Nanoscale Interactions of Humic Acid and Minerals Reveal Mechanisms of Carbon Protection in Soil.

The concentrations of terrestrially sourced dissolved organic matter (DOM) have expanded throughout aquatic ecosystems in recent decades. Although sorption to minerals in soils is one major pathway to sequestrate soil organic matter, the mechanisms of organic matter-mineral interactions are not thoroughly understood. Here, we investigated the effect of calcium phosphate mineralization on humic acid (HA) fixation in simulated soil solutions, either with or without clay mineral montmorillonite (Mt). We found that Mt in solution promoted nucleation and crystallization of calcium phosphate (CaP) due to amorphous calcium phosphate clustering and coalescence on Mt surface, which contributed to the long-term persistence and accumulation of HA. Organic ligands with specific chemical groups on HA have higher binding energies to CaP-Mt than to CaP/Mt, according to dynamic force spectroscopy observations. Moreover, CaP-Mt formed in solution showed a great capacity for HA adsorption with a maximum adsorption quantity of 156.89 mg/g. Our findings directly support that Mt is crucial for DOM sequestration by facilitating CaP precipitation/transformation. This has an impact on how effectively we understand the long-term turnover of DOM and highlights knowledge gaps that might assist in resolving essential soil C sequestration issues.

A Critical Review on the Multiple Roles of Manganese in Stabilizing and Destabilizing Soil Organic Matter.

Manganese (Mn) is a biologically important and redox-active metal that may exert a poorly recognized control on carbon (C) cycling in terrestrial ecosystems. Manganese influences ecosystem C dynamics by mediating biochemical pathways that include photosynthesis, serving as a reactive intermediate in the breakdown of organic molecules, and binding and/or oxidizing organic molecules through organo-mineral associations. However, the potential for Mn to influence ecosystem C storage remains unresolved. Although substantial research has demonstrated the ability of Fe- and Al-oxides to stabilize organic matter, there is a scarcity of similar information regarding Mn-oxides. Furthermore, Mn-mediated reactions regulate important litter decomposition pathways, but these processes are poorly constrained across diverse ecosystems. Here, we discuss the ecological roles of Mn in terrestrial environments and synthesize existing knowledge on the multiple pathways by which biogeochemical Mn and C cycling intersect. We demonstrate that Mn has a high potential to degrade organic molecules through abiotic and microbially mediated oxidation and to stabilize organic molecules, at least temporarily, through organo-mineral associations. We outline research priorities needed to advance understanding of Mn-C interactions, highlighting knowledge gaps that may address key uncertainties in soil C predictions.

From pools to flow: The PROMISE framework for new insights on soil carbon cycling in a changing world.

Soils represent the largest terrestrial reservoir of organic carbon, and the balance between soil organic carbon (SOC) formation and loss will drive powerful carbon-climate feedbacks over the coming century. To date, efforts to predict SOC dynamics have rested on pool-based models, which assume classes of SOC with internally homogenous physicochemical properties. However, emerging evidence suggests that soil carbon turnover is not dominantly controlled by the chemistry of carbon inputs, but rather by restrictions on microbial access to organic matter in the spatially heterogeneous soil environment. The dynamic processes that control the physicochemical protection of carbon translate poorly to pool-based SOC models; as a result, we are challenged to mechanistically predict how environmental change will impact movement of carbon between soils and the atmosphere. Here, we propose a novel conceptual framework to explore controls on belowground carbon cycling: Probabilistic Representation of Organic Matter Interactions within the Soil Environment (PROMISE). In contrast to traditional model frameworks, PROMISE does not attempt to define carbon pools united by common thermodynamic or functional attributes. Rather, the PROMISE concept considers how SOC cycling rates are governed by the stochastic processes that influence the proximity between microbial decomposers and organic matter, with emphasis on their physical location in the soil matrix. We illustrate the applications of this framework with a new biogeochemical simulation model that traces the fate of individual carbon atoms as they interact with their environment, undergoing biochemical transformations and moving through the soil pore space. We also discuss how the PROMISE framework reshapes dialogue around issues related to SOC management in a changing world. We intend the PROMISE framework to spur the development of new hypotheses, analytical tools, and model structures across disciplines that will illuminate mechanistic controls on the flow of carbon between plant, soil, and atmospheric pools.

