Hillslope Positions Research Articles

Application of fallout radionuclide—137Cs for estimating soil erosion in steep hillslopes with diverse land use of North-western Indian Himalayas

Hilly and mountainous regions are significantly impacted by soil erosion, primarily due to rainfall-runoff processes occurring on the hillslope scale. Assessing soil erosion is crucial for quantifying the loss of soil carbon and nutrients, which diminishes the potential of soil ecosystem services and is critical for mitigating the impacts of climate change on food security. However, the Himalayan landscape poses serious challenges for assessing soil erosion due to its steep and rugged terrain, which hinders the use of conventional and modelling methods. The fallout radionuclide—137Cs has been extensively utilized as an environmental marker for investigating soil redistribution processes. Despite its potential, there is a notable lack of 137Cs-based soil erosion studies in the Himalayan region. In this context, we assessed the applicability of the fallout radionuclide—137Cs method in quantifying soil erosion rates and identifying erosion hotspots on two hillslopes of the Higher Himalayas. On the hillslope scale, we observed that soil erosion rates vary based on slope gradient and land use/land cover. Forested areas exhibited the lowest soil erosion rates compared to cultivated areas, while flat hillslope positions experienced lower erosion rates than steeper positions. The average net erosion rate for the Harsil hillslope varied among different hillslope positions, ranging from – 2.9 to – 15.6 t ha−1 yr−1. Similarly, in the Gangnani hillslope, the net erosion rates varied across different positions, ranging from – 5.6 to – 39 t ha−1 yr−1. Our findings confirm that the middle and lower hillslope positions are the most critical source areas with higher soil erosion rates, while hillslope positions with forest cover demonstrate the lowest erosion rates, identified as helpful in controlling soil erosion. The study has demonstrated the applicability of FRN as a soil erosion measurement method in the complex, rugged, and steep terrain of the Himalayas, highlighting the need for targeted conservation efforts to mitigate soil erosion and preserve ecosystem integrity.

Soil Carbon and Nitrogen Stocks Across Hillslopes Underlain by Continuous Permafrost in the Northern Arctic Foothills, Alaska, United States

ABSTRACTConstraining the variability of soil organic carbon (SOC) and total nitrogen (TN) stocks across hillslopes in Low Arctic permafrost‐affected landscapes remains a significant challenge for improving global estimates of permafrost SOC stocks. We investigated SOC and TN stocks across hillslopes at two sites in the Arctic Foothills of Alaska, United States (Happy Valley and Sagwon Hills). Average SOC and TN stocks for the 0–1‐m depth interval were high (52.0 ± 15.1 kg C m−2 and 2.74 ± 0.82 kg N m−2) and linearly related (R2 = 0.74, p < 0.0001). Unlike soils of other permafrost and nonpermafrost landscapes, variability was greatest within rather than between hillslope positions. Furthermore, SOC and TN stocks in the surface 1 m did not exhibit strong patterns by hillslope position and were only weakly associated with major geomorphic parameters that typically predict SOC and TN stocks well in other landscapes. Although sampling at upper hillslope positions was largely limited to depths of less than 1.5 m due to the presence of coarse fragments in reworked glacial till, deeper observations at lower hillslope positions (footslopes, toeslopes, and basins) revealed significantly larger SOC stocks (92.0 ± 18.0 kg C m−2 at 2 m; 117.1 ± 10.4 kg C m−2 at 3 m). The unique small‐scale variability in ice content, cryoturbation, patterned ground, and organic layer thickness on these broad, Low Arctic sites contributes to the relatively homogeneous distribution of SOC and TN stocks across hillslope positions in the top 1 m, but a future focus on deeper sampling may reveal greater differences in SOC and TN stocks.

