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Comparing the influence of wildfire and prescribed burns on watershed nitrogen biogeochemistry using 15N natural abundance in terrestrial and aquatic ecosystem components.

Stephan K, Kavanagh KL, Koyama A - PLoS ONE (2015)

Bottom Line: We compared paired (burned/unburned) watersheds of four wildfires and three spring prescribed burns for three growing seasons post-fire.In-stream moss δ15N in wildfire-burned watersheds was enriched by 1.3 ‰, but there was no response by moss in prescription-burned watersheds, mirroring patterns of streamwater nitrate concentrations.S. betulifolia showed significantly higher nitrate reductase activity two years after wildfires relative to corresponding unburned watersheds, but no such difference was found after prescribed burns.

View Article: PubMed Central - PubMed

Affiliation: Forest, Rangeland, and Fire Sciences, University of Idaho, Moscow, ID, United States of America; Department of Life and Physical Sciences and Cooperative Research, Lincoln University, Jefferson City, Missouri, United States of America.

ABSTRACT
We evaluated differences in the effects of three low-severity spring prescribed burns and four wildfires on nitrogen (N) biogeochemistry in Rocky Mountain headwater watersheds. We compared paired (burned/unburned) watersheds of four wildfires and three spring prescribed burns for three growing seasons post-fire. To better understand fire effects on the entire watershed ecosystem, we measured N concentrations and δ15N in both the terrestrial and aquatic ecosystems components, i.e., soil, understory plants in upland and riparian areas, streamwater, and in-stream moss. In addition, we measured nitrate reductase activity in foliage of Spiraea betulifolia, a dominant understory species. We found increases of δ15N and N concentrations in both terrestrial and aquatic ecosystem N pools after wildfire, but responses were limited to terrestrial N pools after prescribed burns indicating that N transfer from terrestrial to aquatic ecosystem components did not occur in low-severity prescribed burns. Foliar δ15N differed between wildfire and prescribed burn sites; the δ15N of foliage of upland plants was enriched by 2.9 ‰ (difference between burned and unburned watersheds) in the first two years after wildfire, but only 1.3 ‰ after prescribed burns. In-stream moss δ15N in wildfire-burned watersheds was enriched by 1.3 ‰, but there was no response by moss in prescription-burned watersheds, mirroring patterns of streamwater nitrate concentrations. S. betulifolia showed significantly higher nitrate reductase activity two years after wildfires relative to corresponding unburned watersheds, but no such difference was found after prescribed burns. These responses are consistent with less altered N biogeochemistry after prescribed burns relative to wildfire. We concluded that δ15N values in terrestrial and aquatic plants and streamwater nitrate concentrations after fire can be useful indicators of the magnitude and duration of fire effects and the fate of post-fire available N.

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Relationship between soil NH4+-δ15N and foliar δ15N of S. betulifolia in the wildfire sites.Relationships are shown for soil NH4+-δ15N collected in (A) August 2004 and (B) October 2005. Each data point represents one plot since variability within watersheds was higher than variation between watersheds. Open and filled symbols represent unburned (U) and burned (B) plots, respectively. Site abbreviations: DC—Danskin Creek, Ca—Canyon Creek, Ha—Hall, SF—South Fork. No data was available for Ca-U.
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pone.0119560.g004: Relationship between soil NH4+-δ15N and foliar δ15N of S. betulifolia in the wildfire sites.Relationships are shown for soil NH4+-δ15N collected in (A) August 2004 and (B) October 2005. Each data point represents one plot since variability within watersheds was higher than variation between watersheds. Open and filled symbols represent unburned (U) and burned (B) plots, respectively. Site abbreviations: DC—Danskin Creek, Ca—Canyon Creek, Ha—Hall, SF—South Fork. No data was available for Ca-U.

Mentions: Soil inorganic N was significantly enriched in 15N in WB relative to WU but no treatment effect was found in prescribed burn sites (Table 3). Across all four wildfire sites, 15N of NH4+ in soils extracts from August 2004 (post-fire year 1 for three sites, post-fire year 2 for Danskin Creek) was almost 6 ‰ higher in WB relative to WU (P = 0.02), but there was no consistent pattern across prescribed burn sites (P = 0.24, Table 3). This pattern was also found in October 2005 except that in wildfire sites the absolute difference between WB and WU was smaller (P = 0.07), namely 3.2 ‰ ± 0.8 (SE), compared to 5.8 ‰ ± 1.2 in 2004 (Table 3). In both 2004 and 2005, foliar δ15N of S. betulifolia correlated positively (R2 = 0.32, P = 0.004; R2 = 0.34, P = 0.001, respectively) with soil δ15NH4+-N (Fig 4A and 4B) (since the foliage of the other upland species had foliar δ15N pattern similar to S. betulifolia, only S. betulifolia is presented). Soil NO3- data (Table 3) available only for October 2005 had many missing data points due to low N content in extracts from unburned soil preventing the use of the diffusion technique. Statistical analysis of fire treatment effects was therefore not possible; however, it could be shown that soil NO3- was isotopically depleted relative to soil NH4+ by on average 4.5 ‰ (P = 0.002).


