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1. Determining chemical factors controlling abiotic codenitrification

   https://doi.org/https://doi.org/10.1021/acsearthsapcechem.0c00225  

   Wilson, Stephanie J; Song, Bongkeun; Phillips, Rebecca L. (January 2021, ACS earth and space chemistry)

   null (Ed.)

   Codenitrification is a reactive nitrogen (N) removal pathway producing hybrid dinitrogen (N2) by combining nitrite (NO2–) and a partner-N substrate. Abiotic codenitrification also produces hybrid N2 through nitrosation of organic N by NO2–, but it is poorly constrained in soil N cycles. We determined the importance of abiotic codenitrification in soils and examined factors controlling abiotic codenitrification using live soils, sterile soils, and sterile solutions. Abiotic codenitrification in sterile soils ranged from 0.12 ± 0.001 to 0.60 ± 0.08 nmoles 29N2-N g–1 day–1, which accounts for 2.3 to 8.2% of total N2 production measured in live soils. Increased abiotic N2 production was observed in soils with the addition of an organic N partner (glutamine). Consistent with previous work, higher rates were observed in lower-pH soils, but the highest rate was found in the soil with the highest carbon:nitrogen (C:N) ratio. We further investigated a range of organic N partners and the influence of concentration and pH on abiotic codenitrification in solution. Similar to sterile soil incubations, abiotic 29N2 production was negatively correlated with increasing pH in solution. Greater rates of abiotic 29N2 production were measured as the substrate concentration increased and pH decreased. Solution experiments also showed that addition of organic N partners increased abiotic codenitrification rates, which are positively correlated with the C:N ratios of organic N partners. This is the first study demonstrating the importance of N removal through abiotic codenitrification in acidic soils and the C:N ratio of organic N partners as a controlling factor in abiotic codenitrification. 

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2. Divergent controls on carbon concentration and persistence between forests and grasslands of the conterminous US

   https://doi.org/10.1007/s10533-020-00725-z  

   Heckman, K. A.; Nave, L. E.; Bowman, M.; Gallo, A.; Hatten, J. A.; Matosziuk, L. M.; Possinger, A. R.; SanClements, M.; Strahm, B. D.; Weiglein, T. L.; et al (October 2021, Biogeochemistry)

   null (Ed.)

   Abstract Variation in soil organic C (%OC) concentration has been associated with the concentration of reactive Fe- and Al-oxyhydroxide phases and exchangeable Ca, with the relative importance of these two stabilizing components shifting as soil pH moves from acid to alkaline. However, it is currently unknown if this pattern is similar or different with regard to measures of soil C persistence. We sampled soils from 3 horizons (uppermost A, uppermost B, C or lowest B horizons) across a pH gradient of 11 grass-dominated and 13 deciduous/mixed forest-dominated NEON sites to examine similarities and differences in the drivers of C concentration and persistence. Variation in C concentrations in all soils could be linked to abundances of Fe, Al and Ca, but were not significantly linked to variation in soil C persistence. Though pH was related to variation in Δ 14 OC, higher persistence was associated with more alkaline pH values. In forested soils, depth explained 75% of the variation in Δ 14 OC ( p  < 0.0001), with no significant additional correlations with extractable metal phases. In grasslands, soil organic C persistence was not associated with exchangeable Ca concentrations, but instead was explained by depth and inorganic C concentrations (R 2  = 0.76, p  < 0.0001), implying stabilization of organic C through association with carbonate precipitation. In grasslands, measures of substrate quality suggested greater persistence is also associated with a more advanced degree of decomposition. Results suggest that explanatory variables associated with C concentrations differ from those associated with persistence, and that reactive Fe- and Al-oxyhydroxide phases may not be present in high enough concentrations in most soils to offer any significant protective capacity. These results have significant implications for our understanding of how to model the soil C cycle and may suggest previously unrecognized stabilization mechanisms associated with carbonates and forms of extractable Si. 

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3. Pre-agricultural soil erosion rates in the midwestern United States

   https://doi.org/10.1130/G50667.1  

   Quarrier, Caroline L; Kwang, Jeffrey S; Quirk, Brendon J; Thaler, Evan A; Larsen, Isaac J (December 2022, Geology)

   Abstract Erosion degrades soils and undermines agricultural productivity. For agriculture to be sustainable, soil erosion rates must be low enough to maintain fertile soil. Hence, quantifying both pre-agricultural and agricultural erosion rates is vital for determining whether farming practices are sustainable. However, there have been few measurements of pre-agricultural erosion rates in major farming areas where soils form from Pleistocene deposits. We quantified pre-agricultural erosion rates in the midwestern United States, one of the world's most productive agricultural regions. We sampled soil profiles from 14 native prairies and used in situ–produced 10Be and geochemical mass balance to calculate physical erosion rates. The median pre-agricultural erosion rate of 0.04 mm yr–1 is orders of magnitude lower than agricultural values previously measured in adjacent fields, as is a site-averaged diffusion coefficient (0.005 m2 yr–1) calculated from erosion rate and topographic curvature data. The long-term erosion rates are also one to four orders of magnitude lower than the assumed 1 mm yr–1 soil loss tolerance value assigned to these locations by the U.S. Department of Agriculture. Hence, quantifying long-term erosion rates using cosmogenic nuclides provides a means for more robustly defining rates of tolerable erosion and for developing management guidelines that promote soil sustainability. 

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4. Soil NH3 emissions across an aridity, soil pH, and N deposition gradient in southern California

   https://doi.org/10.1525/elementa.2022.00123  

   Krichels, Alexander H.; Homyak, Peter M.; Aronson, Emma L.; Sickman, James O.; Botthoff, Jon; Greene, Aral C.; Andrews, Holly M.; Shulman, Hannah; Piper, Stephanie; Jenerette, G. Darrel (January 2023, Elem Sci Anth)

   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. 

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5. 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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