Hillslope Positions Research Articles

Soil Carbon Dioxide Flux Partitioning in a Calcareous Watershed With Agricultural Impacts

AbstractPredicting the partitioning between aqueous and gaseous C across landscapes is difficult because many factors interact to control carbon dioxide (CO2) concentrations and removal as dissolved inorganic carbon (DIC). For example, carbonate minerals buffer soil pH and allow CO2 dissolution in porewaters, but nitrification of fertilizers may decrease pH so that carbonate weathering results in a gaseous CO2 efflux. Here, we investigate CO2 partitioning in an agricultural, first‐order, mixed‐lithology humid, temperate watershed. We quantified soil mineralogy and measured porewater chemistry, soil moisture, and soil pCO2 and pO2 as a function of depth at three hillslope positions. Variation of soil moisture along the hillslope was the dominant control on the concentration of soil CO2, but mineralogy acted as a secondary control on the partitioning of CO2 between gaseous and aqueous phases. Regression slopes of pCO2 versus pO2 in the carbonate‐bearing soils indicate a deficit of aerobically respired CO2 relative to O2 (p < 0.05). Additionally, nitrification of upslope fertilizers did not lower soil pH and therefore did not cause a gaseous CO2 flux from carbonate weathering. We concluded that in the calcareous soils, up to 43% of respired C potentially dissolves and drains from the soil rather than diffusing out to the atmosphere. To explore the possible implications of the reactions we evaluated, we used databases of carbonate minerals and land uses to map types of soil degassing behaviors. Based on our maps, the partitioning of respired soil CO2 to the aqueous phase could be important in estimating ecosystem C budgets and models.

Deep Learning for Isotope Hydrology: The Application of Long Short-Term Memory to Estimate High Temporal Resolution of the Stable Isotope Concentrations in Stream and Groundwater

Recent advances in laser spectroscopy has made it feasible to measure stable isotopes of water in high temporal resolution (i.e., sub-daily). High-resolution data allow the identification of fine-scale, short-term transport and mixing processes that are not detectable at coarser resolutions. Despite such advantages, operational routine and long-term sampling of stream and groundwater sources in high temporal resolution is still far from being common. Methods that can be used to interpolate infrequently measured data at multiple sampling sites would be an important step forward. This study investigates the application of a Long Short-Term Memory (LSTM) deep learning model to predict complex and non-linear high-resolution (3 h) isotope concentrations of multiple stream and groundwater sources under different landuse and hillslope positions in the Schwingbach Environmental Observatory (SEO), Germany. The main objective of this study is to explore the prediction performance of an LSTM that is trained on multiple sites, with a set of explanatory data that are more straightforward and less expensive to measure compared to the stable isotopes of water. The explanatory data consist of meteorological data, catchment wetness conditions, and natural tracers (i.e., water temperature, pH and electrical conductivity). We analyse the model's sensitivity to different input data and sequence lengths. To ensure an efficient model performance, a Bayesian optimization approach is employed to optimize the hyperparameters of the LSTM. Our main finding is that the LSTM allows for predicting stable isotopes of stream and groundwater by using only short-term sequence (6 h) of measured water temperature, pH and electrical conductivity. The best performing LSTM achieved, on average of all sampling sites, an RMSE of 0.7‰, MAE of 0.4‰, R2 of 0.9 and NSE of 0.7. The LSTM can be utilized to predict and interpolate the continuous isotope concentration time series either for data gap filling or in case where no continuous data acquisition is feasible. This is very valuable in practice because measurements of these tracers are still much cheaper than stable isotopes of water and can be continuously conducted with relatively minor maintenance.

Effects of hillslope position on soil water infiltration and preferential flow in tropical forest in southwest China

The hillslope is an essential natural spatial gradient that influences hydrological processes by affecting water distribution, surface flow, soil erosion, and groundwater recharge. To date, few studies have addressed only the hydrological processes of tropical forest hillslopes. To reveal the effect of hillslope on soil hydrological functioning—including water distribution and exchange, infiltration capacity, and flow behaviour—we conducted 36 field infiltration and nine dye-tracer investigations of different hillslope locations in the natural rainforest of Xishuangbanna, southwest China. The soil physical properties—including soil noncapillary and total porosity, saturated water capacity, and field water capacity—decreased with decreasing elevation from hilltop to middle slope and the valley bottom. The water infiltration capacity—including the initial infiltration rate, saturated soil hydraulic conductivity, and average infiltration rate—decreased from the hilltop to the valley bottom. Preferential flow dominated soil water movement more in the upper locations than in the valley bottom. The infiltration capacity parameters and preferential flow were significantly correlated with soil water content, noncapillary and total porosity, root biomass, and termite holes. These results indicated that along with the soil physical properties, root systems, animal activity, cracks, and stones affected the soil infiltration capacity and preferential flow. Differences in the hydraulic processes of each hillslope position contributed to the redistribution, transportation, and storage of surface and belowground water, resulting in differing availabilities of soil water resources and utilisation by plants. The findings of this study can help understand eco-hydrological processes in the context of water resources management in tropical mountain ecosystems.

