Soil Syst., Volume 2, Issue 1 (March 2018) – 17 articles

Short-term variations of soil conditions affect the form, mobility and bioavailability of metal pollutants. Released metals migrate toward depth where they are intercepted or precipitate, leading to variable spatial metal distribution patterns, at a macro-, meso- and microscale. Studies at a mesoscale give access to trace metal (TM) associations induced by pedological processes. Although scarcely documented, such meso-scale studies represent an essential step for relevant environmental risk assessment, halfway between field- and molecular-scale investigations. We argued for such approach by performing optical microscopy and micro-X-ray fluorescence on thin sections from two soils, contaminated either by industrial zinc-smelter waste or by urban wastewater. Consistent correlation between key indicators of pedological processes (Fe, Mn, and Ca) and trace metals (Zn, Pb, and Cu) on some 20 elemental maps of TM-hosting soil constituents and pedofeatures reveal distinct coinciding localizations, illustrating TM-accumulation via interception or (co)-precipitation processes. Micromorphological interpretation of characteristic pedofeatures in subsurface horizons (crystals, argillans, ferrans, and mangans) containing significant amounts of TM provide valuable insight into the contaminant dynamics in terms of lixiviation, colloidal transport, redox conditions, or fungal activity. Our mesoscale approach stresses the importance of pedology-based sampling strategies, instead of systematic soil-depth sampling, for soil contamination research in natural ecosystems. Full article

Organic carbon in subsoil generally has longer turnover times than that in surface soil, but little is known about how the stability of the specific organic compound classes changes with soil depth. The objective of this study was to analyze the composition and thermal stability of clay-associated organic matter (OM) at varying soil depths in the summit and footslope of a pasture hillslope using C X-ray absorption near edge structure (XANES) and pyrolysis-field ionization mass spectrometry (Py-FIMS). C XANES showed aromatic C was relatively enriched in the subsoil, relative to the surface soil. Py-FIMS demonstrated a relative enrichment of phenols/lignin monomers and alkylaromatics with increasing profile depth in the summit soil, and to a greater extent in the footslope soil, followed by a decreasing abundance of sterols. In surface soil, the thermostability of clay-associated OM increases in the order: carbohydrates and N compounds < phenols/lignin monomers < lignin dimers and alkylaromatics, suggesting the intrinsic chemical nature of OM as a major driver for OM persistent in surface soil. The thermal stability of clay-associated carbohydrates, N compounds, and phenols/lignin monomers increased with profile depth, likely due to stronger organic-organic/organic-mineral binding. In subsoil, the thermal stability of clay-associated carbohydrates and N compounds can be as high as that of alkylaromatic and lignin dimers, implying that persistent subsoil OM could be composed of organic compound classes, like carbohydrates, that were traditionally considered as biochemically labile compounds. In contrast, the thermally-stable compound classes, like lignin dimers and alkylaromatics, showed no changes in the thermal stability with soil depth. This study suggests that stability of the more labile OM compounds may be more strongly influenced by the change in environmental conditions, relative to the more stable forms. Full article

Water transports organic matter through soils, where mineral-organic associations form to retain dissolved organic matter (“DOM”), influencing terrestrial carbon cycling, nutrient availability for plant growth, and other soil organic matter functions. We combined Fourier transform ion cyclotron resonance mass spectrometry with novel data analysis techniques to examine the role of sorptive fractionation in the associations between Fe(III)-montmorillonite and DOM from composted biosolids (“anthropogenic DOM”). To examine the influence of DOM composition on sorption and sorptive fractionation, we used resin-based separation to produce DOM subsamples with different molecular compositions and chemical properties. A large proportion (45 to 64%) of the initial carbon in every DOM solution sorbed to the Fe(III)-montmorillonite. However, when the compositions of the initial solutions were compared to the sorbed organic matter, the computed changes in composition were lower (10 to 32%). In fact, non-selective sorption was more important than selective sorption in every sample, except for the hydrophilic neutral (HiN) fraction, where high nitrogen content and acidic conditions appeared to enhance sorptive fractionation. The results from this study demonstrate that the importance of sorptive fractionation varies with DOM composition and other factors, and that non-selective sorption can contribute substantially to the formation of mineral-organic associations. Full article

