Effects of phosphorus and nitrogen enrichment of mangrove soils, Hinchinbrook Island, north Queensland

Three sites along a 370 m vegetated transect between Coral Creek and Priest Creek, Hinchinbrook Island were monitored at monthly intervals for 12 months, commencing on the 25th July 1979, to determine the response of mangrove trees to soil enrichment with ammonium or phosphate.

Edge sites at each end of the transect and one middle site were chosen for experiments. Edge site 1 and the middle site were treated with phosphate, while edge site 2 was treated with ammonium. At each site, the treatment was applied within a 12 x 7 m marked area. Phosphate enrichment was achieved by applying superphosphate fertilizer, at a rate of 100 kg P/ha. The fertilizer was applied as a solid and buried at 30 cm depth in the soil at a large number of positions within each area. For nitrogen enrichment, ammonium sulphate solution, at a rate of 100 kg N/ha was injected into the soil at 30 cm depth. Treatments were repeated 3 times at 3 monthly intervals.

The forest in each treatment area and its corresponding control area, about 20 m distant, was dominated by Rhizophora stylosa. Other Rhizophoracae occurring at the sites were Rhizophora apiculata, Rhizophora lamarckii, Bruguiera gymnorhiza and Ceriops tagal. The effects of treatments were monitored using 2 litter catchers (2.0 m²) at each site. Litter collections from treatment and control sites were made at approximately monthly intervals beginning August 25, 1979 and continuing until September 2, 1980. The litter was sorted into its various components, dried, and weighed. Stipule fall rates were then compared with rates obtained for identical catcher pairs from a previous year (1977-1978), when no treatment was applied.

Leaf samples were collected at the end of the fertilization study at 8 sites used for previous studies along the transect and at each of the fertilized sites. Rhizophora spp. only were sampled. At each site, 60 mature (but not senescent) leaves and 60 newly formed leaves were collected from 2 to 3 trees in close proximity. Each batch of 60 leaves was made up into 20 samples, with 3 leaves per sample. Each sample was dried and ground to pass through a 250 µm sieve. After digestion, ammonia and phosphate determinations were carried out using standard colorimetric methods. The results were then used to determine whether correlations existed between foliar N and P and average soil conditions for each site (extractable N and P, soil water salinity and EH) obtained from a previous study.