Soil enzymes illustrate the effects of alder nitrogen fixation on soil carbon processes in arctic and boreal ecosystems

Background/Question/Methods

Longer growing seasons and higher average temperatures have contributed to the expansion of woody shrubs throughout the Arctic and Subarctic. This expansion has been linked to several hypothesized feedbacks to climate warming, many of which hinge on changes in the availability of nitrogen (N). Siberian alder (Alnus viridis ssp. fruticosa), a N fixing shrub, is among the species expanding in both the Arctic and boreal forests. By relieving microbial N limitation, alder- fixed N has the potential to increase decomposition of labile soil C, due to the priming effect, while simultaneously decreasing decomposition of recalcitrant soil C, due to the downregulation of microbial N mining. The microbial response to increased N is influenced by the soil organic matter (SOM) chemistry, with the priming or mining responses expected to dominate in soils with labile or recalcitrant SOM respectively. SOM chemistry could influence whether alder N stimulates soil C sequestration or losses. To investigate the microbial responses to alder N, we measured the activities of three extracellular enzymes and, using Fourier transform infrared spectroscopy (FTIR), quantified the SOM chemistry in soils taken under and away from alder in differing stand types in both the arctic and boreal forest regions of Alaska, USA.

Results/Conclusions

In the boreal forest, SOM chemistry differed between aspen (Populus tremuloides) and spruce (Picea glauca) stands with relatively labile soils in the spruce stands. While enzyme activities were generally lower in the spruce stands, alder proximity had no effect on enzyme activities in either boreal forest stand type. This lack of alder effect could be due to a low degree of N limitation potentially brought on by rapid N cycling or legacy effects of alder N fixation. In the Arctic, stand types differed strongly in SOM chemistry with labile SOM at sites where alder had been previously shown to be expanding, and recalcitrant SOM at non-expanding sites. Despite these differences, activities of both labile and recalcitrant SOM degrading enzymes were higher under alder, than away, in both stand types, indicating the dominance of the priming response to alder N. This suggests enhanced labile and recalcitrant SOM decomposition, which could contribute to net soil C losses under alder. However, the net effect of alder N fixation on ecosystem C balance remains unknown. Because high latitude soils store massive quantities of C and are experiencing the rapid expansion of alder, alders’ influence on soil decomposition could have consequences for C budgets.