Effects of wildfire on soil net nitrification in the boreal forest of interior Alaska

Background/Question/Methods

Wildfire frequency and size are increasing in the boreal forests of interior Alaska with climate warming, leading to changes in fluxes of nutrients from terrestrial to aquatic ecosystems. Vegetative nitrogen demand is generally low post fire, which increases the potential of nitrogen leaching to streams. Following a wildfire in 2004 in a subcatchment of the Caribou-Poker Creeks Research Watershed, stream water nitrate (NO₃) concentration increased, and dissolved organic carbon concentration declined. We used this natural experiment to identify potential sources of NO₃ within the watershed that could explain the increase in stream water NO₃ concentration post fire, asking is soil net nitrification higher in the burned watershed compared to an unburned watershed? We hypothesized that post fire decrease in vegetative nitrogen uptake, and changes in soil thermal conditions increase net nitrification rate. We conducted a paired analysis of soil nitrification in the burned and an unburned watershed, sampling along the hill slope, toe slope, and valley bottom. We used a 28-day buried-bag incubation to quantify soil net nitrification rate, repeating the incubations three times in the burned watershed (n=57), and twice in the unburned watershed (n=42) in summer 2012. 

Results/Conclusions

Soil within the burned watershed had a higher soil NO₃ concentration (0.95±0.11 µg NO₃-N/g soil) compared to the unburned watershed (0.53±0.10 µg NO₃-N/g soil; P < 0.01, R²=0.07, as well as a higher net nitrification rate (4.62±1.02µg NO₃-N/g soil/28 days) compared to the unburned watershed (0.22±0.25 µg NO₃-N/g soil/28 days; P<0.01, R²=0.12). Within the burned watershed, the valley bottom had a higher net nitrification rate (7.54±2.31 µg NO₃-N/g soil/28 days) than toe or hill slope positions. The same pattern was observed in the unburned watershed, though the rate was substantially lower than in the same positions of the burned watershed (valley bottom: 1.18±0.61 µg NO₃-N/g soil/28 days). These results support our hypothesis and provide insights into the mechanisms underpinning observed elevated stream NO₃ concentration post fire. Due to the large 
influence of wildfire on soil processes and consequently stream water chemistry, regional-scale nutrient dynamics are likely to be substantially altered as climate warms and fire becomes more frequent. If substantial amounts of nitrogen are lost through post-fire stream exports, there will be profound consequences in the functioning of an ecosystem that is nitrogen limited.