Comparing the influence of wildfire and prescribed burns on watershed nitrogen biogeochemistry using 15N natural abundance in terrestrial and aquatic ecosystem components.

Stephan K, Kavanagh KL, Koyama A - PLoS ONE (2015)

Relationship between soil NH4+-δ15N and foliar δ15N of S. betulifolia in the wildfire sites.Relationships are shown for soil NH4+-δ15N collected in (A) August 2004 and (B) October 2005. Each data point represents one plot since variability within watersheds was higher than variation between watersheds. Open and filled symbols represent unburned (U) and burned (B) plots, respectively. Site abbreviations: DC—Danskin Creek, Ca—Canyon Creek, Ha—Hall, SF—South Fork. No data was available for Ca-U.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4401779&req=5

pone.0119560.g004: Relationship between soil NH4+-δ15N and foliar δ15N of S. betulifolia in the wildfire sites.Relationships are shown for soil NH4+-δ15N collected in (A) August 2004 and (B) October 2005. Each data point represents one plot since variability within watersheds was higher than variation between watersheds. Open and filled symbols represent unburned (U) and burned (B) plots, respectively. Site abbreviations: DC—Danskin Creek, Ca—Canyon Creek, Ha—Hall, SF—South Fork. No data was available for Ca-U.
Mentions: Soil inorganic N was significantly enriched in 15N in WB relative to WU but no treatment effect was found in prescribed burn sites (Table 3). Across all four wildfire sites, 15N of NH4+ in soils extracts from August 2004 (post-fire year 1 for three sites, post-fire year 2 for Danskin Creek) was almost 6 ‰ higher in WB relative to WU (P = 0.02), but there was no consistent pattern across prescribed burn sites (P = 0.24, Table 3). This pattern was also found in October 2005 except that in wildfire sites the absolute difference between WB and WU was smaller (P = 0.07), namely 3.2 ‰ ± 0.8 (SE), compared to 5.8 ‰ ± 1.2 in 2004 (Table 3). In both 2004 and 2005, foliar δ15N of S. betulifolia correlated positively (R2 = 0.32, P = 0.004; R2 = 0.34, P = 0.001, respectively) with soil δ15NH4+-N (Fig 4A and 4B) (since the foliage of the other upland species had foliar δ15N pattern similar to S. betulifolia, only S. betulifolia is presented). Soil NO3- data (Table 3) available only for October 2005 had many missing data points due to low N content in extracts from unburned soil preventing the use of the diffusion technique. Statistical analysis of fire treatment effects was therefore not possible; however, it could be shown that soil NO3- was isotopically depleted relative to soil NH4+ by on average 4.5 ‰ (P = 0.002).

Bottom Line: We compared paired (burned/unburned) watersheds of four wildfires and three spring prescribed burns for three growing seasons post-fire.In-stream moss δ15N in wildfire-burned watersheds was enriched by 1.3 ‰, but there was no response by moss in prescription-burned watersheds, mirroring patterns of streamwater nitrate concentrations.S. betulifolia showed significantly higher nitrate reductase activity two years after wildfires relative to corresponding unburned watersheds, but no such difference was found after prescribed burns.

View Article: PubMed Central - PubMed

Affiliation: Forest, Rangeland, and Fire Sciences, University of Idaho, Moscow, ID, United States of America; Department of Life and Physical Sciences and Cooperative Research, Lincoln University, Jefferson City, Missouri, United States of America.

ABSTRACT
We evaluated differences in the effects of three low-severity spring prescribed burns and four wildfires on nitrogen (N) biogeochemistry in Rocky Mountain headwater watersheds. We compared paired (burned/unburned) watersheds of four wildfires and three spring prescribed burns for three growing seasons post-fire. To better understand fire effects on the entire watershed ecosystem, we measured N concentrations and δ15N in both the terrestrial and aquatic ecosystems components, i.e., soil, understory plants in upland and riparian areas, streamwater, and in-stream moss. In addition, we measured nitrate reductase activity in foliage of Spiraea betulifolia, a dominant understory species. We found increases of δ15N and N concentrations in both terrestrial and aquatic ecosystem N pools after wildfire, but responses were limited to terrestrial N pools after prescribed burns indicating that N transfer from terrestrial to aquatic ecosystem components did not occur in low-severity prescribed burns. Foliar δ15N differed between wildfire and prescribed burn sites; the δ15N of foliage of upland plants was enriched by 2.9 ‰ (difference between burned and unburned watersheds) in the first two years after wildfire, but only 1.3 ‰ after prescribed burns. In-stream moss δ15N in wildfire-burned watersheds was enriched by 1.3 ‰, but there was no response by moss in prescription-burned watersheds, mirroring patterns of streamwater nitrate concentrations. S. betulifolia showed significantly higher nitrate reductase activity two years after wildfires relative to corresponding unburned watersheds, but no such difference was found after prescribed burns. These responses are consistent with less altered N biogeochemistry after prescribed burns relative to wildfire. We concluded that δ15N values in terrestrial and aquatic plants and streamwater nitrate concentrations after fire can be useful indicators of the magnitude and duration of fire effects and the fate of post-fire available N.

Show MeSH
Related in: MedlinePlus