Impact Of Soil Heterogeneity Research Articles

Effect of anisotropy on solute transport in degraded fen peat soils

AbstractPeat soils are heterogeneous, anisotropic porous media. Compared to mineral soils, there is still limited understanding of physical and solute transport properties of fen peat soils. In this study, we aimed to explore the effect of soil anisotropy on solute transport in degraded fen peat. Undisturbed soil cores, taken in vertical and horizontal direction, were collected from one drained and one restored fen peatland both in a comparable state of soil degradation. Saturated hydraulic conductivity (K s) and chemical properties of peat were determined for all soil cores. Miscible displacement experiments were conducted under saturated steady state conditions using potassium bromide as a conservative tracer. The results showed that (1) the K s in vertical direction (K sv) was significantly higher than that in horizontal direction (Ksh), indicating that K s of degraded fen peat behaves anisotropically; (2) pronounced preferential flow occurred in vertical direction with a higher immobile water fraction and a higher pore water velocity; (3) the 5% arrival time (a proxy for the strength of preferential flow) was affected by soil anisotropy as well as study site. A strong correlation was found between 5% arrival time and dispersivity, K s and mobile water fraction; (4) phosphate release was observed from drained peat only. The impact of soil heterogeneity on phosphate leaching was more pronounced than soil anisotropy. The soil core with the strongest preferential flow released the highest amount of phosphate. We conclude that soil anisotropy is crucial in peatland hydrology but additional research is required to fully understand anisotropy effects on solute transport.

Spatial variability of selected soil properties and its impact on the grain yield of oats (Avena sativa L.) in small fields

To investigate spatial variability in topsoil (0–20 cm) pH, available phosphorus (P), potassium (K), total nitrogen (N), and soil organic matter (SOM) of small fields (∼2 ha), and to determine the impact of soil heterogeneity on the spatial variability of crop yield two fields were cropped with spring oats and one with winter wheat under humid-temperate conditions. In the two oat fields, some of the measured soil properties (P, K) and the grain yield varied considerably, and strong spatial trends were recorded for most of the soil traits. In the third field, soil properties showed only a moderate spatial variation, and no spatial trends were found. The spatial distribution of SOM and total N in the topsoil had some influence on the spatial pattern of the oat grain yield in the field of Gränichen; however, spatial relationships between soil chemical properties and grain yield were rather weak in our study.

Impact of Soil Heterogeneity on Forest Structure and Diversity of Tree Species in the Central Congo Basin

Premise of research. The role of soil in shaping plant communities in forest ecosystems has long attracted scientific interest, and many studies have pointed out the importance of edaphic properties on species distributions within the tropics. There is still a need to investigate whether edaphic differentiation affects forest composition and structure and to assess which soil properties can be good predictors of potential differences in species diversity.Methodology. We established 24 1-ha plots in nondisturbed semideciduous forests in the Central Congo Basin, following clear-cut edaphic heterogeneity between sandy soils and clayey substrates. All individual trees with ≥10-cm diameter at breast height were inventoried in order to account for the diametric structures of trees and patterns of their diversity.Pivotal results. Sandy and clayey plots differed significantly in their soil physical and chemical properties. Soil clearly segregates tree species into sand community and clay community, with species abundances differently correlated with soil properties, driving clear cases of edaphic associations in tree species. This trend was confirmed when species data were constrained by edaphic variables through a redundancy analysis, for which statistical tests confirmed the significance of the canonical model and that of the canonical axes. The diametric structure of trees remains the same on clayey and sandy soils, as do their diversity patterns.Conclusions. Edaphic differentiation did not affect the diameter of trees. It also did not have an impact on tree abundances, species richness, and the diversity of trees altogether. Soil pH is an important variable to consider to understand tree diversity patterns in these semideciduous forests, and maybe its effects can be much more evident on the other forest communities (i.e., shrub and herbaceous layers).

Impact of Soil Heterogeneity and NAPL Presence on Stable Carbon Isotope Signature Distribution During Reactive Transport

Multiphase flow and transport simulations were conducted to investigate the impact of soil heterogeneity and non-aqueous phase liquid (NAPL) presence on the distribution of stable carbon isotope signatures during contaminant transport with biodegradation. At a later time during the simulation of a homogeneous case with dense NAPL presence, significant carbon isotope signature (δ13C) values could only be observed in a narrow area at the bottom of the aquifer where NAPL accumulated. After this, the δ13C distribution remained relatively stable for a long time until all NAPL was dissolved into the groundwater and removed via biodegradation and groundwater flushing. These characteristics of δ13C distribution may only be captured when considering NAPL migration and dissolution. The simulation results demonstrated that δ13C values and their distribution significantly differed between the heterogeneous case and the homogeneous case, with respect to the maximum δ13C value and the shape of δ13C contours. When reaction rate constant varied for each soil type (each grid block) by relating it to soil permeability, the δ13C distribution demonstrated different patterns. In addition to geological heterogeneity, this indicates that the distribution of δ13C highly depends on the biological heterogeneity in the field. Therefore, this study suggests that, to avoid misinterpretation of isotope signature changes, geological and biological soil heterogeneities should be investigated. If a NAPL is present in the system, the NAPL phase transport and dissolution should be considered in addition to dissolved phase transport.