Nitrogen-rich microbial products provide new organo-mineral associations for the stabilization of soil organic matter.

Understanding the cycling of C and N in soils is important for maintaining soil fertility while also decreasing greenhouse gas emissions, but much remains unknown about how organic matter (OM) is stabilized in soils. We used nano-scale secondary ion mass spectrometry (NanoSIMS) to investigate the changes in C and N in a Vertisol and an Alfisol incubated for 365 days with 13 C and 15 N pulse labeled lucerne (Medicago sativa L.) to discriminate new inputs of OM from the existing soil OM. We found that almost all OM within the free stable microaggregates of the soil was associated with mineral particles, emphasizing the importance of organo-mineral interactions for the stabilization of C. Of particular importance, it was also found that 15 N-rich microbial products originating from decomposition often sorbed directly to mineral surfaces not previously associated with OM. Thus, we have shown that N-rich microbial products preferentially attach to distinct areas of mineral surfaces compared to C-dominated moieties, demonstrating the ability of soils to store additional OM in newly formed organo-mineral associations on previously OM-free mineral surfaces. Furthermore, differences in 15 N enrichment were observed between the Vertisol and Alfisol presumably due to differences in mineralogy (smectite-dominated compared to kaolinite-dominated), demonstrating the importance of mineralogy in regulating the sorption of microbial products. Overall, our findings have important implications for the fundamental understanding of OM cycling in soils, including the immobilization and storage of N-rich compounds derived from microbial decomposition and subsequent N mineralization to sustain plant growth.

In the presence of the other: How glyphosate and peptide molecules alter the dynamics of sorption on goethite.

The interactions with soil mineral surfaces are among the factors that determine the mobility and bioavailability of organic contaminants and of nutrients present in dissolved organic matter (DOM) in soil and aquatic environments. While most studies focus on high molar mass organic matter fractions (e.g., humic and fulvic acids), very few studies investigate the impact of DOM constituents in competitive sorption. Here we assess the sorption behavior of a heavily used herbicide (i.e., glyphosate) and a component of DOM (i.e., a peptide) at the water/goethite interface, inclusive of potential glyphosate-peptide interactions. We used in-situ ATR-FTIR (attenuated total reflectance Fourier-transform infrared) spectroscopy to study sorption kinetics and mechanisms of interaction as well as conformational changes to the secondary structure of the peptide. NMR (nuclear magnetic resonance) spectroscopy was used to assess the level of interaction between glyphosate and the peptide and changes to the peptide' secondary structure in solution. For the first time, we illustrate competition for sorption sites results in co-sorption of glyphosate and peptide molecules that affects the extent, kinetics, and mechanism of interaction of each with the surface. In the presence of the peptide, the formation of outer-sphere glyphosate-goethite complexes is favored albeit inner-sphere glyphosate-goethite bonds (i.e., POFe) are still formed. The presence of glyphosate induces secondary structural shifts of the sorbed peptide that maximizes the formation of H-bonds with the goethite surface. However, glyphosate and the peptide do not seem to interact with one another in solution nor at the goethite surface upon sorption. The results of this work highlight potential consequences of competition for sorption sites, for example the transport of organic contaminants and nutrient-rich (i.e., nitrogen) DOM components in relevant environmental systems. Predicting the rate and extent with which organic pollutants are removed from solution by a given solid is also one of the most critical factors for the design of effective sorption systems in engineering applications.

FT-IR spectroscopy analysis of HF-treated mineral soils, a direct approach for deciphering organo-mineral interactions.

