Plastics in soil: Analytical methods and possible sources

The HYDRUS-1D and HYDRUS (2D/3D) computer software packages are widely used finite-element models for simulating the one- and two- or three-dimensional movement of water, heat, and multiple solutes in variably saturated media, respectively. In 2008, Simůnek et al. (2008b) described the entire history of the development of the various HYDRUS programs and related models and tools such as STANMOD, RETC, ROSETTA, UNSODA, UNSATCHEM, HP1, and others. The objective of this manuscript is to review selected capabilities of HYDRUS that have been implemented since 2008. Our review is not limited to listing additional processes that were implemented in the standard computational modules, but also describes many new standard and nonstandard specialized add-on modules that significantly expanded the capabilities of the two software packages. We also review additional capabilities that have been incorporated into the graphical user interface (GUI) that supports the use of HYDRUS (2D/3D). Another objective of this manuscript is to review selected applications of the HYDRUS models such as evaluation of various irrigation schemes, evaluation of the effects of plant water uptake on groundwater recharge, assessing the transport of particle-like substances in the subsurface, and using the models in conjunction with various geophysical methods.

/pdf/recent-developments-and-applications-of-the-hydrus-computer-3xmm6e1twr.pdf

Recent Developments and Applications of the HYDRUS Computer Software Packages

The present review covers developments in studies of nanomaterials (NMs) in the environment since our much cited review in 2008. We discuss novel insights into fate and behavior, metrology, transformations, bioavailability, toxicity mechanisms, and environmental impacts, with a focus on terrestrial and aquatic systems. Overall, the findings were that: 1) despite substantial developments, critical gaps remain, in large part due to the lack of analytical, modeling, and field capabilities, and also due to the breadth and complexity of the area; 2) a key knowledge gap is the lack of data on environmental concentrations and dosimetry generally; 3) substantial evidence shows that there are nanospecific effects (different from the effects of both ions and larger particles) on the environment in terms of fate, bioavailability, and toxicity, but this is not consistent for all NMs, species, and relevant processes; 4) a paradigm is emerging that NMs are less toxic than equivalent dissolved materials but more toxic than the corresponding bulk materials; and 5) translation of incompletely understood science into regulation and policy continues to be challenging. There is a developing consensus that NMs may pose a relatively low environmental risk, but because of uncertainty and lack of data in many areas, definitive conclusions cannot be drawn. In addition, this emerging consensus will likely change rapidly with qualitative changes in the technology and increased future discharges. Environ Toxicol Chem 2018;37:2029-2063. © 2018 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc. on behalf of SETAC.

/pdf/nanomaterials-in-the-environment-behavior-fate-1cyiwpi1vp.pdf

Nanomaterials in the environment: behavior, fate, bioavailability, and effects—an updated review

Separation and identification of microplastics from soil and sewage sludge

Transport of polystyrene nanoplastics in natural soils: Effect of soil properties, ionic strength and cation type.

Mobilization of soil colloids has attracted much research attention because of colloids’ potential to facilitate transport of strongly sorbing constituents (e.g., contaminants, pesticides, and nutrients). In redox-dynamic soils or sediments, colloid mobility is largely controlled by Fe3+ dissolution, shifts in pH and ionic strength, and interactions with organic matter (OM). We investigated the complex interplay of colloid release, OM content, and redox conditions via batch experiments to quantify and characterize soil colloid release after the addition of dissolved OM (DOM). Under aerobic conditions, the concentration of released colloids increased from ∼1000 to ∼3000 mg L−1 as the DOM concentration increased from 0 to 20 mg C L−1, respectively. The initial concentrations of released colloids were higher under anaerobic conditions, but they decreased by ∼80 to 90% as incubation continued. In addition, unlike in the aerobic samples, the effects of the added DOM on colloid release in the anaerobic samples were minimal. Iron dissolution under anaerobic conditions coincided with the initial high level of colloid release as well as the release of indigenous DOM (DOMin), which may have led to subsequent colloid aggregation due to bridging. These results demonstrate that (i) DOM stabilized the colloids released under aerobic conditions but had minimal effects on colloids released under anaerobic conditions and (ii) high concentrations of DOMin released under anaerobic conditions decreased subsequent colloid release due to DOMin–colloid association. The findings have implications for colloid release and colloid-associated nutrient and contaminant mobilization and transport in redox-dynamic environments such as wetlands.