Soil redistribution rates along the forested and cultivated steep hillslope in the mid‐Himalayas using fallout—137Cs and RUSLE model

AbstractSoil erosion emerged as a significant land degradation concern, causing serious threat to soil ecosystem services in the Himalayan region. The complex topography of the region poses limitations to the measurement of soil redistribution (erosion, transport, and deposition) rate, necessitated for the effective soil conservation planning. The study investigated soil redistribution processes over a typical complex hillslope of the mid‐Himalayan region using the fallout radionuclide (FRN)—137Cs method and Revised Universal Soil Loss Equation (RUSLE) model. It involved a comparison of 137Cs measured soil redistribution and the RUSLE model estimates, aiming to assess its correspondence over the complex hillslope. Analysis of 137Cs measurements revealed the highest net erosion (−13.2 t ha−1 year−1) at the upper hillslope with a convex shape, while sediment deposition occurred at the lower (36.9 t ha−1 year−1) and valley (32.5 t ha−1 year−1) hillslope positions with a concave shape. The RUSLE model also estimated the highest erosion on the upper hillslope (−12.3 t ha−1 year−1) but the lowest erosion at the lower (−0.88 t ha−1 year−1) and valley (−0.32 t ha−1 year−1) hillslopes, that differed with the 137Cs method. The 137Cs method provided soil redistribution rate (either net erosion or deposition), whereas RUSLE model only showed the gross erosion rate. Thus, the estimate from RUSLE corresponds only to hillslope positions with a convex and straight shapes. The distribution of 137Cs measurements has clearly revealed the influence of slope shape and steepness in governing erosion and deposition processes at the hillslope positions with convex and concave slope shapes, respectively. In addition, terraces effectively trap sediments from upslope areas. Investigation of soil erosion using 137Cs measurement along with the RUSLE model helped to validate erosion and deposition processes over the hillslope positions. The study will help to suggest suitable conservation measures for the various hillslope positions in the mid‐Himalayan region.

Nature versus nurture: Quantifying the effects of management, region, and hillslope position on soil health indicators in an on‐farm survey in Minnesota

AbstractA major challenge to implementing effective soil health assessments is how to distinguish the effects of management from underlying soil variability driven by inherent soil properties. This challenge has important consequences for the use of soil health indicators as tools for monitoring and assessment because soil‐forming factors constrain the range of indicator values and the magnitude of management‐induced changes. Here, we present results from a statewide survey of 15 soil health indicators measured on 30 fields on commercial farms across four major land resource areas in Minnesota. Fields included in the study differed in tillage, cover crop implementation, and crop rotation. Within each field, we collected samples from upper and lower hillslope positions to quantify the effect of topography. We consistently detected differences in soil health indicators between regions (13/15 indicators) and hillslope positions (8/15 indicators). However, only wet aggregate stability was sensitive to management across regions and years, highlighting the importance of physical indicators of soil health. This result was surprising in light of existing literature supporting the sensitivity of many soil health indicators to management, but it is consistent with other studies conducted in high organic matter soils in the Upper US Midwest. Our results highlight the need for regionally representative datasets to guide the development of interpretations and benchmarks for soil health indicators. This is particularly important when soil health indicators are applied outside traditional research contexts (such as in commercial soil health testing), where interpretation must take place without the benefit of historical baseline data.

137Cs radiotracer in investigating influence of hillslope positions and land use on soil erosion and soil organic carbon stock—A case study in the Himalayan region

AbstractThe topography and land use/land cover (LULC) of the hillslope play a significant influence on soil erosion because of water, which is considered as a principal factor for the reduction of soil organic carbon content. Reliable information on the impact of erosion mechanism on soil organic carbon stock (SOCS) is essential for effectively accounting for the carbon flux that influences climate change. The main objectives of this study were to determine soil erosion based on the variation of 137Cs (Radiocesium) radionuclide activity at various hillslope positions and LULC in a hilly and mountainous region of the north‐western Himalayas. Additionally, the relationship between 137Cs concentration, soil erosion rate and SOCS were examined. Fallout radionuclide‐137Cs have emerged as a suitable method for assessing soil erosion in hilly and mountainous regions where rugged topography and extreme weather events restrain the conventional soil erosion assessment. The study revealed very high soil erosion rates of 32.89 and 30.70 t ha−1 year−1 in the lower hillslope positions with cultivated fields. The lowest soil erosion was obtained with a mean of 0.47 t ha−1 year−1 from the ridge with grassland, followed by the upper hillslope (5.50 t ha−1 year−1 under deodar forest and 14.07 t ha−1 year−1 under pine forest), and the middle hillslope (1.58 t ha−1 year−1 for deodar and 7.77 t ha−1 year−1 for pine forest). The soil erosion rates differ significantly between cultivated and forested regions, and there is also a significant difference between deodar and pine forests. Moreover, a significant difference was found between topographic positions concerning 137Cs, SOCS and soil redistribution rate. This difference was more pronounced at hillslope positions with different LULC. In both disturbed (cultivated) (r2 = .111) and undisturbed (forested and grassland) (r2 = .356) soils, positive and statistically significant (p < .005) poor relationships were found between SOCS and 137Cs inventory. This indicates the presence of various factors influencing the soil organic carbon stock (SOCS) mechanism or the indirect contribution of soil erosion‐induced carbon loss. This suggests that forest cover can enhance SOCS in the soil, mitigating the adverse effects of soil erosion and climate change. Consequently, 137Cs could be effectively used to quantify the SOC stock in soil redistribution over the hillslope affected by soil erosion. Statistical analyses indicated that the 137Cs inventory, SOCS and erosion were significantly affected by various hillslope positions and LULC types.