Quantifications of annual soil respiration in switchgrass systems are limited to the growing season or coarse-scale temporal sampling. This study evaluates daily and seasonal soil CO₂ respiration in switchgrass croplands. Hourly measurements during a 12-month period were taken for soil CO₂ flux, soil temperature, and soil moisture. Although both soil temperature and moisture were positively correlated with soil CO₂ flux rates, soil temperature was the primary driver of soil respiration. During winter, lower soil temperatures corresponded with significant decreases in average daily CO₂ flux rates, however, CO₂ pulses associated with precipitation events increased flux rates up to three times the seasonal daily average. Soil temperature influenced both daily and seasonal flux patterns where the highest flux rates, up to 31.0 kg CO₂ ha⁻¹ h⁻¹, were observed during the warmest hours of the day (13:00 to 15:00) and during the warmest season (Summer). Summer and Spring emissions combined accounted for 80.1% of annual flux, indicating that exclusion of non-growing season time periods may result in an underestimation of total annual CO₂ efflux. Our results indicate that inclusion of the non-growing season and a fine-resolution temporal sampling approach provides more accurate quantifications of total annual CO₂ emissions in switchgrass croplands. Full article

Humus forms are the morphological results of organic matter decay and distribution in the topsoil, and thus important indicators for decomposer activities in forest ecosystems. The first aim was to examine if humus forms are suitable indicators of microbiological properties of the topsoil in a high mountain forest (Val di Rabbi, Trentino, Italian Alps). The second aim was to predict microbiological parameters based on the topsoil pH value on two slopes of the study area (ca. 1200-2200 m a.s.l.). We investigated humus forms and determined pH values and microbiological parameters (enzymatic activities, carbon/nitrogen (C/N) ratio and the ratio of bacterial/archaeal abundance) of the uppermost mineral horizon. The results reveal significant correlations between pH value and microbiological parameters (except for bacterial/archaeal abundance), which enable upscaling to the landscape scale using linear models. Based on a random forest with kriging of model residuals, predictive maps of humus form, pH value and microbiological parameters show that decomposition processes in our study area correspond with the topography. As compared to locations on south-facing slopes or close to the valley bottom, locations on north-facing slopes or close to the upper treeline exhibit Moder (scarcely Mull or Amphimull), more acidic topsoil (around pH 4), a lower activity of leucine-aminopeptidase, a lower ratio of alkaline/acid phosphomonoesterase activity and a higher soil C/N ratio (above 20). Our results suggest a high potential of humus forms to indicate soil microbiological properties in a high mountain forest. Together with the pH values of the topsoil, humus forms proved to be a useful tool as a basis for predictive maps of leucine-aminopeptidase activity, ratio of alkaline/acid phosphomonoesterase activity and C/N ratio of the mineral topsoil. Full article

Soil water (θ) dynamics are important parameters to monitor in any field-based drought research. Although apparent electrical conductivity (EC_a) measured by electromagnetic (EM) induction has been used to estimate θ, little research has shown its successful application at the plot-scale for evaluating crop water use. An EM38 conductivity meter was used to collect time-lapse EC_a data at the plot scale across a field cropped with 36 different chickpea genotypes. An empirical multiple linear regression model was established to predict θ measured by neutron probes and depth-specific electrical conductivity (σ) generated by a 1-D EM inversion algorithm. Soil water dynamics and movement were successfully mapped with a coefficient of determination (R²) of 0.87 and root-mean-square-error of 0.037 m³ m⁻³. The rate of soil drying varied with depth and was influenced by chickpea growth stages and genotypes. The results were also used to evaluate the differences in soil water use and rooting depths within- and across-plant species and during the growth stages. Coupled with physiology measurements, the approach can also be used to identify mechanisms of drought tolerance in the field and screening for effective water use in crop breeding programs. Full article

