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1. Promoting success in thin layer sediment placement: effects of sediment grain size and amendments on salt marsh plant growth and greenhouse gas exchange

   https://doi.org/10.1111/rec.14141  

   Wilburn, Brittany P.; Raper, Kirk; Raposa, Kenneth B.; Gray, Andrew B.; Mozdzer, Thomas J. (March 2024, Restoration ecology)

   Thin layer sediment placement (TLP) is used to build elevation in marshes, counteracting effects of subsidence and sea level rise. However, TLP success may vary due to plant stress associated with reductions in nutrient availability and hydrologic flushing or through the creation of acid sulfate soils. This study examined the influence of sediment grain size and soil amendments on plant growth, soil and porewater characteristics, and greenhouse gas exchange for three key US salt marsh plants: Spartina alterniflora (synonym Sporobolus alterniflorus), Spartina patens (synonym Sporobolus pumilus), and Salicornia pacifica. We found that bioavailable nitrogen concentrations (measured as extractable NH4+-N) and porewater pH and salinity were inversely related to grain size, while soil redox was more reducing in finer sediments. This suggests that utilizing finer sediments in TLP projects will result in a more reduced environment with higher nutrient availability, while larger grain sized sediments will be better flushed and oxygenated. We further found that grain size had a significant effect on vegetation biomass allocation and rates of gas exchange, although these effects were species-specific. We found that soil amendments (biochar and compost) did not subsidize plant growth but were associated with increases in soil respiration and methane emissions. Biochar amendments were additionally ineffective in ameliorating acid sulfate conditions. This study uncovers complex interactions between sediment type and vegetation, emphasizing limitations of soil amendments. The findings aid restoration project managers in making informed decisions regarding sediment type, target vegetation, and soil amendments for successful TLP projects. 

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2. Modeling impacts of saltwater intrusion on methane and nitrous oxide emissions in tidal forested wetlands

   https://doi.org/10.1002/eap.2858  

   Wang, Hongqing; Dai, Zhaohua; Krauss, Ken W.; Trettin, Carl C.; Noe, Gregory B.; Burton, Andrew J.; Ward, Eric J. (July 2023, Ecological Applications)

   Abstract Emissions of methane (CH₄) and nitrous oxide (N₂O) from soils to the atmosphere can offset the benefits of carbon sequestration for climate change mitigation. While past study has suggested that both CH₄and N₂O emissions from tidal freshwater forested wetlands (TFFW) are generally low, the impacts of coastal droughts and drought‐induced saltwater intrusion on CH₄and N₂O emissions remain unclear. In this study, a process‐driven biogeochemistry model, Tidal Freshwater Wetland DeNitrification‐DeComposition (TFW‐DNDC), was applied to examine the responses of CH₄and N₂O emissions to episodic drought‐induced saltwater intrusion in TFFW along the Waccamaw River and Savannah River, USA. These sites encompass landscape gradients of both surface and porewater salinity as influenced by Atlantic Ocean tides superimposed on periodic droughts. Surprisingly, CH₄and N₂O emission responsiveness to coastal droughts and drought‐induced saltwater intrusion varied greatly between river systems and among local geomorphologic settings. This reflected the complexity of wetland CH₄and N₂O emissions and suggests that simple linkages to salinity may not always be relevant, as non‐linear relationships dominated our simulations. Along the Savannah River, N₂O emissions in the moderate‐oligohaline tidal forest site tended to increase dramatically under the drought condition, while CH₄emission decreased. For the Waccamaw River, emissions of both CH₄and N₂O in the moderate‐oligohaline tidal forest site tended to decrease under the drought condition, but the capacity of the moderate‐oligohaline tidal forest to serve as a carbon sink was substantially reduced due to significant declines in net primary productivity and soil organic carbon sequestration rates as salinity killed the dominant freshwater vegetation. These changes in fluxes of CH₄and N₂O reflect crucial synergistic effects of soil salinity and water level on C and N dynamics in TFFW due to drought‐induced seawater intrusion. 

