Unusually high levels of bio-available phosphate in the soils of Ogasawara Islands, Japan: Putative influence of seabirds

Ogasawara Islands are important ecosystems sustaining many indigenous spices. To clarify the indigenous soil environments of Ogasawara Islands, we studied the chemistry of the soils. Many surface soils were low in bio-available P (0 to 0.55 g P 2O 5 kg −1, average: 0.04 g P 2O 5 kg −1 as Bray II P, n = 22), but several soils were found to contain extremely large amounts of bio-available P (1.36 to 6.98 g P 2O 5 kg −1, average: 2.93 g P 2O 5 kg −1, n = 5). From soil profile analyses, the authors concluded that the extremely large amount of bio-available P could not be explained by the effects of parent materials with high P contents nor the effect of fertilizations by human activity, but the effects of natural seabird activities in the past could be the cause. The soil profiles with large amounts of bio-available P indicate deep migration of soil materials from A horizons, which could be a result of intensive mixing of upper horizons by seabird activities. The intensive mixing was supported by the low mechanical impedance of the horizons for the P-accumulating soils (8.17 ± 2.54 kg cm −2, n = 8) than those for the non-P-accumulating soils (17.46 ± 3.52 kg cm −2, n = 36). It is likely that in the past seabirds, such as shearwaters, made burrows in the soils for nesting and propagating and inadvertently transported a large amount of P from the sea to the soils, resulting in the extremely large amounts of bio-available P in the present soils. ►Unusually high bio-available phosphates were found in the soils of Ogasawara Islands. ►They increased with soil depth and reached greater than 6 g P 2O 5 kg −1 soil. ►The high bio-available P associated with low mechanical impedance of the horizons. ►They would be derived from natural seabird activities in the past. ►Seabirds would transport a large amount of P from sea to the soils and mix them well.