Processes controlling microbial access to soil organic matter are critical for soil nutrient cycling and C stabilization. The bioavailability of soil organic matter partly depends on the rate that substrates become water-soluble, which is determined by some combination of biological, biochemical, and purely abiotic processes. Our goal was to unravel these biotic and abiotic processes to better understand mechanisms controlling the dynamics of bioavailable soil organic carbon (SOC). We sampled soils in a California annual grassland from manipulated plots with and without plants to help distinguish bioavailable SOC generated from mineral-associated organic matter versus from plant detritus (i.e., the “light fraction”). In the laboratory, soils were incubated for 8 months under all possible combinations of three levels of moisture and two levels of microbial biomass using continuous chloroform sterilization. We measured cumulative carbon dioxide (CO₂) production and the net change in soil water-extractable organic C (WEOC) to quantify C that was accessed biologically or biochemically. Under the driest conditions, microbes appeared to primarily access WEOC from recent plant C, with the other half of CO₂ production explained by extracellular processes. These results suggest that dry, uncolonized conditions promote the adsorption of WEOC onto mineral surfaces. Under wetter conditions, microbial access increased by two orders of magnitude, with a large concomitant decrease in WEOC, particularly in soils without plant inputs from the previous growing season. The largest increase in WEOC occurred in wet sterilized soil, perhaps because exoenzymes and desorption continued solubilizing C but without microbial consumption. A similar amount of WEOC accumulated in wet sterilized soil whether plants were present or not, suggesting that desorption of mineral-associated C was the abiotic WEOC source. Based on these results, we hypothesize that dry-live and wet-uncolonized soil microsites are sources of bioavailable SOC, whereas wet-live and dry-uncolonized microsites are sinks. Full article

The effect of fluctuating soil moisture cycles on soil cadmium (Cd) phytoavailability was investigated in a pot trial with two contrasting soils (Kereone (Allophanic), total Cd 0.79 mg kg⁻¹; and Topehaehae (Gley), total Cd 0.61 mg kg⁻¹) that were either sown with plantain (Plantago lanceolata) or left unseeded. Varying soil moisture contents were established using contrasting irrigation regimes: “flooded” (3 days flooded and then 11 days drained); or “non-flooded” (irrigation to 70% of potted field capacity every 7 days). Overall, there was no significant difference in mean 0.05 M CaCl₂ soil extractable Cd concentrations or plant tissue Cd concentrations between flooded and non-flooded irrigation. However, there was a consistent trend for an increase in soil extractable Cd concentrations following irrigation, regardless of the irrigation regime. Mean soil extractable Cd and plant tissue Cd concentrations were significantly greater (approximately 325% and 183%, respectively) for the Topehaehae soil than the Kereone soil, despite the lower soil total Cd concentration of the Topehaehae soil. These results indicate that Cd solubility is sensitive to increases in soil moisture following periods of soil drainage, but insensitive to short-term periods of soil saturation. Plant tissue Cd concentrations in Cd-sensitive forage crops such as plantain are likely to be greater following large rainfall events over summer and autumn. This has the potential to increase animal dietary Cd exposure and rate of liver/kidney Cd accumulation. Full article

Dissolved organic matter (DOM) chemistry and the potential for organic matter (OM) to self-associate with other OM components are important aspects of understanding the mechanisms of DOM sorption to clay surfaces. To investigate this further, we sorbed DOM isolated from peat humic acid onto either kaolinite, montmorillonite and gibbsite via ten sequential batch equilibration sorption experiments. Dissolved organic carbon (DOC) sorption to all minerals increased consistently, suggesting that sorption occurred via mineral-OM interactions at the beginning of the experiment. After six successive DOM loadings, the concentration of DOC sorbed by kaolinite and gibbsite began to plateau, likely due to the saturation of mineral surface sorption sites. Solution-state nuclear magnetic resonance (NMR) analysis of unbound DOM showed that kaolinite and montmorillonite sorbed aliphatic, protein and lignin components initially and primarily aliphatic and aromatic constituents in later sorption experiments, whereas gibbsite sorbed mostly aliphatic compounds during all DOM loadings. Analysis of the organo-clay complexes using ¹H high resolution–magic angle spinning (HR-MAS) NMR confirmed the preferential sorption of aromatic and aliphatic components to all three minerals. Overall, these results suggest that OM-OM interactions may be important mechanisms of DOM sorption to clay mineral surfaces. Full article

