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1. Climatic and Geochemical Controls on Soil Carbon at the Continental Scale: Interactions and Thresholds

   https://doi.org/10.1029/2020GB006781  

   Yu, Wenjuan; Weintraub, Samantha R.; Hall, Steven J. (March 2021, Global Biogeochemical Cycles)

   Abstract Previous studies found conflicting results on the importance of temperature and precipitation versus geochemical variables for predicting soil organic carbon (SOC) concentrations and trends with depth, and most utilized linear statistical models. To reconcile the controversy, we used data from 2574 mineral horizons from 675 pits from National Ecological Observatory Network sites across North America, typically collected to 1 m depth. Climate was a fundamental predictor of SOC and played similarly important roles as some geochemical predictors. Yet, this only emerged in the generalized additive mixed model and random forest model and was obscured in the linear mixed model. Relationships between water availability and SOC were strongest in very dry ecosystems and SOC increased most strongly at mean annual temperature < 0°C. In all models, depth, oxalate‐extractable Al (Al_ox), pH, and exchangeable calcium plus exchangeable magnesium were important while silt + clay, oxalate‐extractable Fe (Fe_ox), and vegetation type were weaker predictors. Climate and pH were independently related to SOC and also interacted with geochemical composition: Fe_oxand Al_oxrelated more strongly to SOC in wet or cold climates. Most predictors had nonlinear threshold relationships with SOC, and a saturating response to increasing reactive metals indicates soils where SOC might be limited by C inputs. We observed a mostly constant relative importance of geochemical and climate predictors of SOC with increasing depth, challenging previous statements. Overall, our findings challenge the notion that climate is redundant after accounting for geochemistry and demonstrate that considering their nonlinearities and interactions improves spatial predictions of SOC. 

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2. What do relationships between extractable metals and soil organic carbon concentrations mean?

   https://doi.org/10.1002/saj2.20343  

   Hall, Steven_J; Thompson, Aaron (January 2022, Soil Science Society of America Journal)

   Abstract Aluminum (Al)‐bearing and iron (Fe)‐bearing minerals, especially short‐range‐ordered (SRO) phases, are thought to protect soil organic C (SOC). However, it remains methodologically challenging to assess the influence of Al vs. Fe minerals or metal complexes. Whereas SRO Al and Fe phases share some properties, Al dissolved by oxalate (Al_ox) often correlates stronger with SOC than Fe dissolved by oxalate (Fe_ox) or citrate–dithionite (Fe_cd). To further evaluate these relationships, we analyzed a large North American soil dataset from the National Ecological Observatory Network. A strong relationship between Al_oxand SOC (and weaker Fe_ox‐SOC relationship) persisted even after excluding soils rich in SRO minerals (Andisols and Spodosols). Al dissolved by oxalate was strongly correlated with citrate–dithionite‐extractable Al (Al_cd; slope = 0.92,R² = .69), and discrepancies could be explained (R² = .87) by greater dissolution of Al‐substituted Fe phases by citrate–dithionite and greater dissolution of aluminosilicates by oxalate. Aluminum dissolved by oxalate and Al_cdwere both strong SOC predictors despite their differing relationships with silicon (Si). Al dissolved by oxalate and Si_oxstrongly covaried (R² = .79), but Al_cdwas inconsistently related to Si_cd(R² = .18). Similar relationships of Al_oxand Al_cdwith SOC, despite differences in minerals extracted by oxalate and citrate–dithionite, suggest that Al‐OC complexes (as opposed to aluminosilicate or iron‐bearing minerals) were the best SOC predictor. This raises important questions: do Al‐OC complexes indicate protection from decomposition or simply reflect greater intensity of mineral weathering by organic acids; and, if the latter, then perhaps SOC input is driving Al_oxand SOC correlations rather than Al phase composition or abundance. 

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3. Mass and Chemistry of Organic Horizons and Surface Mineral Soils on Watershed 1 at the Hubbard Brook Experimental Forest 1996-present

   https://doi.org/10.6073/pasta/7395cf86c134440f38d800ca59a4857b  

   Johnson, Chris E (January 2022, Environmental Data Initiative)