The influence of hillslope topography on beech water use: a comparative study in two different climates

Abstract. Understanding the interrelation between topography and vegetation across different environments is important to assess how hydrological and climatic conditions affect tree physiological activity. This becomes especially important given the expected reduction in water availability and the increase in water demand driven by climate change. These extremes could enhance the thermal and hydrologic gradients along slopes. Here, we aimed to test if and how different climatic and hydrological conditions affect the physiological response of beech trees (Fagus sylvatica L.) to environmental variables along two different topographic sequences. For this purpose, we set up a comparative study on a gentle hillslope in the Weierbach catchment in Luxembourg (oceanic climate) and on a steep hillslope in the Lecciona catchment in Italy (Mediterranean climate). We combined sap velocity measurements with isotopic measurements of soil, precipitation, stream water, groundwater, and xylem over 2019 and 2020 for the Luxembourgish site and over 2021 for the Italian site. We found that, in the Weierbach catchment, trees' responses to environmental variables (i.e. vapour pressure deficit and relative extractable water in the soil) were similar among hillslope positions and between the two monitored years, resulting from homogeneous growing conditions along the topographic sequence. We also did not find any statistical difference in the isotopic composition of xylem water between positions, suggesting that beech trees relied on similar water sources across the landscape. In the Lecciona catchment, we observed lower sap velocities and shorter growing season in trees growing in the upper portions of the hillslope, likely related to water redistribution and different soil moisture along the hillslope catena. Xylem isotopic composition was significantly lighter at the footslope location throughout the growing season than in the upslope locations, suggesting location-specific water use. These results emphasize how differing hydrometeorological processes occurring at the hillslope scale can lead to contrasting tree responses.

Responses of soil moisture at different topographic positions to rainfall events along a steep subtropical forested hillslope

AbstractUnderstanding the dynamic response of soil moisture to rainfall is crucial for describing hydrological processes at the hillslope scale. However, because of sparse monitoring coupled with the complexity of water movement and steep topography, the findings of rainfall‐related soil moisture dynamics have not always been consistent, indicating a need for systematic investigations of soil moisture dynamics and infiltration patterns following rainfall inputs at multiple topographic positions along a hillslope. This study aimed to examine the nature of these responses by characterizing and quantifying the response amplitude, rate and time for 37 large rainfall events at 25 combinations of topographic positions and soil depths along a steep forested hillslope. Our results showed that soil moisture responses under different rainfall patterns could be attributed to one or the other rainfall characteristics, such as rainfall intensity and amount. However, soil moisture dynamics at different hillslope positions after rainfall varied widely due to the controls of soil properties, topography, and non‐equilibrium flow. Preferential flow was more evident under dry initial soil conditions than under wet initial soil conditions. Findings of this study reveal that the dynamic response patterns of soil moisture to rainfall do not always follow topographic controls, which can improve our understanding of water cycling related to the infiltration process at the hillslope scale, and support water resources management in subtropical mountain ecosystems.

