Plant and soil microfaunal biodiversity across the borders between arable and forest ecosystems in a Mediterranean landscape

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The distribution of organisms across ecosystem borders can be indicative of trophic interactions, food-web dynamics, and the potential for recovery after disturbance. Yet relatively little is known regarding patterns and ecology of belowground organisms across borders. Our hypothesis was that incremental zonation of vegetation and soil properties at the interface between cultivated fields and forests may facilitate the recolonization of a more complex soil faunal assemblage after disturbance ceases. Vegetation, soil characteristics, and soil nematodes (indicators of disturbance) were studied at the interface between arable and natural ecosystems (oak forest and maquis shrubland) in southwestern France. Sampling was along 23-m long transects, at six positions (center and edge of grain fields, both sides of field borders, and bands of shrub and forest vegetation) at four sites. Plant functional groups changed more markedly than species richness. Total soil carbon (C) and nematode biomass were 3.5 and 6 times higher in the forest than in the center of the cultivated fields. The nematode Structure Index gradually increased from fields to forests, along with higher total and labile soil C pools, litter, root C, and root C:N, and more negative root δ15N. Microbivore nematodes were related to labile and total soil C. Structural equation modeling indicated that nematode predators and prey were both affected by total soil C, but proximity to the forest was important for predators, whereas plant community complexity was important for prey (i.e., microbivorous nematodes). The forested borders had minor effects on zonation of nematode assemblages and soil ecosystem services within the fields, yet woody vegetation may have facilitated recolonization by plants and soil fauna after tillage ceased and probably provided benefits for production of livestock (i.e., shade and erosion reduction) that were not measured. During plant succession, litter C and N apparently decomposed slowly into active forms in the soil, creating habitats for more K-selected, larger-bodied nematodes. Due to less cultivation and higher C inputs during the past 50 years, the more homogeneous landscape may promote more complex soil food webs, but less total agrobiodiversity, compared to the mosaic of diverse ecosystems that occurred in the ancient cultural landscape of the past.