Multiple gradient effects on spatial distribution of island soil microbial biomass

Soil microbial biomass (SMB) rapidly responds to the environment and can function as an important indicator for monitoring archipelago ecological vulnerability. An archipelago is featured by distinct and unique gradients between and within islands. The multiple gradient effects on the island ecosystem are worth exploring. We established a multiple gradient system comprising natural, human, and ecological gradients in island and site scales. The natural gradients consist of island area, shape, and distance to mainland in island scale, and terrain and distance to shoreline in site scale; the human gradients include population and exploitation in island scale, and distance to hardened ground in site scale; and the ecological gradients are composed of community type, biomass, biodiversity, and soil physical-chemical properties in site scale. SMB, including SMB carbon (SMBC), SMB nitrogen (SMBN), and SMB phosphorus (SMBP), was used to verify the multiple gradient effects on the island ecosystem, and the northern Miaodao Archipelago in North China was selected as the study area. Results revealed that SMB exhibited distinct spatial heterogeneities. SMBC was sensitive to the natural and human gradients in both scales; SMBN was influenced by parts of natural, human, and ecological gradients in both scales; and SMBP did not show regular changes along the gradients in island scale, yet responded sensitively to the ecological gradients in site scale. The multiple gradient system is unique and fundamental in the spatial heterogeneities of the island ecosystem, and fully revealing the multiple gradient effects is essential for reasonable island conservation and exploitation.