Soil acidification is a global issue that often results in increased aluminum (Al) toxicity. While no-till (NT) management has many benefits regarding sustainability, a discrete zone of acidification often occurs when ammoniacal fertilizers are banded below the seed. The full agroecological consequences of NT stratification and impacts on bacterial communities are largely unknown. Using next-generation sequencing (NGS) and Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt), we characterized the influence of liming amendment and soil stratification on bacterial community composition and predicted function in 2-cm depth increments. Soil depth, pH, DTPA extractable aluminum (DTPA-Al), and KCl extractable Al (KCl-Al) were all significantly correlated with bacterial community structure and function. In soils with the lowest pH and greatest extractable Al, bacterial community was distinct, with highest relative abundance of the Koribacteraceae family, an indicator of soil degradation. Additionally, aspects of bacterial metabolism and nutrient turnover were impacted in the lowest pH zones, including secondary metabolite, carbohydrate, and energy metabolism. These results suggest that soil stratification (Al and pH) in NT systems has direct impacts on microbial community structure and function, potentially influencing ecosystem services at a highly resolved spatial scale within surface depths relevant to seed germination and emergence. Full article

The dynamics of soil organic carbon (SOC) storage and turnover are a critical component of the global carbon cycle. Mechanistic models seeking to represent these complex dynamics require detailed SOC compositions, which are currently difficult to characterize quantitatively. Here, we address this challenge by using a novel approach that combines Fourier transform infrared spectroscopy (FT-IR) and bulk carbon X-ray absorption spectroscopy (XAS) to determine the abundance of SOC functional groups, using elemental analysis (EA) to constrain the total amount of SOC. We used this SOC functional group abundance (SOC-fga) method to compare variability in SOC compositions as a function of depth across a subalpine watershed (East River, Colorado, USA) and found a large degree of variability in SOC functional group abundances between sites at different elevations. Soils at a lower elevation are predominantly composed of polysaccharides, while soils at a higher elevation have more substantial portions of carbonyl, phenolic, or aromatic carbon. We discuss the potential drivers of differences in SOC composition between these sites, including vegetation inputs, internal processing and losses, and elevation-driven environmental factors. Although numerical models would facilitate the understanding and evaluation of the observed SOC distributions, quantitative and meaningful measurements of SOC molecular compositions are required to guide such models. Comparison among commonly used characterization techniques on shared reference materials is a critical next step for advancing our understanding of the complex processes controlling SOC compositions. Full article

The increasing presence of successful invasive plant species can have an impact on wetlands capacity to store and release C. We have investigated the relationships between stocks of different soil organic carbon (SOC) along the soil vertical profile and invasive plant success in a China wetland. In stands dominated by the exotic invasive species Spartina alterniflora and the native invasive Phragmites australis soil organic-carbon concentrations (SOC) were higher (12% and 9%, respectively) than in plots of a native species, Cyperus malaccensis, whereas SOC content (g m⁻²) was 18% and 17% lower under P. australis than under S. alterniffolia and C. malaccensis, respectively. Soils under both invasive species had the concentrations and contents of light-fraction organic carbon (LFOC), light-fraction organic nitrogen (LFON) at 30–60 cm of soil depth and labile organic carbon (LOC) concentrations at 0–10 cm higher than soils under native species. The invasive species had higher total aboveground, total biomasses and lower shoot:root ratios than the native species. The success of both invasive species was associated with higher growth rates and accumulation of nutrients in biomass than in the native species and also accumulation of C in plant soil system. The stands currently dominated by the invasive species were recently occupied by monospecific stands of the native C. malaccensis, strongly suggesting that all or most of the current soil differences were due to the invasions. Higher sand fraction in C. malaccensis community and higher clay fraction in P. australis community relative to the native species, were correlated with higher soil N and P concentrations in invaded stands. The results suggest that different vegetation cover with distinct shoot/root ratio can change soil structure by favoring sedimentation of different particle size classes. Thus, despite both invasive species have some common traits, the results also showed that different invasive species with partially distinct impacts on soil and nutrient uses can succeed under the same conditions. The traits conferring invasive success are thus not necessarily species-specific. A clear change in the general accumulation of C, N and P in the plant-soil system was related to the invasive plant success in this wetland areas. Full article

