it appears that nitrogen limitation is common throughout with phosphorus limitation also evident at the higher elevation areas. Foliar analyses showed that mature Rhizophora leaf nitrogen and phosphorus levels were highly significantly correlated with average soil ammonium and extractable phosphorus respectively. Mature leaves are therefore likely to be useful indicators of mangrove forest nutritional status in remote area surveys. Newly formed leaves showed much higher N and P levels and the leaf parameters showed a complex set of correlations with other soil factors such as redox potential and salinity.