Are spatial patterns of soil moisture at plot scales generalisable across catchments, climates, and other characteristics? A synthesis of synoptic soil moisture across theMid‐Atlantic

AbstractThe spatial variation of soil moisture over very small areas (<100 m2) can have nonlinear impacts on cycling and flux rates resulting in bias if it is not considered, but measuring this variation is difficult over extensive temporal and spatial scales. Most studies examining spatial variation of soil moisture were conducted at hillslope (0.01 km2) to multi‐catchment spatial scales (1000 km2). They found the greatest variation at mid wetness levels and the smallest variation at wet and dry wetness levels forming a concave down relationship. There is growing evidence that concave down relationships formed between spatial variation of soil moisture and average soil moisture are consistent across spatial scales spanning several orders of magnitude, but more research is needed at very small, plot scales (<100 m2). The goal of this study was to characterise spatial variation in shallow soil moisture at the plot scale by relating the mean of measurements collected in a plot to the standard deviation (SD). We combined data from a previous study with thousands of new soil moisture measurements from 212 plots in eight catchments distributed across the US Mid‐Atlantic Region to (1) test for a generalisable mean–SD relationship at plot scales, (2) characterise how landcover, land use, season, and hillslope position contribute to differences in mean–SD relationships, and (3) use these generalised mean–SD relationships to quantify their impacts on catchment scale nitrification and denitrification potential. Our study found that 98% of all measurements formed a generalised mean–SD relationship like those observed at hillslope and catchment spatial scales. The remaining 2% of data comprised a mean–SD relationship with greater spatial variation that originated from two riparian plots reported in a previous study. Incorporating the generalised mean–SD relationship into estimates of nitrification and denitrification potential revealed strong bias that was even greater when incorporating mean–SD observations from the two riparian plots with significantly greater spatial variation.

A sensors-based profile heterogeneity index for soil characterization

The technological and computational development has raised new possibilities for soil scientists to describe and infer about soil-forming processes and functions. Thus, why not to insert the contrasts between soil layers in a profile revealed by proximal sensing? We proposed to study the soil profile heterogeneity by reflectance (Vis-NIR-SWIR), X-ray fluorescence and magnetic susceptibility aiming to quantify it with the proposed “Profile Heterogeneity Index” (PHI) through seven soil sequences in twenty-seven locations over different hillslope positions and parent materials. The study was conducted in a 165-ha farm, at Rio das Pedras municipality, São Paulo State, Brazil. The differences between two consecutive soil layers were calculated by subtracting the data to the bottom and the immediately above for each soil profile. The results are depicted as a depth function with proportional colors to highlight the heterogeneity in the soil profiles. This approach provided a new method to identify where the transitions of soil attributes occur exactly and improve the understanding about soil-formation factors and processes. In this study, greater PHI values were mainly related to erosion and redeposition process through the landscape, differences in soil parent materials and also soil-forming processes. The sensors can furnish important and complementary information to the characterization of soil profile heterogeneity.

Efficient prediction of profile mean soil water content for hillslope-scale Caragana korshinskii plantation using temporal stability analysis

Spatio-temporal variability in soil water content (SWC) increases the difficulty in soil water resource management and vegetation construction in arid and semi-arid regions. Our objectives were to investigate the spatial variability and temporal stability of SWC and identify representative locations for reliably predicting the profile mean SWC (MSWC) of a 33-year-old Caragana korshinskii plantation on typical sunny and shady hillslopes on the Loess Plateau, China. The profile SWC (0–180 cm) in three sampling belts and five hillslope positions (upper position, UP; upper-middle position, UMP; middle position, MP; lower-middle position, LMP; lower position, LP) were measured 27 times for both the hillslopes in 2018 and 2019. The mean relative difference (MRD), the standard deviation of the MRD (SDRD), and an index of temporal stability (ITS) were used to identify representative locations of the MSWC. The results indicated that the SWC presented different horizontal but similar vertical change trends in the five hillslope positions. The profile MSWC on the sunny hillslope (8.89%) was lower than that on the shady hillslope (9.44%). The mean Spearman correlation coefficients of the temporal patterns ranged from 0.6 to 0.8 for the five soil layers on both hillslopes. The characteristics of temporal stability differed in both temporal persistence and the representative locations for the five soil layers on the sunny and shady hillslopes. The mean ITS values generally decreased with increasing soil depth. Ultimately, the representative locations for estimating the profile MSWC were determined to be the MP-2 location on the sunny hillslope and the MP-3 location on the shady hillslope based on the MRD, SDRD, and ITS values. The representative locations have the lowest ITS values and represent the MSWC accurately, with root mean square error (RMSE) and mean error (ME) values of less than 1%. In addition, the soil organic carbon, clay, silt, and sand contents greatly affect the spatial and temporal distributions of the soil water at the hillslope scale. Our study provides a rapid and accurate method for determining profile MSWC in C. korshinskii plantations on typical sunny and shady hillslopes and could also support valuable guidance for the rational management of hillslope-scale SWC resources for similar dry regions.

