Soil Syst., Volume 3, Issue 3 (September 2019) – 19 articles

Bulk soil phosphorus speciation by X-ray absorption spectroscopy (XAS) using fluorescence yield-mode measurements is an important tool for phosphorus research because of the low soil P contents. However, when measuring in fluorescence mode, increasing the concentration of the absorbing atom can dampen the XAS spectral features because of self-absorption and affect the linear combination (LC) fitting results. To reduce the self-absorption for samples of high P contents, thick boron nitride diluted samples are produced, yet the effects of self-absorption on P speciation results via LC fitting of P K-edge XANES spectroscopy, and the possible benefits of data processing optimization are unknown. Toward this end, we produced a series of ternary standard mixtures (calcium-iron-aluminum phosphates) and an example soil sample both diluted using boron nitride over a range from 1 to ~900 mmol kg⁻¹ for the soil sample and up to ~6000 mmol kg⁻¹ for the standard mixture. We show that by optimizing background subtraction and normalization values, consistent results with less than 10% error can be obtained for samples with up to 300 mmol kg⁻¹ P. Our results highlight the applicability of optimized P K-edge XANES fitting across a wide range of concentrations encountered in natural environments. Full article

Topographic depressions in upland soils experience anaerobic conditions conducive for iron (Fe) reduction following heavy rainfall. These depressional areas can also accumulate reactive Fe compounds, carbon (C), and nitrate, creating potential hot spots of Fe-mediated carbon dioxide (CO₂) and nitrous oxide (N₂O) production. While there are multiple mechanisms by which Fe redox reactions can facilitate CO₂ and N₂O production, it is unclear what their cumulative effect is on CO₂ and N₂O emissions in depressional soils under dynamic redox. We hypothesized that Fe reduction and oxidation facilitate greater CO₂ and N₂O emissions in depressional compared to upslope soils in response to flooding. To test this, we amended upslope and depressional soils with Fe(II), Fe(III), or labile C and measured CO₂ and N₂O emissions in response to flooding. We found that depressional soils have greater Fe reduction potential, which can contribute to soil CO₂ emissions during flooded conditions when C is not limiting. Additionally, Fe(II) addition stimulated N₂O production, suggesting that chemodenitrification may be an important pathway of N₂O production in depressions that accumulate Fe(II). As rainfall intensification results in more frequent flooding of depressional upland soils, Fe-mediated CO₂ and N₂O production may become increasingly important pathways of soil greenhouse gas emissions. Full article

Soil carbon storage is affected by particle-size fractions and Fe oxides. We assessed soil carbon concentrations in different particle-size fractions, determined the soil chemical composition of the soil, and weathering and mineralogy of sandy soils of the Wisconsin Central Sands, USA. Three land uses were studied (agriculture, forest, and prairie). The soils contained a minimum of 830 g sand kg⁻¹ up to 190 cm soil depth. Approximately 46% of the sand was in the 250–500 μm fraction, and 5% was <125 μm. Soil carbon ranged from 5 to 13 g kg⁻¹ in the topsoil, and decreased with depth. The <45 μm fraction tended to have high concentrations of carbon, ranging from 19 to 43 g kg⁻¹ in the topsoil. Silicon content was over 191 g Si kg⁻¹, and was lowest in the Bt horizons (191–224 g Si kg⁻¹). Up to 29 g Fe kg⁻¹ and 39 g Al kg⁻¹ were present in the soil, and were highest in the Bt horizons. These soils were mostly quartz, and diopside was found throughout the soil profiles. Weathering indices, such as the Ruxton Ratio, showed that the C horizons were the least weathered and the Bt horizons were more weathered. We conclude that most of the carbon in these soils is held in the <45 μm fraction, and soil carbon and total Fe were lowest in the coarser size fractions. Full article

