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Warming‐induced changes in precipitation regimes, coupled with anthropogenically enhanced nitrogen (N) deposition, are likely to increase the prevalence, duration, and magnitude of soil respiration pulses following wetting via interactions among temperature and carbon (C) and N availability. Quantifying the importance of these interactive controls on soil respiration is a key challenge as pulses can be large terrestrial sources of atmospheric carbon dioxide (CO2) over comparatively short timescales. Using an automated sensor system, we measured soil CO2 flux dynamics in the Colorado Desert—a system characterized by pronounced transitions from dry‐to‐wet soil conditions—through a multi‐year series of experimental wetting campaigns. Experimental manipulations included combinations of C and N additions across a range of ambient temperatures and across five sites varying in atmospheric N deposition. We found soil CO2 pulses following wetting were highly predictable from peak instantaneous CO2 flux measurements. CO2 pulses consistently increased with temperature, and temperature at time of wetting positively correlated to CO2 pulse magnitude. Experimentally adding N along the N deposition gradient generated contrasting pulse responses: adding N increased CO2 pulses in low N deposition sites, whereas adding N decreased CO2 pulses in high N deposition sites. At a low N deposition site, simultaneous additions of C and N during wetting led to the highest observed soil CO2 fluxes reported globally at 299.5 μmol CO2 m−2 s−1. Our results suggest that soils have the capacity to emit high amounts of CO2 within small timeframes following infrequent wetting, and pulse sizes reflect a non‐linear combination of soil resource and temperature interactions. Importantly, the largest soil CO2 emissions occurred when multiple resources were amended simultaneously in historically resource‐limited desert soils, pointing to regions experiencing simultaneous effects of desertification and urbanization as key locations in future global C balance. [ABSTRACT FROM AUTHOR]

For tidal marshes of the US Northeast, the late twentieth century decline of Spartina patens (Aiton) Muhl. has been attributed to increased flooding associated with accelerated sea level rise and nitrogen over-enrichment from cultural eutrophication. The objective of this study was to examine the impacts of inundation and nutrient availability on growth, photosynthesis, and interactions of S. patens and Distichlis spicata (L.) Greene, which co-occur and are common marsh species. Plants were grown in a factorial greenhouse experiment, where flow-through seawater was used to simulate semidiurnal tides. Field surveys were additionally conducted to relate plant distributions to environmental conditions. For S. patens grown in monoculture, nutrient additions did not enhance growth for the high inundation treatment. In addition, the combination of high nutrient availability and high inundation adversely affected S. patens tiller density, photosynthetic efficiency, and leaf C[O.sub.2] uptake. For D. spicata, nutrient additions enhanced growth for both inundation treatments with respect to aboveground biomass and tiller density. For species pairings, S. patens expanded relative to D. spicata under low inundation, low nutrient availability conditions, but declined relative to D. spicata under daily inundation in combination with nutrient amendments. These findings were additionally supported by field data, which indicated that D. spicata was more common than S. patens where nutrient availability was high. These results suggest that S. patens persistence is favored by low nutrient inputs and well-drained conditions, and supports the interpretation that this species is vulnerable to loss where high nutrient loads coincide with accelerated sea level rise. Key words: eutrophication, salinity, marsh loss, New England, sea level rise. Dans les marais littoraux du nord-est des Etats-Unis, le declin de Spartina patens (Aiton) Muhl. au cours de la fin du vingtieme siecle a ete attribue a des inondations accrues associees a l'acceleration de l'elevation du niveau de la mer et du sur-enrichissement en azote par l'eutrophisation due aux cultures. L'objectif de cette etude etait d'examiner les impacts de l'inondation et de la disponibilite des nutriments sur la croissance, la photosynthese et les interactions de S. patens et Distichlis spicata (L.) Greene, qui cohabitent et qui constituent des especes communes du marais littoral. Les plantes ont ete cultivees en serres dans un plan d'experiences factorielles, oU une circulation continue d'eau de mer a ete utilisee pour simuler les marees semi-diurnes. Des evaluations sur le terrain ont aussi ete conduites afin de relier la distribution des plantes aux conditions environnementales. Chez S. patens cultivee en monoculture, l'ajout de nutriments n'augmentait pas la croissance lors du traitement simulant une inondation forte. De plus, la combinaison d'une haute disponibilite en nutriments et une forte inondation affectait de facon negative la densite des talles de S. patens, son efficacite photosynthetique et la capture foliaire de C[O.sub.2]. Chez D. spicata, l'ajout de nutriments augmentait la croissance dans les deux conditions d'inondation relativement a la biomasse aerienne et la densite des talles. Lors du pairage des especes, S. patens s'etendait relativement a D. spicata en condition de faible inondation et de faible disponibilite en nutriments, mais elle declinait relativement a D. spicata sous des conditions d'inondation quotidienne combinee a des amendements nutritifs. Ces donnees ont ete de plus appuyees par les donnees sur le terrain, qui indiquaient que D. spicata etait plus courante que S. patens lorsque la disponibilite en nutriments etait elevee. Ces resultats suggerent que la persistance de S. patens est favorisee par de faibles apports en nutriments et un bon drainage, et appuient l'interpretation que cette espece est en danger de disparition lorsque une charge nutritive elevee coincide avec une acceleration de l'elevation du niveau de la mer. [Traduit par la Redaction] Mots-cles : eutrophisation, salinite, disparition des marais, Nouvelle-Angleterre, elevation du niveau de la mer., Introduction Healthy tidal wetlands provide a wide variety of economic, cultural, ecological, and aesthetic benefits to coastal communities, including protection from coastal flooding, shoreline stabilization, water quality improvements for adjacent [...]