Impact of Hillslope Agriculture on Soil Compaction and Seasonal Water Dynamics in a Temperate Vineyard

Major losses of agricultural production and soils are caused by erosion, which is especially pronounced on hillslopes due to specific hydrological processes and heterogeneity. Therefore, the aim of this study was to assess the impact of agricultural management on the compaction, infiltration, and seasonal water content dynamics of the hillslope. Measurements were made at the hilltop and footslope, i.e., soil water content and potential were measured using sensors, wick lysimeters were used to quantify water flux, while a mini-disk infiltrometer was used to measure the infiltration rate and calculate the unsaturated hydraulic conductivity (K_unsat). Soil texture showed differences between hillslope positions, i.e., at the hilltop after 50 cm depth, the soil is classified as silty clay loam, and from 75 cm onward, the soil is silty clay, while at the footslope, the soil is silt loam even at the deeper depths. The results show a higher K_unsat at the footslope as well as higher average water volumes collected in wick lysimeters compared to the hilltop. Average water volumes showed a statistically significant difference at p < 0.01 between the hilltop and the footslope. The soil water content and water potential sensors showed higher values at the footslope at all depths, i.e., 8.0% at 15 cm, 8.4% at 30 cm, and 27.3% at 45 cm. The results show that, even though the vineyard is located in a relatively small area, soil heterogeneity is present, affecting the water flow along the hillslope. This suggests the importance of observing water movement in the soil, especially today when facing extreme weather (e.g., short-term high-intensity rainfall events) in order to protect soil and water resources.

Conducting an in situ evaluation of erodibility in a Mediterranean semi-arid and conventional vineyard in Granada province (Southern Spain) through rainfall simulation experiments

AbstractVineyards in Europe has been fundamental for food, drink and cosmetic production, and job creation; however, in recent decades due to increased cultivation intensity, numerous negative consequences, including erosion, have been observed. Erodibility or susceptibility to erosion is a little-studied parameter in vineyards but is crucial for analyzing the vulnerability of this crop. Therefore, in this research, a small portable rainfall simulator was used as a useful tool for assessing erodibility, combined with other methods such as soil analyses or infiltration measurements in a semi-arid vineyard located in the Granada province (Spain) considering 20 different hotspots at diverse hillslope positions along the inter-rows and close to the traffic roads. The experiments were conducted in spring 2022 under dry soil conditions. Our results display susceptibility to erosion, particularly on steeper parts such as the shoulder and backslopes. In these areas, runoff gained momentum, carrying a significant sediment load, diminishing the effectiveness of stone cover, and occasionally leading to its removal, especially near the roads. Nevertheless, it is observed that increased roughness plays a mitigating role by slowing down runoff. Using linear correlation analysis and Spearman rank coefficient, we observed this effect is linked to factors such as stoniness, vegetation, and moderate tillage. Conversely, in the flatter zones, primarily in lower areas, reduced runoff and delayed onset are primarily influenced by factors such as roughness, type of cover, material composition, and organic matter content. We concluded that this study case can demonstrate that erodibility in vineyards can shed light as an extra parameter to inform farmers, rural inhabitants, and policymakers about the extreme problem of the vulnerable soils of vineyards.

Topography influences diurnal and seasonal microclimate fluctuations in hilly terrain environments of coastal California.

Understanding the topographic basis for microclimatic variation remains fundamental to predicting the site level effects of warming air temperatures. Quantifying diurnal fluctuation and seasonal extremes in relation to topography offers insight into the potential relationship between site level conditions and changes in regional climate. The present study investigated an annual understory temperature regime for 50 sites distributed across a topographically diverse area (>12 km2) comprised of mixed evergreen-deciduous woodland vegetation typical of California coastal ranges. We investigated the effect of topography and tree cover on site-to-site variation in near-surface temperatures using a combination of multiple linear regression and multivariate techniques. Sites in topographically depressed areas (e.g., valley bottoms) exhibited larger seasonal and diurnal variation. Elevation (at 10 m resolution) was found to be the primary driver of daily and seasonal variations, in addition to hillslope position, canopy cover and northness. The elevation effect on seasonal mean temperatures was inverted, reflecting large-scale cold-air pooling in the study region, with elevated minimum and mean temperature at higher elevations. Additionally, several of our sites showed considerable buffering (dampened diurnal and seasonal temperature fluctuations) compared to average regional conditions measured at an on-site weather station. Results from this study help inform efforts to extrapolate temperature records across large landscapes and have the potential to improve our ecological understanding of fine-scale seasonal climate variation in coastal range environments.