Soil organic matter (SOM) constitutes roughly 60% organic carbon (OC) and therefore plays a crucial role in regulating global climate. However, our understanding of the long-term dynamics of the soil carbon pool remains constrained by limitations in analytical approaches capable of providing high resolution molecular-level information from arguably the most complex biomaterial on the planet. In this contribution, we combine hydrofluoric acid (HF) treatment with a spectroscopic approach as a strategy to provide refined molecular-level information on the interactions between soil minerals and SOM. Critically, we have not seen the use of this combined approach anywhere in the literature and strongly believe that it could allow us to improve our overall understanding to the mechanisms and pathways that regulate SOM transformation. Results clearly illustrates which organic structures are preferentially adsorbed to soil minerals and are likely to be protected from degradation, as well as spatial co-variations of SOM with specific mineral components such as Al3+, Si4+ and dibasic cations such as Mg2+as a function of their importance in the interaction process.•Soil samples were collected from different land-use types in rural farming communities of the Upper Rio Grande Valley.•Samples were oven dried, disaggregated, sieved, treated with 10% HF, rinsed and oven dried.•Oven dried samples were subjected to Mid-infrared (4000-400 cm-1), XRD and ED-XRF analyses.

Effects of long-term fertilization on calcium-associated soil organic carbon: Implications for C sequestration in agricultural soils.

Although the contribution of calcium ion (Ca2+) to stabilizing organic carbon (OC) in soils has been known for years, we still have a limited understanding of the quantity and molecular composition of Ca2+ bound SOC (Ca-OC) evolution in response to long-term fertilization. Here we report the role of Ca2+ in the accumulation of OC in the topsoil (0-20 cm) from two long-term (25-37 years) fertilization experiment sites. Approximately 4.54-19.27% and 9.00-25.15% of SOC was bound with Ca2+ in the Ferric Acrisol and Fluvic Cambisol, respectively. The application of NPK mineral fertilizers (NPK) decreased (p < 0.05) the Ca-OC stocks from 3.40 t ha-1 to 0.96 t ha-1 and from 2.03 t ha-1 to 1.17 t ha-1 in the Ferric Acrisol and Fluvic Cambisol, respectively. Swine manure (M) addition did not change (p > 0.05) the Ca-OC stock in Ferric Acrisol, but enhanced (p < 0.05) that from 2.03 t ha-1 to 9.75 t ha-1 in Fluvic Cambisol. Fourier transform infrared and carbon (1s)-near X-ray absorption spectroscopies showed that Ca2+ was mainly bound with aromatic carbon and carboxylic carbon. Long-term M fertilization facilitated the binding of Ca2+ with O-alkyl C, suggesting an increment of Ca-linked polysaccharide. Calcium ion was preferentially associated with 13C enriched organic matter (OM). Mineral fertilization promoted the 13C-enriched organic compounds in the Ca-OC, while organic fertilization facilitated the binding of 13C-depleted organic C with Ca2+. This study suggests that Ca-OC may be a potentially vital and stable OC pool in arable soils, and provides direct evidence for the preferential association of OC with Ca2+ in edaphic environments.

The effects of organic matter-mineral interactions and organic matter chemistry on diuron sorption across a diverse range of soils.

Sorption of non-ionic organic compounds to soil is usually expressed as the carbon-normalized partition coefficient (KOC), because it is assumed that the main factor that influences the amount sorbed is the organic carbon content of the soil. However, KOC can vary by a factor of at least ten across a range of soils. We investigated two potential causes of variation in diuron KOC - organic matter-mineral interactions and organic matter chemistry - for a diverse set of 34 soils from Sri Lanka, representing a wide range of soil types. Treatment with hydrofluoric acid (HF-treatment) was used to concentrate soil organic matter. HF-treatment increased KOC for the majority of soils (average factor 2.4). We attribute this increase to the blocking of organic matter sorption sites in the whole soils by minerals. There was no significant correlation between KOC for the whole soils and KOC for the HF-treated soils, indicating that the importance of organic matter-mineral interactions varied greatly amongst these soils. There was as much variation in KOC across the HF-treated soils as there was across the whole soils, indicating that the nature of soil organic matter is also an important contributor to KOC variability. Organic matter chemistry, determined by solid-state (13)C nuclear magnetic resonance (NMR) spectroscopy, was correlated with KOC for the HF-treated soils. In particular, KOC increased with the aromatic C content (R=0.64, p=1×10(-6)), and decreased with O-alkyl C (R=-0.32, p=0.03) and alkyl C (R=-0.41, p=0.004) content.