Soil Colloid Release Affected by Dissolved Organic Matter and Redox Conditions

Erosion is a fundamental phenomenon that governs the biogeochemical cycle of carbon (C) in the Earth system. Understanding the fate of eroded C at the watershed scale thus provides essential guidance for balancing the C budget and evaluating soil management strategies. In this study, we collected a total of 229 topsoils of source materials [land use types (forests, grassland, cropland, orchard, and fallow) and gully bank] and a total of 56 surface sediment samples [main river channel (18) and three tributaries (38) with different erosion intensity]. Multiple tracers (C and nitrogen (N) isotopes, molecular n-alkane biomarker, and spectroscopic indices) applied widely were then analyzed to determine relative source contributions to eroded organic matter (OM) (including bulk OM and water-extractable OM) in sediments at a broader watershed on the Loess Plateau of China. The results from an isotopic mixing model analysis showed that gully bank was the primary contributor of bulk OM in sediments, accounting for mean probability estimate (MPE) 55.3%, followed by “grassland” (MPE 21.0%), “cropland” (MPE 14.4%), and “forests” (MPE 9.3%). The contributions of grassland and cropland showed an increasing trend from upstream to downstream sections of the studied watershed. The contribution of forests was found higher in upstream than those of midstream and downstream. Along with previous evidences, our results show that erosion intensity mainly affects the proportion of C source, where gully bank was found as the highest contributor in three associated catchments, especially in region with light erosion intensity. We further demonstrated that the sediment bulk OM deposited was mainly derived from the region with light erosion intensity at the broader watershed scale. Moreover, results of n-alkane ratios and spectroscopic indices also implied that biological production of OM other than terrestrial sources (i.e., soil OM) might play a critical role in source contribution of water-extractable OM in sediments of main river channel.

Identifying eroded organic matter sources in sediments at fluvial system using multiple tracers on the Loess Plateau of China

. Most plants derive their water and nutrient needs from soils where the resources are often scarce, patchy, and ephemeral. It is not uncommon for plant roots to encounter mismatched patches of water-rich and nutrient-rich regions in natural environments. Such an uneven distribution of resources necessitates plant reliance on strategies for exploring and acquiring nutrients from relatively dry patches. We conducted a laboratory study that elucidates the biophysical mechanisms that enable this adaptation. The roots of tomato (Solanum lycopersicum) seedlings were laterally split and grown in two adjacent, hydraulically disconnected pots, which permitted precise control of water and nutrient applications to each compartment. We observed that the physical separation of water-rich and nutrient-rich compartments (one received 90 % water and 0 % nutrients and the other received 10 % water and 100 % nutrients) does not significantly stunt plant growth and productivity compared to two control treatments (control 1: 90 % water and 100 % nutrients versus 10 % water and 0 % nutrients; control 2: 50 % water and 50 % nutrients in each compartment). Specifically, we showed that soil dryness does not reduce nutrient uptake, vegetative growth, flowering, and fruiting compared to control treatments. We identified localized root proliferation in nutrient-rich dry soil patches as a critical strategy that enabled nutrient capture. We observed nocturnal rewetting of the nutrient-rich but dry soil zone (10 % water and 100 % nutrients) but not in the nutrient-free and dry zone of the control experiment (90 % water and 100 % nutrients). We interpreted the rewetting as the transfer of water from the wet to dry zones through roots, a process commonly known as hydraulic redistribution (HR). The occurrence of HR likely prevents the nutrient-rich soil from drying due to permanent wilting and the subsequent decline of root functions. Sustaining rhizosphere wetness is also likely to increase nutrient mobility and uptake. Lack of HR in the absence of nutrients suggests that HR is not entirely a passive, water-potential-gradient driven flow. The density and size of root hairs appeared to be higher (qualitative observation) in the nutrient-rich and dry compartments than in the nutrient-free and dry compartments. We also observed organic coating on sand grains in the rhizosphere of the nutrient-rich and dry compartments. The observations are consistent with prior observations that root hairs and rhizodeposition aid rhizosphere wetting. These findings were synthesized in a conceptual model that explains how plants of dry regions may be adapted to mismatched resources. This study also suggests that separating the bulk of applied nutrients from the frequently irrigated soil region can increase nutrient use efficiency and curtail water pollution from intensive agricultural systems.

/pdf/root-uptake-under-mismatched-distributions-of-water-and-1yha2c8sva.pdf

Root uptake under mismatched distributions of water and nutrients in the root zone

Mobile soil colloids play an important role in affecting soil hydraulic properties and soil formation, as well as in facilitating the transport of strongly sorbing constituents (e.g., contaminants, pesticides, nutrients) in subsurface environments. Accurate quantification of mobile colloids is thus essential for understanding and prediction of soil element migration, changes in soil hydraulic properties, and colloid-associated contaminants in natural and managed systems. However, quantification of colloids in field samples, especially the

Size-Dependent Turbidimetric Quantification of Suspended Soil Colloids

Quantification and characterization of mobile colloids: their potential role in carbon cycling under varying redox conditions

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