Landscape position mediates drought vulnerability in California valley oak (Quercus lobata)

Future climate change will exacerbate drought stress in water-limited ecosystems. However, topography can alter the fine-scale climatic and hydrologic conditions that mediate plant response to meteorological drought. Here, with six new valley oak (Quercus lobata) tree-ring width chronologies, we assess how topography acts as a mediating factor on tree growth and drought sensitivity. Because valley oaks are known to be highly dependent on subsurface water, we predicted that trees growing in riparian sites would be less sensitive to precipitation variability due to greater access to groundwater. Trees were sampled in the Tehachapi Mountains of California across a landscape gradient of sites ranging from 375–1650 m elevation and across upland (55–69 m mean height above nearest drainage) and riparian (2–6 m mean height above the nearest drainage) hillslope positions. Interannual tree growth patterns and drought sensitivity varied substantially in association with hillslope position and elevation. Valley oak radial growth showed a consistently weaker response to precipitation at riparian sites. The influence of hillslope position on drought sensitivity varied with elevation, such that the riparian buffering effect was weakest at sites higher in the watershed and strengthened, progressively, toward the lower elevation sites with greater climatic water deficit. Upland tree growth exhibited a strong response to high-frequency interannual precipitation variability at the high elevation site, whereas trees at lower elevation upland sites responded more to low-frequency decadal trends in precipitation, possibly reflecting hydrogeological processes by which precipitation feeds groundwater lower in the watershed. Our results are consistent with groundwater-dependence of valley oak and indicate that riparian habitats are the most likely refugia for the species during prolonged drought.

Universal microbial reworking of dissolved organic matter along environmental gradients.

Soils are losing increasing amounts of carbon annually to freshwaters as dissolved organic matter (DOM), which, if degraded, can offset their carbon sink capacity. However, the processes underlying DOM degradation across environments are poorly understood. Here we show DOM changes similarly along soil-aquatic gradients irrespective of environmental differences. Using ultrahigh-resolution mass spectrometry, we track DOM along soil depths and hillslope positions in forest catchments and relate its composition to soil microbiomes and physico-chemical conditions. Along depths and hillslopes, we find carbohydrate-like and unsaturated hydrocarbon-like compounds increase in abundance-weighted mass, and the expression of genes essential for degrading plant-derived carbohydrates explains >50% of the variation in abundance of these compounds. These results suggest that microbes transform plant-derived compounds, leaving DOM to become increasingly dominated by the same (i.e., universal), difficult-to-degrade compounds as degradation proceeds. By synthesising data from the land-to-ocean continuum, we suggest these processes generalise across ecosystems and spatiotemporal scales. Such general degradation patterns can help predict DOM composition and reactivity along environmental gradients to inform management of soil-to-stream carbon losses.

Potential of pine forest in controlling soil erosion in Himalayan region - Investigation using fallout radionuclide (137Cs) measurements