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−1 and 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−1 in 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. [ABSTRACT FROM AUTHOR]

Aims: Mediterranean shrublands adjacent to urbanization experience nitrogen deposition and exotic grass invasions which likely have downstream consequences for carbon and nitrogen emissions from soils. We tested a hypothesis that soil wetting produces trace gas pulses modified by seasonal wetting history, grass litter availability, and cover type in these systems. Methods: Over two seasons, we conducted 48-h wetting experiments and measured CO2, N2O, and NO pulses at an invaded, polluted California shrubland following grass litter addition to sites dominated by either an invasive grass (Schismus barbatus) or a native shrub (Erioginum fasciculatum). Results: CO2 and N2O pulses consistently appeared 15 min post-wetting and diminished within 12 h; NO peaked later and remained elevated at 24 h. All pulses were stronger in the dry season than wet season. Grass litter amendments increased CO2 and dry-season N2O pulses without significantly modifying NO pulses. Grass cover reduced CO2 pulses compared to shrub cover. Conclusions: Our results support the hypothesis that shrubland soils produce stronger pulses of CO2, N2O, and NO during the dry season when wetting is less frequent. We show that invasive grass litter can provide a labile C source that stimulates CO2 and N2O, but not NO, emissions from shrubland soils. [ABSTRACT FROM AUTHOR]

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, δ15Nbulk = 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 soilwater content of <1%--bacterial denitrifiers can account for most of the N2O emitted when dry soils are wetted. [ABSTRACT FROM AUTHOR]

The critically endangered Cumberland Plain woodland within the greater Sydney metropolitan area hosts a dwindling refuge for melaleuca trees, an integral part of Australia's native vegetation. Despite their high carbon stocks, melaleucas have not explicitly been targeted for studies assessing their carbon sequestration potential, and especially little is known about their energy cycling or their response to increasing climate stress, precluding a holistic assessment of the resilience of Australia's forests to climate change. To improve our understanding of the role of melaleuca forest responses to climate stress, we combined forest inventory and airborne LiDAR data to identify species distribution and associated variations in forest structure, and deployed flux towers in a melaleuca-dominated (AU-Mel) and in a eucalypt-dominated (AU-Cum) stand to simultaneously monitor carbon and energy fluxes under typical growing conditions, as well as during periods with high atmospheric demand and low soil water content. We discovered that the species distribution at our study site affected the vertical vegetation structure, leading to differences in canopy coverage (75% at AU-Cum vs. 84% at AU-Mel) and plant area index (2.1 m2 m−2 at AU-Cum vs. 2.6 m2 m−2 at AU-Mel) that resulted in a heterogeneous forest landscape. Furthermore, we identified that both stands had comparable net daytime carbon exchange and sensible heat flux, whereas daytime latent heat flux (115.8 W m−2 at AU-Cum vs 119.4 W m−2 at AU-Mel, respectively) was higher at the melaleuca stand, contributing to a 0.3 °C decrease in air temperature and reduced vapor pressure deficit above the melaleuca canopy. However, increased canopy conductance and higher latent heat flux during moderate VPD or when soil moisture was low indicated a lack of water preservation at the melaleuca stand, highlighting the potential for increased vulnerability of melaleucas to projected hotter and drier future climates. Unlabelled Image • Integrating remote sensing with plot inventories and eddy covariance measurements • Monitoring ecosystem function in a fragmented, peri-urban environment • Ecosystem function linked to ecosystem structure • Species distribution affects stand structure, canopy coverage and leaf area index • Melaleuca stands more vulnerable to heat and moisture stress than eucalypt stands [ABSTRACT FROM AUTHOR]