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3. The ratio of denitrification end-products were influenced by soil pH and clay content across different texture classes in Oklahoma soils

   https://doi.org/10.3389/fsoil.2024.1342986  

   Khalifah, Shaima; Foltz, Mary E. (February 2024, Frontiers in Soil Science)

   Nitrous oxide (N₂O) is a potent greenhouse gas that contributes to stratospheric ozone depletion and global climate change. Soil denitrification has two potential end-products, N₂O and dinitrogen (N₂), and the ratio of these end-products (N₂O:(N₂O+N₂) or the N₂O ratio) is controlled by various factors. This study aims to quantify the influence of soil pH on the ratio of denitrification end-products in Oklahoma soils with different soil textures. Six natural grassland soils encompassing three distinct soil textures were incubated in the laboratory under natural and modified pH with an overall tested pH ranging from 2 to 10. Denitrification end-products were measured in the laboratory using the acetylene inhibition technique and further estimated using a process-based biogeochemical model. Both the laboratory and model results showed that soil pH and texture influenced the ratio of the denitrification end-products. Generally, as soil pH increased the N₂O ratio decreased, although both lab and model results indicated that this relationship was not linear. Soil texture may have an indirect effect on the N₂O ratio, as two soils of the same texture could have different N₂O ratios. However, clay percentage of the soil did show a linear positive correlation with the N₂O ratio, suggesting components of soil texture may be more influential than others. Overall, soil pH was a controlling factor in the ratio of denitrification end-products and the newly observed nonlinear relationship warrants further study, particularly when considering its effects in different soil textures. 

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4. Bacterial denitrification drives elevated N ₂ O emissions in arid southern California drylands

   https://doi.org/10.1126/sciadv.adj1989  

   Krichels, Alexander H; Jenerette, G Darrel; Shulman, Hannah; Piper, Stephanie; Greene, Aral C; Andrews, Holly M; Botthoff, Jon; Sickman, James O; Aronson, Emma L; Homyak, Peter M (December 2023, Science Advances)

   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. 

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5. Isotopic signals in an agricultural watershed suggest denitrification is locally intensive in riparian areas but extensive in upland soils

   https://doi.org/10.1007/s10533-022-00898-9  

   Sigler, W. A.; Ewing, S. A.; Wankel, S. D.; Jones, C. A.; Leuthold, S.; Brookshire, E. N.; Payn, R. A. (March 2022, Biogeochemistry)

   Abstract Nitrogen loss from cultivated soils threatens the economic and environmental sustainability of agriculture. Nitrate (NO 3 − ) derived from nitrification of nitrogen fertilizer and ammonified soil organic nitrogen may be lost from soils via denitrification, producing dinitrogen gas (N 2 ) or the greenhouse gas nitrous oxide (N 2 O). Nitrate that accumulates in soils is also subject to leaching loss, which can degrade water quality and make NO 3 − available for downstream denitrification. Here we use patterns in the isotopic composition of NO 3 − observed from 2012 to 2017 to characterize N loss to denitrification within soils, groundwater, and stream riparian corridors of a non-irrigated agroecosystem in the northern Great Plains (Judith River Watershed, Montana, USA). We find evidence for denitrification across these domains, expressed as a positive linear relationship between δ 15 N and δ 18 O values of NO 3 − , as well as increasing δ 15 N values with decreasing NO 3 − concentration. In soils, isotopic evidence of denitrification was present during fallow periods (no crop growing), despite net accumulation of NO 3 − from the nitrification of ammonified soil organic nitrogen. We combine previous results for soil NO 3 − mass balance with δ 15 N mass balance to estimate denitrification rates in soil relative to groundwater and streams. Substantial denitrification from soils during fallow periods may be masked by nitrification of ammonified soil organic nitrogen, representing a hidden loss of soil organic nitrogen and an under-quantified flux of N to the atmosphere. Globally, cultivated land spends ca. 50% of time in a fallow condition; denitrification in fallow soils may be an overlooked but globally significant source of agricultural N 2 O emissions, which must be reduced along-side other emissions to meet Paris Agreement goals for slowing global temperature increase. 

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