The Himalayas possess a distinctive topography owe to the dynamic interplay of tectonic activity, geological erosion and sedimentation, glacial processes, and climatic influences over the millions of years. The rugged, steep terrain and poor land management make it more prone to water erosion, negatively impacts the soil, affecting the goods and services supported by the soil ecosystems. Traditional methods used in soil erosion assessment face limitations when dealing with topographically complex hillslopes. The use of Fallout Radionuclide (FRN) -137Cs provides a feasible alternative for measurement of soil erosion in the region with such topography. However, there is lack of 137Cs-based soil erosion studies in the north-west Himalayas. Pine (Pinus roxburghii) is the predominant forest type in the Himalayas, offering numerous benefits to both natural ecosystems and human beings. In this study, we selected a typical steep hillslope covered with pine forest in the Himalayas for soil erosion assessment. The study measured 137Cs reference inventory of 1409 Bq m−2 in the landscape. Importantly, the concentration of 137Cs along the hillslope positions showed a significant variation attributed to topographic variability. Topographic factors, such as the slope shape and gradient, were identified as the major governing parameters of soil erosion in the hilly and mountainous region. The net soil erosion rate over hillslope positions revealed highest at upper hillslope followed by ridge, middle and valley hillslope positions. The net soil erosion rate under the pine forest ranged from 8.0 to 14.6 t ha−1 yr−1, with an average rate of 9.9 t ha−1 yr−1. Erosion rate over the hillslope positions were found in accordance to the soil loss tolerance limit (SLTL) except for the upper hillslope, indicating it as critical slope position requires to adopt suitable conservation measures. The study signifies the role of the forest in mitigating soil erosion and, in turn, conserving soil resources. The findings provide crucial insights and guidance to land managers and decision-makers, emphasizing the necessity of conserving and restoring forests in the Himalayas.

Varying particle size selectivity of soil erosion along a cultivated catena

Abstract Sheet erosion is a complex multi-factor-dependent process with high spatial heterogeneity on hillslopes. Although the individual factors have been well studied, their aggregated effect on size-selective erosional processes is highly uncertain. Therefore, this study concentrates on the aggregate size distribution and effective particle size distribution (PSD) of the aggregates in the soil loss, collected from different simulated hillslope positions and surface conditions. These simulated hillslope positions combine moisture content from the extremely dry to the saturated with related slope positions of 2, 5, and 12% steepness and different surface roughness (tilled and crusted surfaces) modelled in a laboratory rainfall simulator. Using hierarchical cluster analysis, the PSD of the aggregates was separated into three groups based on the differences in the 59–116 µm range of the PSD histograms, namely, macro-aggregates, 50–250 µm sized micro-aggregates, and <50 µm sized fractions were classified into distinct groups, although some micro-aggregate samples were classified into the macro-aggregate group. PSDs from the 50–250 µm aggregate size fraction were clustered into a group of macro-aggregates if the PSD changed with time (during the rainfall event), notably on rough surfaces. The role of the specified size range in the classification is believed to be due to the parallel presence of aggregates and single particles in this range. As aggregates have a lower density than mineral particles, they tend to be enriched in soil loss under low-energy runoff conditions. Moreover, all samples in the <50 µm fraction clustered into the macro-aggregate group were eroded from the smooth/crusted surface, probably due to the presence of larger particles. The results indicate that the combined effect of erosional factors is not apparent, and the impact of the crust and extreme moisture content on the selectivity and size distribution of the sediment requires further investigation.

Soil Biological Properties along a Topographic Gradient in Brazil’s Atlantic Forest Biome

The Atlantic Forest exhibits remarkable floristic diversity over short distances, and when combined with altitude, it results in variations in soil biological properties and hydrological dynamics. This study aimed to quantify the distribution pattern of soil biological properties along a small topographic gradient and identify how these properties are related to soil chemical, physical, and topographical attributes in an Atlantic Forest fragment in Brazil. Plots were established along hillslope positions varying from 100 to 180 m of altitude. Soil biological characterization was performed at a depth of 0.0–0.10 m in both summer and winter seasons. The lowland showed higher microbial biomass nitrogen in summer (57.28 ± 5.57 μg g−1 soil) and higher anaerobically mineralizable nitrogen in summer and winter (42.70 ± 2.14 mg kg−1 and 41.06 ± 3.37 mg kg−1, respectively). The midland showed lower microbial biomass carbon, enzymatic activity, and soil moisture in both seasons, and higher metabolic coefficients in summer. Soil chemical properties exerted a greater influence on the variability of biological properties in both seasons. The land slope conditioned lower microbial activity and organic cycling in the midland. Soil biological properties were affected by seasonality. Even a small altitudinal gradient (up to 100 m) in the Sea of Hills regions of the Atlantic Forest can lead to significant changes in soil biological and chemical attributes.