In the Southeast United States (U.S.), the climate is predicted to be warmer and have more severe drought in the summer. Decreasing rainfall in summer months should create more severe soil drying, which will eventually affect re-wetting cycles deeper in the soil profile. Changing drying-wetting cycles in this deeper portion of the profile may impact the soil C pool, the largest pool of terrestrial C globally. The aim of this research is to study the effect of drying-wetting cycles on deep soil C. A soil incubation experiment was established using four soils that are part of a simulated drought experiment in Oklahoma, Virginia, Georgia, and Florida. Soils were incubated from as many as eight layers up to a depth of 3.0 m. During incubations, soil respiration was generally greatest in surface soils and declined with depth. When compared to soils that were kept constantly moist, drying-wetting cycles did not consistently stimulate more soil respiration. Soil respiration as a proportion of total soil C, however, was higher in soils below 1 m than above. Total C (R² = 0.82) and hydrolysable C (R² = 0.77) were the best predictors for soil respiration. Assuming that there was no other factor (i.e., new carbon inputs) affecting soil respiration at depth other than soil moisture cycles, this study indicates that there would be no significant change to soil respiration in deep soils under more severe drying-wetting cycles. Full article

Processes underlying soil organic matter (SOM) transformations are meeting growing interest as SOM contains more carbon (C) than global vegetation and the atmosphere combined. Therefore, SOM is a crucial element of the C cycle, especially in ecosystems rich in organic matter, such as boreal forests. However, climate change may shift the fate of this SOM from C sink into C source, accelerating global warming. These processes require a better understanding of the involved mechanisms driving both the C cycle and the interlinked nitrogen (N) cycle. SOM transformations are balanced by a network of interactions between biological, chemical and physical factors. In this review, we discuss the findings of the most recent studies to the current state of knowledge about the main drivers in SOM transformations. We focus on plant-derived secondary metabolites, as their biochemical traits, especially interactions with soil microbial communities, organic N compounds and enzymes make them potential regulators of SOM decomposition. However, these regulatory abilities of plant-derived compounds are not fully explored. Full article

Adsorption processes at mineral–water interfaces control the fate and transport of arsenic in soils and aquatic systems. Mechanistic and thermodynamic models to describe this phenomenon only consider inner-sphere complexes but recent observation of the simultaneous adsorption of inner- and outer-sphere arsenate on single crystal surfaces complicates this picture. In this study, we investigate the ionic strength-dependence of the macroscopic adsorption behavior and molecular-scale surface speciation of arsenate bound to gibbsite and bayerite. Arsenate adsorption decreases with increasing ionic strength on both minerals, with a larger effect at pH 4 than pH 7. The observed pH-dependence corresponds with a substantial decrease in surface charge at pH 7, as indicated by ζ-potential measurements. Extended X-ray absorption fine structure (EXAFS) spectroscopy finds that the number of second shell Al neighbors around arsenate is lower than that required for arsenate to occur solely as an inner-sphere surface complex. Together, these observations demonstrate that arsenate displays macroscopic and molecular-scale behavior consistent with the co-occurrence of inner- and outer-sphere surface complexes. This demonstrated that outer-sphere species can be responsible for strong adsorption of ions and suggests that environments experiencing an increase in salt content may induce arsenic release to water, especially under weakly acidic conditions. Full article