Which factor determines the spatial variance of soil fertility on uninhabited islands?

Abstract

Uninhabited islands serve as a natural and unique laboratory for the study of soil fertility under varying natural conditions and potential anthropogenic influences, and how the soil fertility spatially varies and which factor determines the spatial variance remain to be studied. In this study, a total of 15 uninhabited islands in Miaodao Archipelago, a typical archipelago in northern China, were selected to demonstrate the study. The soil fertility was represented by seven soil factors and a composite index, namely, soil fertility indicator (SFI). Different environmental factors that covered natural and anthropogenic aspects were identified at island and local scales, and the spatial pattern of soil fertility responding to the environmental factors was analyzed. Then, the influences of different environmental factors on the spatial variances of soil fertility at the dual scales were quantified. Results revealed that the soils in the study area had considerably higher values of total carbon, total nitrogen, organic matter, available phosphorus, and available potassium compared with the soils on other China’s islands, the recognized standards, and the national and regional mean values, indicating the extremely high fertility. Great spatial differences in the soil fertility were observed among different islands and sampling sites. At island scale, the island isolation, which was represented by the distance to the inhabited islands and the mainland (DTI), determined the spatial variance of soil fertility. The SFI distinctly increased with the increase in DTI, which could be explained partly by the isolation from human activities and the vegetation condition at island scale, and partly by the cross-scale influence from altitude (Al). At local scale, Shannon-Wiener index (H’), Al, slope (Sl), and distance to the shoreline (DTS) showed clear positive correlations with SFI. The Al and DTS determined the spatial variance of soil fertility under the cross-scale influences from island area and DTI, while H’ and Sl didn’t involve cross-scale influences.