Arsenic availability to rice is tied to biogeochemical cycling of Fe and Mn in rice soils. Two strategies to minimize As uptake by rice—increasing Si and decreasing water—affect soil Fe and Mn pools. We synthesized data from several soil-based experiments with four rice cultivars across pot and field trials with manipulations of Si, water, or both. Increasing Si alters the mineral composition of Fe plaque more than decreasing water, with the former promoting relatively more ferrihydrite and less lepidocrocite. Nonflooded conditions decrease lepidocrocite but slightly increase goethite compared to flooded rice. Plaque As, which was a mixture of arsenite (15–40%) and arsenate (60–85%), was correlated positively with ferrihydrite and negatively with lepidocrocite and goethite. Plaque As was also positively correlated with F1 and F2 soil As, and F2 was correlated positively with porewater As, total grain As, and grain organic As (oAs). Grain inorganic As (iAs) was negatively correlated with oxalate-extractable Fe and Mn. Our data and multiple linear regression models suggest that under flooded conditions iAs is released by poorly crystalline Fe oxides to porewater mainly as iAs(III), which can either be taken up by the plant, adsorbed to Fe plaque, oxidized to iAs(V) or methylated to oAs. Increasing Si can promote more desorption of iAs(III) and promote more poorly-ordered phases in plaque and in bulk soil. The ultimate effectiveness of a Si amendment to decrease As uptake by rice depends upon it being able to increase exogenous Si relative to As in porewater after competitive adsorption/desorption processes. Our data further suggest that poorly crystalline Fe and Mn soil pools can retain inorganic As and decrease plant uptake, but these pools in bulk soil and plaque control grain organic As. Full article

Biotic interactions structure ecological communities but abiotic factors affect the strength of these relationships. These interactions are difficult to study in soils due to their vast biodiversity and the many environmental factors that affect soil species. The McMurdo Dry Valleys (MDV), Antarctica, are relatively simple soil ecosystems compared to temperate soils, making them an excellent study system for the trophic relationships of soil. Soil microbes and relatively few species of nematodes, rotifers, tardigrades, springtails, and mites are patchily distributed across the cold, dry landscape, which lacks vascular plants and terrestrial vertebrates. However, glacier and permafrost melt are expected to cause shifts in soil moisture and solutes across this ecosystem. To test how increased moisture and salinity affect soil invertebrates and their biotic interactions, we established a laboratory microcosm experiment (4 community × 2 moisture × 2 salinity treatments). Community treatments were: (1) Bacteria only (control), (2) Scottnema (S. lindsayae + bacteria), (3) Eudorylaimus (E. antarcticus + bacteria), and (4) Mixed (S. lindsayae + E. antarcticus + bacteria). Salinity and moisture treatments were control and high. High moisture reduced S. lindsayae adults, while high salinity reduced the total S. lindsayae population. We found that S. lindsayae exerted top-down control over soil bacteria populations, but this effect was dependent on salinity treatment. In the high salinity treatment, bacteria were released from top-down pressure as S. lindsayae declined. Ours was the first study to empirically demonstrate, although in lab microcosm conditions, top-down control in the MDV soil food web. Full article

This paper addresses the role of soil erosion and mass movements on mountainous trails due to human trampling on steep slopes. This is the case of several trails situated on forested areas in South-East Brazil, even those located in protected areas. Two methods were used to achieve the research objectives. Firstly, analyses of microtopography using erosion bridges, which was monitored four times on Caixa D’Aço natural pool trails in Serra da Bocaina National Park. Secondly, disturbed and undisturbed soil samples were collected at 0–10 cm depth at four sites on Água Branca trail in Serra do Mar State Park. Using this methodology, we assessed soil degradation in two different humid tropical environments. Generally, trampling combined with deficient trail management, play important roles in degrading soils in both areas. Bioengineering techniques should be used to recuperate these trails, which are used by tourists and local residents. We hope this research work may contribute towards improved management in Brazilian protected areas. Full article