Environmental controls on observed spatial variability of soil pore water geochemistry in small headwater catchments underlain with permafrost

Abstract. Soil pore water (SPW) chemistry can vary substantially across multiple scales in Arctic permafrost landscapes. The magnitude of these variations and their relationship to scale are critical considerations for understanding current controls on geochemical cycling and for predicting future changes. These aspects are especially important for Arctic change modeling where accurate representation of sub-grid variability may be necessary to predict watershed-scale behaviors. Our research goal is to characterize intra- and inter-watershed soil water geochemical variations at two contrasting locations in the Seward Peninsula of Alaska, USA. We then attempt to identify the key factors controlling concentrations of important pore water solutes in these systems. The SPW geochemistry of 18 locations spanning two small Arctic catchments was examined for spatial variability and its dominant environmental controls. The primary environmental controls considered were vegetation, soil moisture and/or redox condition, water–soil interactions and hydrologic transport, and mineral solubility. The sampling locations varied in terms of vegetation type and canopy height, presence or absence of near-surface permafrost, soil moisture, and hillslope position. Vegetation was found to have a significant impact on SPW NO3- concentrations, associated with the localized presence of nitrogen-fixing alders and mineralization and nitrification of leaf litter from tall willow shrubs. The elevated NO3- concentrations were, however, frequently equipoised by increased microbial denitrification in regions with sufficient moisture to support it. Vegetation also had an observable impact on soil-moisture-sensitive constituents, but the effect was less significant. The redox conditions in both catchments were generally limited by Fe reduction, seemingly well-buffered by a cache of amorphous Fe hydroxides, with the most reducing conditions found at sampling locations with the highest soil moisture content. Non-redox-sensitive cations were affected by a wide variety of water–soil interactions that affect mineral solubility and transport. Identification of the dominant controls on current SPW hydrogeochemistry allows for qualitative prediction of future geochemical trends in small Arctic catchments that are likely to experience warming and permafrost thaw. As source areas for geochemical fluxes to the broader Arctic hydrologic system, geochemical processes occurring in these environments are particularly important to understand and predict with regards to such environmental changes.

Hillslope‐Channel Transitions and the Role of Water Tracks in a Changing Permafrost Landscape

AbstractThe Arctic is experiencing rapid climate change, and the effect on hydrologic processes and resulting geomorphic changes to hillslopes and channels is unclear because we lack quantitative models and theory for rapid changes resulting from thawing permafrost. The presence of permafrost modulates water flow and the stability of soil‐mantled slopes, implying that there should be a signature of permafrost processes, including warming‐driven disturbance, in channel network extent. To inform understanding of hillslope‐channel dynamics under changing climates, we examined soil‐mantled hillslopes within a ∼300 km2 area of the Seward Peninsula, western Alaska, where discontinuous permafrost is particularly susceptible to thaw and rapid landscape change. In this study, we pair high‐resolution topographic and satellite data to multi‐annual observations of InSAR‐derived surface displacement over a 5‐year period to quantify spatial variations in topographic change across an upland landscape. We find that neither the basin slope nor the presence of knickzones controls the magnitude of recent surface displacements within the study basin, as may be expected under conceptual models of temperate hillslope evolution. Rather, the highest displacement magnitudes tended to occur at the broad hillslope‐channel transition zone. In this study area, this zone is occupied by water tracks, which are zero‐order ecogeomorphic features that concentrate surface and subsurface flow paths. Our results suggest that water tracks, which appear to occupy hillslope positions between saturation and incision thresholds, are vulnerable to warming‐induced subsidence and incision. We hypothesize that gullying within water tracks will outpace infilling by hillslope processes, resulting in the growth of the channel network under future warming.

Mapping Variations in Bedrock Weathering With Slope Aspect Under a Sedimentary Ridge‐Valley System Using Near‐Surface Geophysics and Drilling

