Soil fertility underlies the positive relationship between island area and litter decomposition in a fragmented subtropical forest landscape

Abstract

Habitat fragmentation has transformed many once contiguous habitats into smaller patches varying in size, especially in forest ecosystems. However, while the consequences of habitat fragmentation for biodiversity and ecosystem functioning (e.g., productivity) have been widely documented, we know much less about how habitat fragmentation affects litter decomposition, a key component of biogeochemical cycling. Here we investigated the effect of forest fragment size on leaf litter decomposition by conducting a litterbag experiment on a group of land-bridge islands in the Thousand Island Lake in subtropical China. We also explored the mechanisms underlying litter decomposition-island area relationship by assessing the bottom-up (via soil nutrients) and top-down (via soil fauna) effects on litter decomposition. We found that the leaf litter of six common plant species with different litter quality, including Castanopsis sclerophylla, Dalbergia hupeana, Liquidambar formosana, Pinus massoniana, Quercus fabri, and Schima superba, consistently decomposed faster on larger islands, where greater soil fertility facilitated decomposition. Soil faunal diversity also enhanced litter decomposition, but its effect was mainly associated with litter quality and unaffected by island size. Our study demonstrates the deleterious effect of habitat fragmentation on litter decomposition, as well as the importance of bottom-up regulation by soil fertility in modulating habitat fragmentation effect on biogeochemical processes.