To investigate the influence of residence time on molybdenum [Mo(VI)] adsorption behavior in soil environments, kinetic batch experiments coupled with X-ray near-edge structure (XANES) spectroscopy were performed for a neutral-pH soil (Webster loam) and two acidic soils (Mahan sand and Windsor sand) at different time scales (1 day–1 year). Batch-type experiments indicated that retention of Mo(VI) was rate limited and typical biphasic for soils. Initial rapid retention was followed by a continued slow retention with increasing aging time for Mahan and Windsor soils. In contrast, the reaction for Webster soil was nearly complete after 8 h, reflecting difference in soil properties. XANES analysis for Webster soil confirmed that most of Mo was bound to montmorillonite during long-term reaction time, whereas kaolinite constitutes a very important host phase for Mahan and Windsor soils. Sequential extraction results indicated that the percentages of Fe/Al oxide and residual fractions increased at the advanced time periods for Mahan and Windsor soils. The goodness-of-fit of numerical modeling results indicated that a simple version of multi-reaction model (MRM) with equilibrium and kinetic sorption sites was capable of describing Mo(VI) retention data for Webster loam. However, for Windsor and Mahan soils, an additional irreversible sorption site was required to simulate Mo(VI) retention over time. Although each site from MRM model cannot be unequivocally clarified from each other by either XANES analysis or sequential extraction results, their finding provided evidence of surface irreversible reactions at long residence times. Full article

The ability of fertilizer phosphates to desorb arsenates from soils is not yet adequately studied especially in cases of mining lands severely contaminated with arsenic (As). In this study, two soils with different physicochemical properties and heavily contaminated with As equilibrated with solutions containing various rates of phosphates either in the form of triple superphosphate fertilizer (TSP) or as NH₄H₂PO₄ using NaNO₃ as background electrolyte. A treatment with TSP in water was also applied to mimic agronomic practices. In general, increased P rates resulted in higher As release and to lower P sorption. Depending on the P rate, desorbed As ranged between 8 and 64.4 mg/kg for soil 1 and between 16.5 and 35.3 mg/kg for soil 2, corresponding to more than 50% of the potentially available As, as defined by the sum of the two first fractions of Wenzel sequential extraction scheme. Arsenic desorption patterns substantially differ between the two soils, mainly affected by active carbonates, organic matter and Fe and Al oxides contents. Though the differences between P treatments were not always significant, the presence of NaNO₃ increased the desorbing strength of the solutions. Phosphorus sorption capacity was high for both soils, but excess P addition led to high P concentrations in the equilibrium solutions, implying leaching hazard. Full article

Boreal agriculture struggles with soils of lower agronomic value, most of which are sandy with a low water holding capacity (WHC) and prone to nutrient leaching. Biochar amendments are associated with positive effects on soil hydraulic properties and enhanced nutrient retention. However, these effects are strongly related to feedstock type and pyrolysis parameters and depend on biochar application rates and soil types. While biochar could increase the productivity of boreal agriculture by improving water and nutrient use efficiency, little is known about its effects on hydraulic processes in podzol. In this study, we investigated the effects of biochar rates (10, 20, 40, 80 Mg carbon ha⁻¹) and maturity on soil hydrology for an agriculturally used Podzol in Labrador, Canada. The in-situ soil water content (SWC) and weather data over an entire growing season were analysed. Hydrus 1D simulations were used to estimate changes in water fluxes. SWC showed clear differentiation among storage parameters (i.e., initial, peak and final SWC) and kinetic parameters (i.e., rate of SWC change). Storage parameters and soil wetting and drying rates were significantly affected by biochar rates and its maturity. The magnitude of the changes in SWC after either wetting or drying events was statistically not affected by the biochar rate. This confirms that biochar mostly affected the WHC. Nevertheless, reductions in cumulative lower boundary fluxes were directly related to biochar incorporation rates. Overall, biochar had positive effects on hydrological properties. The biochar rate of 40 Mg C ha⁻¹ was the most beneficial to agriculturally relevant hydraulic conditions for the tested Podzol. Full article

