Soil microbial co-occurrence networks become less connected with soil development in a high Arctic glacier foreland succession.

Classically, ecologists have considered that biota becomes more integrated and interdependent with ecosystem development in primary successional environments. However, recent work on soil microbial communities suggests that there may in fact be no change in network integration over successional time series. Here, we performed a test of this principle by identifying network-level topological features of the soil microbial co-occurrence networks in the primary successional foreland environment of the retreating high-Arctic glacier of Midtre Lovénbreen, Svalbard. Soil was sampled at sites along the foreland of inferred ages 10-90 years since deglaciation. DNA was extracted and amplicon sequenced for 16 s rRNA genes for bacteria and ITS1 region for fungi. Despite the chronologically-related soil pH decline and organic C/N accumulation, analysis on network-level topological features showed network integration did not change with inferred chronological ages, whereas network integration declined with decreasing pH and increasing total organic carbon (TOC) - both factors that can be viewed as an indicator of soil development. We also found that bacteria played a greater role in the network structure than fungi, with all keystone species in the microbial co-occurrence network being bacteria species. Both number and relative abundance of the keystone species were significantly higher when soil pH increased or TOC decreased. It appears that in the more extreme and less productive conditions of early primary succession, integration between members of soil biota into consortia may play a greater role in niche adaptation and survival. Our finding also emphasizes that ecosystem development is not simply a product of time but is influenced by locally heterogeneous factors.