Phosphorus limits the growth of bacterial and eukaryotic primary producers in recently deglaciated High Arctic soils

Glaciers are receding at unprecedented rates worldwide, exposing large areas of barren sediment that are quickly colonized by microbes. Nutrient availability is a critical driver of early ecosystem succession, but in the High Arctic its role has remained uncertain. This limits mechanistic projections of ecosystem development and response to glacial retreat and warming. Here we combine field observations and microcosm experiments on recently deglaciated (<20 years) proglacial soils from Midtre Lovénbreen, Svalbard. Geochemical analyses revealed that soils were highly oligotrophic, and had C:N:P ratios (~300:15:1) suggesting stronger limitation by nitrogen (N) and phosphorus (P) than by carbon (C). To distinguish between N and P limitation, we caried out ex‐situ microcosm incubations under simulated summer light and temperature regimes, treating soils with nutrients (N, P or NP). In NP microcosms there was a bloom of rapidly growing soil algae (e.g. Ulotrichales) and predatory bacteria (e.g. Myxococcaceae) resulting in significant shifts in eukaryotic and prokaryotic community structure. Addition of N alone had a slight stimulatory effect on moss growth but did not change the structure of microbial communities or the C:N ratio of the biomass. In contrast, addition of P alone decreased the C:N ratio, changed the structure of the prokaryotic community, and increased the growth of soil algae, mosses, and N‐fixing cyanobacteria (Nostocales), suggesting that moss and algae were receiving N from Nostoc . Overall, these results indicate that P availability, rather than N availability, limits the establishment of early microbial primary producers, with N supplied largely through biological fixation. Given the rapid deglaciation of High Arctic regions, these findings highlight the central role of phosphorus in structuring these emerging ecosystems.