Pheretimoid earthworms are invasive in hardwood forests of formerly glaciated regions in the USA. They alter the forest floor structure by creating an extensive, several cm-deep casting layer comprising loose macro-aggregates. Little is known about the physical properties of the casting layer and how they relate to earthworm ecology. Here, thermal and macropore properties of three forest soil textures (clay, silt, and sandy soils, with and without pheretimoids) were measured and compared to explore the possible relationships to their ecology. Thermal properties were significantly different between the casting layer (CAST) and original soil (NOCAST). Results indicate that CAST soils dampen temperature fluctuations occurring at the surface more than NOCAST soil. The increased dampening may be of particular importance to pheretimoid survival in forest fires and during spring when surface fluctuations could expose the hatchlings to fatal temperatures. Macropore volume, an indicator of ease of movement of pheretimoids, was significantly greater in CAST than NOCAST soil. Together, the ease of movement and greater temperature dampening of CAST soils may provide thermal refuges to pheretimoids from temperature variations outside the optimal range. This may improve their chances of survival in newly colonized areas where the climate differs from the original range. Full article

Soil erosion by water is one of the main soil degradation processes worldwide, which leads to declines in natural soil fertility and productivity especially on arable land. Despite advances in soil erosion modelling, the effects of compacted tramlines are usually not considered. However, tramlines noticeably contribute to the amount of soil eroded inside a field. To quantify these effects we incorporated high-resolution spatial tramline data into modelling. For simulation, the process-based soil erosion model EROSION3D has been applied on different fields for a single rainfall event. To find a reasonable balance between computing time and prediction quality, different grid cell sizes (5, 1, and 0.5 m) were used and modelling results were compared against measured soil loss. We found that (i) grid-based models like E3D are able to integrate tramlines, (ii) the share of measured erosion between tramline and cultivated areas fits well with measurements for resolution ≤1 m, (iii) tramline erosion showed a high dependency to the slope angle and (iv) soil loss and runoff are generated quicker within tramlines during the event. The results indicate that the integration of tramlines in soil erosion modelling improves the spatial prediction accuracy, and therefore, can be important for soil conservation planning. Full article

There are multiple mechanisms by which enhanced diversity of plant communities improves soil structure and function. One critical pathway mediating this relationship is through changes to soil prokaryotic communities. Here, nine different cropping systems were studied to evaluate how legume and grass cover crops influence soil fertility and microbial communities in a maize-based no tillage system. The soil’s bacterial and archaeal communities were sequenced (Illumina GAIIx, 12 replicates for treatment) and correlated with eight different soil features. The microbial community composition differed widely between planting treatments, with three primary “community types” emerging in multivariate space: (1) A community type associated with bare soil linked with low P, low pH, and high aluminum [Al]; (2) a community type associated with Lablab beans linked with high soil N, total organic carbon and other base cation concentrations, and high pH; and (3) a community type of all other non-lablab planting arrangements linked with higher soil P (relative to bare soil), but lower soil fertility (N and base cations). Lablab-based arrangements also expressed the highest microbial richness and alpha diversity. The inclusion of Lablab in maize-based cropping systems represents a potential alternative to reduce the use of chemical fertilizers and increase the chemical and biological quality in agricultural soils under the no-tillage system. Full article

