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The Carbon in Permafrost Experimental Heating Research (CiPEHR) project addresses the following questions: 1) Does ecosystem warming cause a net release of C from the ecosystem to the atmosphere?, 2) Does the decomposition of old C, that comprises the bulk of the soil C pool, influence ecosystem C loss?, and 3) How do winter and summer warming alone, and in combination, affect ecosystem C exchange? We are answering these questions using a combination of field and laboratory experiments to measure ecosystem carbon balance and radiocarbon isotope ratios at a warming experiment located in an upland tundra field site near Healy, Alaska in the foothills of the Alaska Range. This data set includes weekly thaw depth measurements collected from winter warming, summer warming, and control treatment plots at CiPEHR. Additional measurements from on-plot gas flux wells, water table monitoring wells, and off-plot locations are also reported. Note that the experimental warming portion of this experiment concluded in 2022. These data are a continuation of measurements taken at previously warmed plots but plots were not actively manipulated after 2022. At the Gradient Thaw Site, in this larger study, we are asking the question: Is old carbon that comprises the bulk of the soil organic matter pool released in response to thawing of permafrost? We are answering this question by using a combination of field and laboratory experiments to measure radiocarbon isotope ratios in soil organic matter, soil respiration, and dissolved organic carbon, in tundra ecosystems. The objective of these proposed measurements is to develop a mechanistic understanding of the SOM sources contributing to C losses following permafrost thawing. We are making these measurements at an established tundra field site near Healy, Alaska in the foothills of the Alaska Range. Field measurements center on a natural experiment where permafrost has been observed to warm and thaw over the past several decades. This area represents a gradient of sites each with a different degree of change due to permafrost thawing. As such, this area is unique for addressing questions at the time and spatial scales relevant for change in arctic ecosystems. 

Soils are the largest source of atmospheric nitrous oxide (N₂O), a powerful greenhouse gas. Dry soils rarely harbor anoxic conditions to favor denitrification, the predominant N₂O-producing process, yet, among the largest N₂O emissions have been measured after wetting summer-dry desert soils, raising the question: Can denitrifiers endure extreme drought and produce N₂O immediately after rainfall? Using isotopic and molecular approaches in a California desert, we found that denitrifiers produced N₂O within 15 minutes of wetting dry soils (site preference = 12.8 ± 3.92 per mil, δ¹⁵N^bulk= 18.6 ± 11.1 per mil). Consistent with this finding, we detected nitrate-reducing transcripts in dry soils and found that inhibiting microbial activity decreased N₂O emissions by 59%. Our results suggest that despite extreme environmental conditions—months without precipitation, soil temperatures of ≥40°C, and gravimetric soil water content of <1%—bacterial denitrifiers can account for most of the N₂O emitted when dry soils are wetted. 

Soil ammonia (NH3) emissions are seldom included in ecosystem nutrient budgets; however, they may represent substantial pathways for ecosystem nitrogen (N) loss, especially in arid regions where hydrologic N losses are comparatively small. To characterize how multiple factors affect soil NH3 emissions, we measured NH3 losses from 6 dryland sites along a gradient in soil pH, atmospheric N deposition, and rainfall. We also enriched soils with ammonium (NH4+), to determine whether N availability would limit emissions, and measured NH3 emissions with passive samplers in soil chambers following experimental wetting. Because the volatilization of NH3 is sensitive to pH, we hypothesized that NH3 emissions would be higher in more alkaline soils and that they would increase with increasing NH4+ availability. Consistent with this hypothesis, average soil NH3 emissions were positively correlated with average site pH (R2 = 0.88, P = 0.004), ranging between 0.77 ± 0.81 µg N-NH3 m−2 h−1 at the least arid and most acidic site and 24.2 ± 16.0 µg N-NH3 m−2 h−1 at the most arid and alkaline site. Wetting soils while simultaneously adding NH4+ increased NH3 emissions from alkaline and moderately acidic soils (F1,35 = 14.7, P < 0.001), suggesting that high N availability can stimulate NH3 emissions even when pH is less than optimal for NH3 volatilization. Thus, both pH and N availability act as proximate controls over NH3 emissions suggesting that these N losses may limit how much N accumulates in arid ecosystems. 

Abstract Soil drying and wetting cycles can produce pulses of nitric oxide (NO) and nitrous oxide (N₂O) emissions with substantial effects on both regional air quality and Earth’s climate. While pulsed production of N emissions is ubiquitous across ecosystems, the processes governing pulse magnitude and timing remain unclear. We studied the processes producing pulsed NO and N₂O emissions at two contrasting drylands, desert and chaparral, where despite the hot and dry conditions known to limit biological processes, some of the highest NO and N₂O flux rates have been measured. We measured N₂O and NO emissions every 30 min for 24 h after wetting soils with isotopically-enriched nitrate and ammonium solutions to determine production pathways and their timing. Nitrate was reduced to N₂O within 15 min of wetting, with emissions exceeding 1000 ng N–N₂O m⁻² s⁻¹and returning to background levels within four hours, but the pulse magnitude did not increase in proportion to the amount of ammonium or nitrate added. In contrast to N₂O, NO was emitted over 24 h and increased in proportion to ammonium addition, exceeding 600 ng N–NO m⁻² s⁻¹in desert and chaparral soils. Isotope tracers suggest that both ammonia oxidation and nitrate reduction produced NO. Taken together, our measurements demonstrate that nitrate can be reduced within minutes of wetting summer-dry desert soils to produce large N₂O emission pulses and that multiple processes contribute to long-lasting NO emissions. These mechanisms represent substantial pathways of ecosystem N loss that also contribute to regional air quality and global climate dynamics.