Warming-induced changes in precipitation regimes, coupled with anthropogenically enhanced nitrogen (N) deposition, are likely to increase the prevalence, duration, and magnitude of soil respiration pulses following wetting via interactions among temperature and carbon (C) and N availability. Quantifying the importance of these interactive controls on soil respiration is a key challenge as pulses can be large terrestrial sources of atmospheric carbon dioxide (CO 2 ) over comparatively short timescales. Using an automated sensor system, we measured soil CO 2 flux dynamics in the Colorado Desert-a system characterized by pronounced transitions from dry-to-wet soil conditions-through a multi-year series of experimental wetting campaigns. Experimental manipulations included combinations of C and N additions across a range of ambient temperatures and across five sites varying in atmospheric N deposition. We found soil CO 2 pulses following wetting were highly predictable from peak instantaneous CO 2 flux measurements. CO 2 pulses consistently increased with temperature, and temperature at time of wetting positively correlated to CO 2 pulse magnitude. Experimentally adding N along the N deposition gradient generated contrasting pulse responses: adding N increased CO 2 pulses in low N deposition sites, whereas adding N decreased CO 2 pulses in high N deposition sites. At a low N deposition site, simultaneous additions of C and N during wetting led to the highest observed soil CO 2 fluxes reported globally at 299.5 μmol CO 2  m -2  s -1 . Our results suggest that soils have the capacity to emit high amounts of CO 2 within small timeframes following infrequent wetting, and pulse sizes reflect a non-linear combination of soil resource and temperature interactions. Importantly, the largest soil CO 2 emissions occurred when multiple resources were amended simultaneously in historically resource-limited desert soils, pointing to regions experiencing simultaneous effects of desertification and urbanization as key locations in future global C balance., (© 2023 John Wiley & Sons Ltd.)

Tidal salt marsh is a key defense against, yet is especially vulnerable to, the effects of accelerated sea level rise. To determine whether salt marshes in southern New England will be stable given increasing inundation over the coming decades, we examined current loss patterns, inundation-productivity feedbacks, and sustaining processes. A multi-decadal analysis of salt marsh aerial extent using historic imagery and maps revealed that salt marsh vegetation loss is both widespread, and accelerating, with vegetation loss rates over the past four decades summing to 17.3%. Seaward retreat of the marsh edge, widening and headward expansion of tidal channel networks, loss of marsh islands, and the development and enlargement of interior depressions found on the marsh platform contributed to vegetation loss. Inundation due to sea level rise is strongly suggested as a primary driver: vegetation loss rates were significantly negatively correlated with marsh elevation ( r 2 =0.96; p =0.0038), with marshes situated below mean high water (MHW) experiencing greater declines than marshes sitting well above MHW. Growth experiments with Spartina alterniflora , the Atlantic salt marsh ecosystem dominant, across a range of elevations and inundation regimes further established that greater inundation decreases belowground biomass production of Spartina alterniflora and thus negatively impacts organic matter accumulation. These results suggest that southern New England salt marshes are already experiencing deterioration and fragmentation in response to sea level rise, and may not be stable as tidal flooding increases in the future.