Considering the increased interests in biochar (BC) as a soil amendment and a growing media substrate in agriculture, we evaluated the effect of BC incorporation on TDR (time-domain reflectometer)-based volumetric soil moisture content (VSMC) estimations in a loamy sand podzolic soil. Two commercial BC types (powdered—BC_P, and granular—BC_G) were mixed in different rates (w/w) with a podzolic soil. The dielectric constants measured using a TDR cable tester (MOHR CT 100) were converted to VSMC. Three commonly used models: (i) Topp’s equation, M-1; (ii) mixing model, M-2; and (iii) the forest soil model, M-3, were used. The accuracy of the estimated VSMC using these three models was statistically compared with measured VSMC. BC_P at lower rates produced very similar results to the actual VSMC with M-1 and M-2 but deviated with increasing rates. The M-3 showed a non-linear relationship with measured VSMC. In BC_G treatments, all models overestimated the VSMC. BC_G rates higher than 15% (w/w) resulted in highly attenuated TDR waveforms and the signal was completely dissipated when rates higher than 50% (w/w) were used (typical application for field soils is less than 5% w/w). These results showed that predictions of the soil moisture content based on the soil dielectric constant might not be feasible for tested podzolic soils amended at high BC rates. Full article

This study investigates the effects of pumice excavation on runoff formation and soil erosion processes in a forested catchment in SW-Germany. The underlying questions are, if (a) backfilled soils have different properties concerning runoff generation and erodibility and if (b) clear-cutting prior to excavation triggers runoff and erosion. Four adjacent sub-areas were observed, which represented different pre- and post-excavation-stages. The basis of the investigation was a comprehensive field sampling that delivered the data for physical erosion modeling using the Water Erosion Prediction Project (WEPP). Modeling took place for standardized conditions (uniform slope geometry and/or uniform land management) and for actual slope geometry and land management. The results show that backfilled soils exhibited 53% increase of annual runoff and 70% increase of annual soil loss under standardized conditions. Storm runoff was increased by 6%, while storm soil loss was reduced by 9%. Land management changes also triggered shifts in annual runoff and soil erosion: Clear-cut (+1.796% runoff, +4.205% soil loss) and bare (+5.958% runoff, +21.055% soil loss) surfaces showed the most distinct changes when compared to undisturbed forest. While reforestation largely diminished post-excavation runoff and soil erosion, the standardized results statistically prove that soil erodibility and runoff generation remain increased after backfilling. Full article

Elemental hyperaccumulation protects plants from many aboveground herbivores. Little is known about effects of hyperaccumulation on belowground herbivores or their ecological interactions. To examine effects of plant selenium (Se) hyperaccumulation on nematode root herbivory, we investigated spatial distribution and speciation of Se in hyperaccumulator roots using X-ray microprobe analysis, and effects of root Se concentration on root-associated nematode communities. Perennial hyperaccumulators Stanleya pinnata and Astragalus bisulcatus, collected from a natural seleniferous grassland contained 100–1500 mg Se kg⁻¹ root dry weight (DW). Selenium was concentrated in the cortex and epidermis of hyperaccumulator roots, with lower levels in the stele. The accumulated Se consisted of organic (C-Se-C) compounds, indistinguishable from methyl-selenocysteine. The field-collected roots yielded 5–400 nematodes g⁻¹ DW in Baermann funnel extraction, with no correlation between root Se concentration and nematode densities. Even roots containing > 1000 mg Se kg⁻¹ DW yielded herbivorous nematodes. However, greenhouse-grown S. pinnata plants treated with Se had fewer total nematodes than those without Se. Thus, while root Se hyperaccumulation may protect plants from non-specialist herbivorous nematodes, Se-resistant nematode taxa appear to associate with hyperaccumulators in seleniferous habitats, and may utilize high-Se hyperaccumulator roots as food source. These findings give new insight into the ecological implications of plant Se (hyper)accumulation. Full article

