Climate warming in northern high-latitude regions is triggering widespread permafrost thaw, whereby large amounts of previously frozen soil carbon and nitrogen become available for biological uptake, while the ensuing land subsidence leads to the conversion of forests into bog systems. Increased microbial activity post-thaw also stimulates soil carbon mineralization and the release of greenhouse gases, contributing to further warming through the permafrost carbon-climate feedback, a well-documented feedback mechanism projected to have a pronounced, accelerating effect on future climate change. However, few studies have addressed whether wetland plants can access new nitrogen sources made available post-thaw, which could potentially increase primary productivity and offset some permafrost carbon losses.
Two research questions were explored through an in vivo ammonium uptake experiment on Carex aquatilis roots excavated from bogs that developed following permafrost thaw in an interior Alaskan forest: 1) does C. aquatilis ammonium uptake vary with depth and time-after-thaw; 2) does variation in C. aquatilis growth characteristics and ammonium uptake correlate with aboveground primary production?
Results/Conclusions
Deep C. aquatilis roots had similar rates of ammonium uptake to shallow roots, while maximum rooting depth, but not belowground biomass, was positively correlated with aboveground biomass. Our findings indicate that plants are accessing deep soil resources near the thaw front, but we found no decline in deep root ammonium uptake with time-after-thaw, as would be expected if plants were responding to a short-term pulse release of nitrogen from thawing permafrost organic matter. Our results therefore suggest that the deep roots of C. aquatilis are not significantly affected by anoxic conditions typical of bog systems, can contribute to plant nitrogen uptake in the long term, and may impact primary productivity.
As permafrost thaw and disproportionately fast climate warming are both occurring in the same regions globally, northern communities will likely be interested in any factors affecting future climate projections. This work supplements information on the permafrost carbon-climate feedback with an understanding of how subarctic plants respond to warming and how ecosystems might help stabilize future warming through enhanced carbon sequestration.