The Impact of Soil Heterogeneity on Nitrate Dynamic and Losses in Tile-Drained Arable Fields

Intensive agriculture and the related application of fertilizers are one of the main sources for diffuse nitrate (NO3–N) inputs into surface- and groundwater. It is assumed that especially subsurface tile drainage is an essential source. To examine the impact of heterogeneous soil properties on NO3–N leaching via drains, field experiments in the northern Altmark region in Saxony-Anhalt (Germany) were realized on two neighboring, tile-drained arable fields with adjusted agronomic conditions but different soil physical properties from 1 November 2013 until 31 October 2015. Field campaigns, the implemented measuring technology, and numerical simulations provided the detailed characterization of the subsurface, the flow regime, and the NO3–N dynamic. The results revealed that field I was groundwater influenced and dominated by ponding and surface runoff, whereas at field II, the whole amount of seepage water percolated freely to the drains. Thus, the amount of drained water and discharged NO3–N loads differed significantly between both sites although their comparable size (field I/field II, 17.1 mm and 2.5 kg N ha−1/150.2 mm and 40.4 kg N ha−1). Furthermore, NO3–N accumulated in the top soil of field I; whereas, this site showed a huge denitrification potential due to long hydraulic residence times. These aspects could not be reproduced at field II. A site-specific N management is absolutely required to reduce further N losses at field II as a requirement to fulfill the objectives of the European water protection policy.

Soil heterogeneity affects ramet placement of Hydrocotyle vulgaris

Aims Soil heterogeneity is common in natural habitats. It may trigger foraging responses (placing more ramets and/or roots in nutrient-rich patches than in nutrient-poor patches) and further affect the growth of plants. However, the impact of soil heterogeneity on competitive interactions has been little tested. Methods We conducted a greenhouse experiment to investigate the effects of soil heterogeneity on intraspecific competition with a stoloniferous herb Hydrocotyle vulgaris. We grew one (without competition) or nine ramets (with competition) of H. vulgaris under a homogeneous environment and two heterogeneous environments differing in patch size (large or small patches). In the heterogeneous treatment, the soil consisted of the same number of nutrient-rich and nutrient-poor patches arranged in a chessboard manner, and in the homogeneous treatment, the soil was an even mixture of the same amount of the nutrient-rich and the nutrientpoor soil. Important Findings Irrespective of intraspecific competition, H. vulgaris showed foraging responses to soil heterogeneity in the large patch treatment, e.g. it produced significantly more biomass, ramets, aboveground mass and root mass in the nutrient-rich patches than in the nutrientpoor patches. In the small patch treatment, foraging responses were observed when intraspecific competition was present, but responses were not observed when there was no competition. However, we find a significant effect of soil heterogeneity on neither overall growth nor competitive intensity of H. vulgaris. Our results suggest that foraging responses to soil heterogeneity may not necessarily be adaptive and intraspecific competition may not be influenced by soil heterogeneity.

Impact of soil heterogeneity on the functioning of horizontal ground heat exchangers

The impact of heterogeneity in soil thermal properties on the performance of horizontal ground heat exchangers (HGHE) is closely examined using a custom-made finite element model. Ensembles of heterogeneous soil fields were generated with spatial correlation parameters derived from similar studies of heterogeneity in hydraulic parameters. Within these soil fields, a single loop HGHE was modelled as discrete pipes placed in a heterogeneous soil continuum. The effect of heterogeneity was found to be minimal relative to uncertainty of the mean soil thermal conductivity, supporting the continued use of the assumption of homogeneity when modelling and designing these systems. Multiple design techniques were identified which would allow HGHE designers to further mitigate any deleterious effects, such as preferential heat transfer between adjacent pipes which may act as a ‘short-circuit’ mechanism, reducing HGHE effectiveness.

