Simulating the Role of Biogeochemical Hotspots in Driving Nitrogen Export From Dryland Watersheds
Abstract Climate change and nitrogen (N) pollution are altering biogeochemical and ecohydrological processes in dryland watersheds, increasing N export, and threatening water quality. While simulation models are useful for projecting how N export will change in the future, most models ignore biogeoc...
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| Format: | Article |
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Wiley
2024-03-01
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| Series: | Water Resources Research |
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| Online Access: | https://doi.org/10.1029/2023WR036008 |
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| author | Jianning Ren Erin J. Hanan Aral Greene Christina Tague Alexander H. Krichels William D. Burke Joshua P. Schimel Peter M. Homyak |
| author_facet | Jianning Ren Erin J. Hanan Aral Greene Christina Tague Alexander H. Krichels William D. Burke Joshua P. Schimel Peter M. Homyak |
| author_sort | Jianning Ren |
| collection | DOAJ |
| description | Abstract Climate change and nitrogen (N) pollution are altering biogeochemical and ecohydrological processes in dryland watersheds, increasing N export, and threatening water quality. While simulation models are useful for projecting how N export will change in the future, most models ignore biogeochemical “hotspots” that develop in drylands as moist microsites in the soil become hydrologically disconnected from plant roots when soils dry out. These hotspots enable N to accumulate over dry periods and rapidly flush to streams when soils wet up. To better project future N export, we developed a framework for representing hotspots using the ecohydrological model RHESSys. We then conducted a series of virtual experiments to understand how uncertainties in model structure and parameters influence N export to streams. Modeled N export was sensitive to three major factors (a) the abundance of hotspots in a watershed: N export increased linearly and then reached an asymptote with increasing hotspot abundance; this occurred because carbon and N inputs eventually became limiting as hotspots displaced vegetation cover, (b) the soil moisture threshold required for subsurface flow from hotspots to reestablish: peak streamflow N export increased and then decreased with an increasing threshold due to tradeoffs between N accumulation and export that occur with increasingly disconnected hotspots, and (c) the rate at which water diffused out of hotspots as soils dried down: N export was generally higher when the rate was slow because more N could accumulate in hotspots over dry periods, and then be flushed more rapidly to streams at the onset of rain. In a case study, we found that when hotspots were modeled explicitly, peak streamflow nitrate export increased by 29%, enabling us to better capture the timing and magnitude of N losses observed in the field. N export further increased in response to interannual precipitation variability, particularly when multiple dry years were followed by a wet year. This modeling framework can improve projections of N export in watersheds where hotspots play an increasingly important role in water quality. |
| format | Article |
| id | doaj-art-988136c3e49a4ae5abd79a47799c45e4 |
| institution | DOAJ |
| issn | 0043-1397 1944-7973 |
| language | English |
| publishDate | 2024-03-01 |
| publisher | Wiley |
| record_format | Article |
| series | Water Resources Research |
| spelling | doaj-art-988136c3e49a4ae5abd79a47799c45e42025-08-20T03:22:16ZengWileyWater Resources Research0043-13971944-79732024-03-01603n/an/a10.1029/2023WR036008Simulating the Role of Biogeochemical Hotspots in Driving Nitrogen Export From Dryland WatershedsJianning Ren0Erin J. Hanan1Aral Greene2Christina Tague3Alexander H. Krichels4William D. Burke5Joshua P. Schimel6Peter M. Homyak7Department of Natural Resources and Environmental Science University of Nevada Reno NV USADepartment of Natural Resources and Environmental Science University of Nevada Reno NV USADepartment of Environmental Sciences University of California Riverside CA USABren School of Environmental Science & Management University of California Santa Barbara CA USAUSDA Forest Service Rocky Mountain Research Station Albuquerque NM USADepartment of Natural Resources and Environmental Science University of Nevada Reno NV USADepartment of Ecology Evolution and Marine Biology University of California Santa Barbara CA USADepartment of Environmental Sciences University of California Riverside CA USAAbstract Climate change and nitrogen (N) pollution are altering biogeochemical and ecohydrological processes in dryland watersheds, increasing N export, and threatening water quality. While simulation models are useful for projecting how N export will change in the future, most models ignore biogeochemical “hotspots” that develop in drylands as moist microsites in the soil become hydrologically disconnected from plant roots when soils dry out. These hotspots enable N to accumulate over dry periods and rapidly flush to streams when soils wet up. To better project future N export, we developed a framework for representing hotspots using the ecohydrological model RHESSys. We then conducted a series of virtual experiments to understand how uncertainties in model structure and parameters influence N export to streams. Modeled N export was sensitive to three major factors (a) the abundance of hotspots in a watershed: N export increased linearly and then reached an asymptote with increasing hotspot abundance; this occurred because carbon and N inputs eventually became limiting as hotspots displaced vegetation cover, (b) the soil moisture threshold required for subsurface flow from hotspots to reestablish: peak streamflow N export increased and then decreased with an increasing threshold due to tradeoffs between N accumulation and export that occur with increasingly disconnected hotspots, and (c) the rate at which water diffused out of hotspots as soils dried down: N export was generally higher when the rate was slow because more N could accumulate in hotspots over dry periods, and then be flushed more rapidly to streams at the onset of rain. In a case study, we found that when hotspots were modeled explicitly, peak streamflow nitrate export increased by 29%, enabling us to better capture the timing and magnitude of N losses observed in the field. N export further increased in response to interannual precipitation variability, particularly when multiple dry years were followed by a wet year. This modeling framework can improve projections of N export in watersheds where hotspots play an increasingly important role in water quality.https://doi.org/10.1029/2023WR036008nitrogen exportdenitrificationnitrogen leachingecohydrologyhotspotdryland ecosystem |
| spellingShingle | Jianning Ren Erin J. Hanan Aral Greene Christina Tague Alexander H. Krichels William D. Burke Joshua P. Schimel Peter M. Homyak Simulating the Role of Biogeochemical Hotspots in Driving Nitrogen Export From Dryland Watersheds Water Resources Research nitrogen export denitrification nitrogen leaching ecohydrology hotspot dryland ecosystem |
| title | Simulating the Role of Biogeochemical Hotspots in Driving Nitrogen Export From Dryland Watersheds |
| title_full | Simulating the Role of Biogeochemical Hotspots in Driving Nitrogen Export From Dryland Watersheds |
| title_fullStr | Simulating the Role of Biogeochemical Hotspots in Driving Nitrogen Export From Dryland Watersheds |
| title_full_unstemmed | Simulating the Role of Biogeochemical Hotspots in Driving Nitrogen Export From Dryland Watersheds |
| title_short | Simulating the Role of Biogeochemical Hotspots in Driving Nitrogen Export From Dryland Watersheds |
| title_sort | simulating the role of biogeochemical hotspots in driving nitrogen export from dryland watersheds |
| topic | nitrogen export denitrification nitrogen leaching ecohydrology hotspot dryland ecosystem |
| url | https://doi.org/10.1029/2023WR036008 |
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