Getting the dirt on phosphorus loading in mountain lakes: are rising soil temperatures contributing to eutrophication in mountain watersheds?

Background/Question/Methods

: As global temperatures climb, the mountain regions that act as water towers for millions of people across the globe are facing unprecedented ecological and biogeochemical changes. Simultaneously, evidence from across North America suggests that remote mountain watersheds are undergoing significant eutrophication driven by increasing phosphorus (P) concentrations. While eutrophication in mountain watersheds has clear negative consequences for both ecosystem integrity and downstream water quality, the factors driving mountain watershed P increases remain uncertain. Dust deposition and recovery from acidification have been implicated in P increases in some watersheds, but fail to explain P increases in watersheds where neither of these processes are present. In this study, we tested whether faster soil organic matter (SOM) decomposition driven by increasing temperatures could be increasing labile P export from mountain soils into adjacent lakes and streams. Kinetic theory predicts that soil P solubilization will increase with temperature, but the extent to which this increase actually manifests in mountain soils is unclear. In order to evaluate how warming impacts labile P production in mountain soils, we collected intact soil cores from five North American mountain ranges, conducted incubations at three different temperatures, and measured rain-labile P concentrations in each soil following the incubation period.

Results/Conclusions

: Preliminary results suggest that temperature does not have a significant effect on rain-labile soluble reactive phosphorus (rain-SRP) concentrations in collected soils. Regardless of incubation temperature, the strongest predictors of rain-SRP were SOM content, elevation, duff depth, and vegetation type. Soil respiration rates during incubation were also unaffected by temperature but were significantly correlated with soil moisture. Notably, rain-SRP concentrations were not correlated with soil moisture content or respiration rate. These findings suggest that moisture content may exert a stronger control on microbial activity in mountain soils than temperature and that for the soils we tested, microbial decomposition of SOM was not a major driver of rain-SRP production – possibly because microbial P immobilization rates exceeded or equaled P mineralization rates as respiration rates increased. These findings contrast other studies that have found increases in SRP with increasing incubation temperature – possibly due to differences in study design or to differences in nutrient limitation relationships in the soils used in this study. In future work, we plan to analyze the effects of year-long reciprocal transplant along an elevation gradient on rain-P and phosphatase enzyme activity to investigate the effects of warming on soil P production and utilization in a field setting.