Comparing different approaches to calculate the effects of heterogeneous root distribution on nutrient uptake: a case study on subsoil nitrate uptake by a barley root system

Simulation models of nutrient uptake of root systems starting with one-dimensional single root approaches up to complex three-dimensional models are increasingly used for examining the interacting of root distribution and nutrient uptake. However, their accuracy was seldom systematically tested. The objective of the study is to compare one-dimensional and two-dimensional modelling approaches and to test their applicability for simulation of nutrient uptake of heterogeneously distributed root systems giving particular attention to the impact of spatial resolution. Therefore, a field experiment was carried out with spring barley (Hordeum vulgare L. cv. Barke) in order to obtain data of in situ root distribution patterns as model input. Results indicate that a comparable coarse spatial resolution can be used with sufficient modelling results when a steady state approximation is applied to the sink cells of the two-dimensional model. Furthermore, the accuracy of the model was clearly improved compared to a simple zero sink approach assuming both near zero concentrations within the sink cell and a linear gradient between the sink cell and its adjacent neighbours. However, for modelling nitrate uptake of a heterogeneous root system a minimum number of grid cells is still necessary. The tested single root approach provided a computational efficient opportunity to simulate nitrate uptake of an irregular distributed root system. Nevertheless, two-dimensional models are better suited for a number of applications (e.g. surveys made on the impact of soil heterogeneity on plant nutrient uptake). Different settings for the suggested modelling techniques are discussed.

Soil water balance scenario studies using predicted soil hydraulic parameters

Pedotransfer functions (PTFs) have become a topic drawing increasing interest within the field of soil and environmental research because they can provide important soil physical data at relatively low cost. Few studies, however, explore which contributions PTFs can make to land-use planning, in terms of examining the expected outcome of certain changes in soil and water management practices. This paper describes three scenario studies that show some aspects of how PTFs may help improve decision making about land management practices. We use an exploratory research approach using simulation modelling to explore the potential effect of alternative solutions in land management. We: (i) evaluate benefits and risks when irrigating a field, and the impact of soil heterogeneity; (ii) examine which changes can be expected (in terms of soil water balance and supply) if organic matter content is changed as a result of an alternative management system; (iii) evaluate the risk of leaching to deeper horizons in some soils of Hungary. Using this research approach, quantitative answers are provided to 'what if?' type questions, allowing the distinction of trends and potential problems, which may contribute to the development of sustainable management systems. Copyright (c) 2005 John Wiley & Sons, Ltd.

Impact of soil heterogeneity in a mixed-layer model of the planetary boundary layer

The dynamics of soil moisture and temperature states are forced by land surface fluxes that form the coupling between the soil and the atmosphere. This two-way interaction between the soil and the atmosphere results in feedback mechanisms that affect the temporal patterns of variability of the system. To understand these characteristics of soil moisture and temperature variability, a one-dimensional coupled mixed-layer model of the land surface and the planetary boundary layer energy and humidity budgets is constructed. To allow integrations on the order of days, the model includes both the growth phase and the collapse phase of mixed layer evolution. The model includes the radiative effects due to the presence of clouds. Given the incoming shortwave radiation, lapse rates of temperature and humidity above the mixed layer and lateral windspeed, the model predicts all longwave radiative and turbulent surface heat fluxes. The model is shown to capture accurately observations taken during the FIFE field experiment. With this model, the impact of soil heterogeneity on the evolution of the surface and mixed-layer energy and humidity budgets is examined, using hydraulic properties from sand, loam and clay soils. It is assumed that the small correlation length scale of soil hydraulic properties and surface inhomogeneities are averaged out in the mixed layer. Relative to a uniform soil, heterogeneity increases the spatial mean latent heat flux for conductive soils and decreases it for unconductive soils, due to decreased and increased percolation respectively. Locally decreasing latent heat fluxes cause a warmer and drier mixed layer and through that an increase of the latent heat flux over other areas. This mixed-layer feedback reduces the impact of heterogeneous surface fluxes.

Impact of soil heterogeneity on the water budget of the unsaturated zone

The impact of soil heterogeneity on the spatially average water budget of the unsaturated zone is investigated using a framework of analytical solutions. This framework was previously shown to adequately represent the long‐term average water budget as well as its transient evolution. Nine combinations of three climate and three soil types are considered, covering a wide range of different conditions. In highly conductive soils, where evapotranspiration is limited by percolation through the lower boundary, heterogeneity increases the spatially average evapotranspiration relative to the uniform (scaled average) soil. In contrast, for less conductive soils the decreasing infiltration rates due to soil heterogeneity cause evapotranspiration to become smaller. Formally derived “equivalent” parameters, which are intended to capture the effect of soil heterogeneity by means of one set of soil hydraulic parameters, are shown to depend not only on the spatially heterogeneous parameters, but on climate as well. Moreover, the “equivalent” parameters derived for the long‐term average water budgets are not valid for transient behavior.