AbstractUnderstanding how soil thickness and bedrock weathering vary across ridge and valley topography is needed to constrain the flowpaths of water and sediment production within a landscape. Here, we investigate saprolite and weathered bedrock properties across a ridge‐valley system in the Northern California Coast Ranges, USA, where topography varies with slope aspect such that north‐facing slopes have thicker soils and are more densely vegetated than south‐facing slopes. We use active source seismic refraction surveys to extend observations made in boreholes to the hillslope scale. Seismic velocity models across several ridges capture a high velocity gradient zone (from 1,000 to 2,500 m/s) located ∼4–13 m below ridgetops that coincides with transitions in material strength and chemical depletion observed in boreholes. Comparing this transition depth across multiple north‐ and south‐facing slopes, we find that the thickness of saprolite does not vary with slope aspects. Additionally, seismic survey lines perpendicular and parallel to bedding planes reveal weathering profiles that thicken upslope and taper downslope to channels. Using a rock physics model incorporating seismic velocity, we estimate the total porosity of the saprolite and find that inherited fractures contribute a substantial amount of pore space in the upper 6 m, and the lateral porosity structure varies strongly with hillslope position. The aspect‐independent weathering structure suggests that the contemporary critical zone structure at Rancho Venada is a legacy of past climate and vegetation conditions.

Transpiration rates decline under limited moisture supply along hillslopes in a humid karst terrain

Topographic positions can mediate subsurface water availability, but its effects on tree transpiration are controversial. In humid karst regions, climax forests are usually not limited by moisture supply, even at the summit, through absorbing water from deep layers. However, little is known on the transpiration pattern and its limiting factor on the shrubland widely distributed along the karst hillslopes. In the current study, Rhus chinensis, a widely spread constructive species in natural restoration was selected. Meteorological factors, 0–300 cm soil–epikarst moisture, sap flow, and root water uptake were studied during an entire growing season to assess how hillslope positions affected transpiration. We found the mean water content in uphill was only around 60 % of that in downhill, indicating a contrasting water supply along the slope. However, there were no significant differences in the xylem isotopic composition and lc–excess which suggested the similar water uptake strategies in both uphill and downhill. R. chinensis primarily relied on the soil water rather than epikarst water (groundwater) along the hillslope because of the MixSIAR model results and more negative lc–excess values (−13.18 ‰). R. chinensis exhibited decreases of nearly half in the transpiration rate and amount in uphill compared to those in downhill. In downhill with sufficient water availability, transpiration followed the variation in atmospheric water demand. In uphill, a poor moisture supply limited tree transpiration and its response to atmospheric water demand. Our findings revealed that the early successional species did not entirely depend on atmospheric water demand, absorbing deep epikarst water as the mature forest. The transpiration rates of those species declined by nearly half to adapt to the water–limited environment along the hillslope in the humid karst region. This study can contribute to the evaluation of eco–hydrological functions during natural restoration.

Using Dye and Bromide Tracers to Identify Preferential Water Flow in Agricultural Hillslope Soil under Controlled Conditions

Processes in hillslope soils present a particular challenge for agricultural production and soil management due to their hydropedological specifics and high soil erosion risk. Soil heterogeneities can cause preferential and/or lateral flow on the entire hillslope resulting in the off-site movement of water, fertilizers and chemicals used in crop production. A study was conducted under controlled conditions in a laboratory with undisturbed soil cores (250 cm3), which were used to estimate the soil hydraulic properties (SHP) using HYPROP and WP4C devices, while undisturbed soil columns (diameter = 16 cm, length = 25 cm) were used for the evaluation of preferential flow pathways using potassium bromide and Brilliant Blue. Samples were excavated in triplicate from the hilltop, backslope and footslope regions within the inter-rows of a vineyard from a critical zone observatory, SUPREHILL, in Croatia in Dystric Luvic Stagnosol. The aim of this study was to determine if the erosion-affected hillslope position affected the physical, chemical and hydraulic properties of soil and to identify water flow and possible preferential flow using dye and bromide tracers. The results of the sensor measurements and estimated SHPs were in agreement, showing a faster leaching of the irrigated rainwater in the footslope column. The tracer experiments showed variability even in the columns taken from the same position on the hillslope, which can be linked to plant roots and soil fauna activity. Altogether, the results showed a deeper loose layer at the footslope as a consequence of the soil erosion, which then resulted in higher hydraulic conductivity and the leached mass of the bromide due to better soil structure and pore connectivity. Thus, due to significant differences in the leached mass of bromide, this research should be later expanded in field experiments to reveal the impact of surface runoff, subsurface preferential and lateral flow on a larger scale.