The nature of depth distribution of soil organic carbon (SOC) was examined in 85 soils across New South Wales with the working hypothesis that the depth distribution of SOC is controlled by processes that vary with depth in the profile. Mathematical functions were fitted to 85 profiles of SOC with SOC values at depth intervals typically of 0–5, 5–10, 10–20, 20–30, 30–40, 40–50, 50–60, 60–70, 70–80, 80–90 and 90–100 cm. The functions fitted included exponential functions of the form SOC = A exp (Bz); SOC = A + B exp (Cz) as well as two phase exponential functions of the form SOC = A + B exp (Cz) + D exp (Ez). Other functions fitted included functions where the depth was a power exponent or an inverse term in a function. The universally best-fitting function was the exponential function SOC = A + B exp (Cz). When fitted, the most successful function was the two-phase exponential, but in several cases this function could not be fitted because of the large number of terms in the function. Semi-log plots of log values of the SOC against soil depth were also fitted to detect changes in the mathematical relationships between SOC and soil depth. These were hypothesized to represent changes in dominant soil processes at various depths. The success of the exponential function with an added constant, the two-phase exponential functions, and the demonstration of different phases within the semi-log plots confirmed our hypothesis that different processes were operating at different depths to control the depth distributions of SOC, there being a surface component, and deeper soil component. Several SOC profiles demonstrated specific features that are potentially important for the management of SOC profiles in soils. Woodland and to lesser extent pasture soils had a definite near surface zone within the SOC profile, indicating the addition of surface materials and high rates of fine root turnover. This zone was much less evident under cropping. Full article

Soils are biodiversity-dense and constantly carry chemical flows of information, with our mental image of soil being dark and quiet. But what if soil biota tap sound, or more generally, vibrations as a source of information? Vibrations are produced by soil biota, and there is accumulating evidence that such vibrations, including sound, may also be perceived. We here argue for potential advantages of sound/vibration detection, which likely revolve around detection of potential danger, e.g., predators. Substantial methodological retooling will be necessary to capture this form of information, since sound-related equipment is not standard in soils labs, and in fact this topic is very much at the fringes of the classical soil research at present. Sound, if firmly established as a mode of information exchange in soil, could be useful in an ‘acoustics-based’ precision agriculture as a means of assessing aspects of soil biodiversity, and the topic of sound pollution could move into focus for soil biota and processes. Full article

The geometry of rooting systems is important for modeling water flows in the soil-plant-atmosphere continuum. Measured information about root density can be summarized in adjustable equations applied in hydrological models. We present such descriptive functions used to model root density distribution over depth and evaluate their quality of fit to measured crop root density profiles retrieved from the literature. An equation is presented to calculate the mean root half-distance as a function of depth from root length density profiles as used in single root models for water uptake. To assess the importance of the shape of the root length density profile in hydrological modeling, the sensitivity of actual transpiration predictions of a hydrological model to the shape of root length density profiles is analyzed using 38 years of meteorological data from Southeast Brazil. The cumulative root density distributions covering the most important agricultural crops (in terms of area) were found to be well described by the logistic function or the Gompertz function. Root length density distribution has a consistent effect on relative transpiration, hence on relative yield, but the common approach to predict transpiration reduction and irrigation requirement from soil water storage or average water content is shown to be only partially supported by simulation results. Full article

A recent conversion of the grasslands to cropland degrading the soil quality (SQ), and impacting the soil erosion and crop productivity in the West Corn Belt (WCB) of the USA. The current study was conducted to estimate the spatial distribution of soil erosion at Big Sioux River (BSR) watershed scale using the Geographical Information System (GIS)-enabled Revised Universal Soil Loss Equation (RUSLE). Spatial data such as weather, a digital elevation model (DEM), land use maps and soils were used for assessment of soil erosion was downloaded from the easily available online sources. Data showed that about 7% of grassland acreage reduced from 2008 (24%) to 2015 (17%), whereas, about 7.4% of cropland acreage increased from 2008 (64.6%) to 2015 (72%) in the BSR watershed. This grassland conversion to cropland increased the soil erosion (estimated using the RUSLE model) from 12.2 T ha⁻¹ year⁻¹ in 2008 to 14.8 T ha⁻¹ year⁻¹ in 2015. The present study concludes that grassland conversion to cropland in the BSR watershed increased the soil erosion, therefore, management practices essential to be applied to reduce the erosion risk and various other ecosystem services. Full article