   {"Abstract":["This data set includes chemistry of O-horizons ("forest floor") and the 0-10 cm \nmineral soil layer in Watershed 1 at Hubbard Book. Calcium in the form of wollastonite \n(CaSiO3) was added to Watershed 1 in October 1999. The application rate was 1028 kg \nCa per ha, and the application was relatively uniform across the watershed. Pre-treatment \nforest floor surveys were completed in 1996 and 1998. The first post-treatment forest \nfloor survey was completed in 2000. This data set includes mass and thickness data for \nthe sampled layers. Chemical data include concentrations and pools of organic matter, \nC, N, Ca, Mg, K, P, Mn, Fe, Al, Cu, Pb, and Zn. Soil pH and exchangeable Al, Ca, Mg, K, \nand H are also included. Sampling is intended to continue at 4 or 5 year intervals.\n These data were gathered as part of the Hubbard Brook Ecosystem Study (HBES). \nThe HBES is a collaborative effort at the Hubbard Brook Experimental Forest, \nwhich is operated and maintained by the USDA Forest Service, Northern Research Station."]} 

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4. Copper oxide nanoparticle dissolution at alkaline pH is controlled by dissolved organic matter: influence of soil-derived organic matter, wheat, bacteria, and nanoparticle coating

   https://doi.org/10.1039/d0en00574f  

   Hortin, J. M.; Anderson, A. J.; Britt, D. W.; Jacobson, A. R.; McLean, J. E. (January 2020, Environmental Science: Nano)

   Dissolution of CuO nanoparticles, releasing Cu ions, is a primary mechanism of Cu interaction in the rooting zone of plants. CuO dissolution is sometimes incorrectly considered negligible at high pH, since complexation of Cu with dissolved organic matter may enhance nanoparticle dissolution. Therefore data on the effects of plant-microbial-soil interactions on nanoparticle dissolution, particularly in alkaline soils, are needed. Dissolution of CuO nanoparticles (100 mg kg −1 Cu) was studied in sand supplemented with factorial combinations of wheat growth, a root-colonizing bacterium, and saturated paste extracts (SPEs) from three alkaline, calcareous soils. In control sand systems with 3.34 mM Ca(NO 3 ) 2 solution, dissolved Cu was low (266 μg L −1 Cu). Addition of dissolved organic matter via wheat root metabolites and/or soil SPEs increased dissolved Cu to 795–6250 μg L −1 Cu. Dissolution was correlated with dissolved organic carbon ( R = 0.916, p < 0.0001). Ligands >3 kDa, presumably fulvic acid from the SPEs, complexed Cu driving solubility; the addition of plant exudates further increased solubility 1.5–3.5×. The root-colonizing bacterium decreased dissolved Cu in sand pore waters from planted systems due to metabolism of root exudates. Batch solubility studies (10 mg L −1 Cu) with the soil SPEs and defined solutions containing bicarbonate or fulvic acid confirmed elevated CuO nanoparticle solubility at >7.5 pH. Nanoparticle dissolution was suppressed in batch experiments compared to sand, via nanoparticle organic matter coating or homoconjugation of dissolved organic matter. Alterations of CuO nanoparticles by soil organic matter, plant exudates, and bacteria will affect dissolution and bioavailability of the CuO nanoparticles in alkaline soils. 

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5. Weathering Intensity and Presence of Vegetation Are Key Controls on Soil Phosphorus Concentrations: Implications for Past and Future Terrestrial Ecosystems

   https://doi.org/10.3390/soilsystems4040073  

   Dzombak, Rebecca M.; Sheldon, Nathan D. (December 2020, Soil Systems)

   null (Ed.)

   Phosphorus (P) is an essential limiting nutrient in marine and terrestrial ecosystems. Understanding the natural and anthropogenic influence on P concentration in soils is critical for predicting how its distribution in soils may shift as climate changes. While it is known that P is sourced from bedrock weathering, relationships between weathering, P, and other soil-forming factors have not been quantified at continental scales, limiting our ability to predict large-scale changes in P concentrations. Additionally, while we know that Fe oxide-associated P is an important P phase in terrestrial environments, the range in and controls on soil Fe concentrations and species (e.g., Fe in oxides, labile Fe) are poorly constrained. Here, we explore the relationships between soil P and Fe concentrations, soil order, climate, and vegetation in over 5000 soils, and Fe speciation in ca. 400 soils. Weathering intensity has a nuanced control on P concentrations in soils, with P concentrations peaking at intermediate weathering intensities (Chemical Index of Alteration, CIA~60). The presence of vegetation (but not plant functional types) affected soils’ ability to accumulate P. Contrary to expectations, P was not more strongly associated with Fe in oxides than other Fe phases. These results are useful both for predicting changes in potential P fluxes from soils to rivers under climate change and for reconstructing changes in terrestrial nutrient limitations in Earth’s past. In particular, soils’ tendency to accumulate more P with the presence of vegetation suggests that biogeochemical models invoking the evolution and spread of land plants as a driver for increased P fluxes in the geological record may need to be revisited. 

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