Nitrogen cycling in the soil–plant system along a series of coral islands affected by seabirds in the South China Sea

Abstract

The nitrogen (N) utilization strategy of plants has become a topic of interest within the field of phytoecology. However, few studies have considered N cycling on coral island ecosystems from the perspective of their evolution. The aim of this study was to test the impacts of biological transport by seabirds, on the sources and uses of N by plants, and pathways of N cycling in soil–plant ecosystems on coral islands. A series of eight coral islands were investigated, five of which were affected to a varying extent by seabirds. The total phosphorus (TP) concentration from avian sources and the δ15N values of total nitrogen (TN) and inorganic nitrogen (IN: NH4+-N, and NO3−-N), δ18O of NO3−-O, in soils were determined, as well as proxies in plant leaves of two dominant plant species, including TN, the carbon/nitrogen ratio (C/N), and δ13C and δ15N values. The results show that, with an increase of TP, the TN and IN content, and δ15N values in soils all increased. Plant C/N and δ15N values decreased and increased, respectively, as the soil N content increased. When the TN content of the soil was low, the δ15N value in plant leaves was similar to that in soil NO3−, but was much lower than that in soil NH4+. When the soil TN content was high, the δ15N values were similar. Both plants and soil were probably N-limited prior to seabird colonization, with the N source on the barren coral islands originating primarily from atmospheric deposition. With seabird guano input and subsequent pedogenesis, the source of N switched to guano. Under these conditions, most of the N utilized by plants originated from NH4+, while nitrate is dominant for non-seabirds islands. Seabird activities have played a key role in the N dynamics of soil–plant ecosystems at coral islands.