Wetland restoration following agricultural abandonment supports diversity and reduces stochasticity in soil fungal communities due to anaerobic-induced interspecific associations

Abstract

Soil fungi play a pivotal role in regulating various functions within terrestrial ecosystems. Understanding the progression of soil fungal communities during wetland restoration following agricultural abandonment is essential for assessing the sustainability of rehabilitated ecosystems. Soils along a chronosequence of paddy fields and wetlands restored for 1, 3, and 4 years were studied to investigate the dynamics of fungal diversity and assembly processes, focusing on how these changes relate to species associations and soil environmental factors, particularly redox condition and carbon (C) supply, which predominantly regulate a range of ecological services. Wetland restoration increased both soil fungal taxonomic and phylogenetic diversity. The relative contribution of nestedness increased with restoration duration, emphasising the importance of richness increases in soil fungal community succession during wetland restoration. Concurrently, stochastic assembly processes of soil fungal communities decreased with restoration duration, as indicated by a decline in the phylogenetic normalised stochasticity ratio. However, soil fungal community assembly was decoupled from anaerobic condition (indicated by ferrous iron/ferric iron ratio) and labile C supply (indicated by organic C content and the relative abundance of the di-O-alkyl C functional group). In contrast, the decrease in stochasticity was associated with enhanced species associations, reflected by increases in node number, edge number, and average degree, particularly positive associations, as shown by the rise in the positive-to-negative edge ratio during wetland restoration. This pattern might be because that despite stronger soil anaerobic condition, enriched anaerobic species (e.g., Mortierellales) could provide nutrients to aerobic species (e.g., Helotiales) through positive associations, thereby broadening fungal niche breadth. This, in turn, might cascade into more diversified and less stochastic fungal communities during wetland restoration. Overall, our results suggest that anaerobic-induced interspecific associations reduce stochasticity in soil fungal communities and provide a mechanistic perspective on the protection of soil fungal diversity during wetland restoration following agricultural abandonment.