Abstract
ABSTRACTGlaciers retreating in response to climate warming are progressively exposing primary mineral substrates to surface conditions. As primary production is constrained by nitrogen (N) availability in these emerging ecosystems, improving our understanding of how N accumulates with soil formation is of critical concern. In this study, we quantified how the distribution and speciation of N, as well as rates of free-living biological N fixation (BNF), change along a 2000-year chronosequence of soil development in a High Arctic glacier forefield. Our results show the soil N pool increases with time since exposure and that the rate at which it accumulates is influenced by soil texture. Further, all N increases were organically bound in soils which had been ice-free for 0–50 years. This is indicative of N limitation and should promote BNF. Using the acetylene reduction assay technique, we demonstrated that microbially mediated inputs of N only occurred in soils which had been ice-free for 0 and 3 years, and that potential rates of BNF declined with increased N availability. Thus, BNF only supports N accumulation in young soils. When considering that glacier forefields are projected to become more expansive, this study has implications for understanding how ice-free ecosystems will become productive over time.
Highlights
Since ∼1850, glacier coverage in high latitude and altitude regions has continued to decline in response to climate warming (Stocker and others, 2013)
Soil clay content was significantly influenced by soil age (DT, p < 0.001), where clay particles decrease from 6 ± 1% to 4 ± 1% between the 0- and 50-year-old soils (DT, p < 0.001)
This discovery is corroborated by a study of the Damma glacier forefield, Switzerland, by Dümig and others (2011) which failed to identify a clear trend in the soil clay content with age, and demonstrated that the accumulation of soil organic matter did not share a linear relationship with soil texture
Summary
Since ∼1850, glacier coverage in high latitude and altitude regions has continued to decline in response to climate warming (Stocker and others, 2013). Glacier forefields are comprised from linear arrays of soil development stages which provide unique opportunities to investigate long-term primary succession and ecosystem development (Bradley and others, 2014). The development of soils at high latitudes occurs over relatively long timescales as soil forming processes are highly constrained by low temperatures, short growing seasons and slow weathering rates (Ellis and Mellor, 1995). Principal inputs which contribute to the accumulation of N in these ecosystems include biological N fixation (BNF) by free-living (asymbiotic) soil bacteria and plant–microbe associations, mineralisation of organic matter previously overridden by the ice, and allochthonous loadings from aerial deposition and glacial runoff (Brankatschk and others, 2011). While the relative contributions of these inputs may vary, asymbiotic BNF is widely considered to be the dominant source of assimilatory N during the initial stages of soil development (Bradley and others, 2014)
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