Impacts of oak deforestation and rainfed cultivation on soil redistribution processes across hillslopes using 137Cs techniques

BackgroundAs one of the main components of land-use change, deforestation is considered the greatest threat to global environmental diversity with possible irreversible environmental consequences. Specifically, one example could be the impacts of land-use changes from oak forests into agricultural ecosystems, which may have detrimental impacts on soil mobilization across hillslopes. However, to date, scarce studies are assessing these impacts at different slope positions and soil depths, shedding light on key geomorphological processes.MethodsIn this research, the Caesium-137 (137Cs) technique was applied to evaluate soil redistribution and soil erosion rates due to the effects of these above-mentioned land-use changes. To achieve this goal, we select a representative area in the Lordegan district, central Iran. 137Cs depth distribution profiles were established in four different hillslope positions after converting natural oak forests to rainfed farming. In each hillslope, soil samples from three depths (0–10, 10–20, and 20–50 cm) and in four different slope positions (summit, shoulder, backslope, and footslope) were taken in three transects of about 20 m away from each other. The activity of 137Cs was determined in all the soil samples (72 soil samples) by a gamma spectrometer. In addition, some physicochemical properties and the magnetic susceptibility (MS) of soil samples were measured.ResultsErosion rates reached 51.1 t·ha− 1·yr− 1 in rainfed farming, whereas in the natural forest, the erosion rate was 9.3 t·ha− 1·yr− 1. Magnetic susceptibility was considerably lower in the cultivated land (χhf = 43.5 × 10− 8 m3·kg− 1) than in the natural forest (χhf = 55.1 × 10− 8 m3·kg− 1). The lower soil erosion rate in the natural forest land indicated significantly higher MS in all landform positions except at the summit one, compared to that in the rainfed farming land. The shoulder and summit positions were the most erodible hillslope positions in the natural forest and rainfed farming, respectively.ConclusionsWe concluded that land-use change and hillslope positions played a key role in eroding the surface soils in this area. Moreover, land management can influence soil erosion intensity and may both mitigate and amplify soil loss.

Wavelet analysis of soil water state variables for identification of lateral subsurface flow: Lysimeter vs. field data

AbstractPreferential and lateral subsurface flow (LSF) may be responsible for the accelerated transport of water and solutes in sloping agricultural landscapes; however, the process is difficult to observe. One idea is to compare time series of soil moisture observations in the field with those in lysimeters, where flow is vertically oriented. This study aims at identifying periods of deviations in soil water contents and pressure heads measured in the field and in a weighing lysimeter with the same soil profile. Wavelet coherency analysis (WCA) was applied to time series of hourly soil water content and pressure head data (15‐, 32‐, 60‐, 80‐, and 140‐cm depths) from Colluvic Regosol soil profiles. The phase shifts and periodicities indicated by the WCA plots reflected the response times to rain events in the same depth of lysimeter and field soil. For many rain events and depths, pressure and moisture sensors installed in the field soil responded earlier than those in the lysimeter. This could be explained by either vertical preferential flow or LSF from upper hillslope positions. Vice versa, a faster response in the lysimeter soil could be indicative for vertical preferential flow effects. Dry weather conditions and data gaps limited the number of periods with elevated soil moisture in 2016–2018, in which LSF was likely to occur. The WCA plots comprise all temporal patterns of time shifts and correlations between larger data time series in a condensed form to identify potentially relevant periods for more detailed analyses of subsurface flow dynamics.

Topographical Controls on Hillslope‐Scale Hydrology Drive Shrub Distributions on the Seward Peninsula, Alaska

AbstractObservations indicate shrubs are expanding across the Arctic tundra, mainly on hillslopes and primarily in response to climate warming. However, the impact topography exerts on hydrology, nutrient dynamics, and plant growth can make untangling the mechanisms behind shrub expansion difficult. We examined the role topography plays in determining shrub expansion by applying a coupled transect version of a mechanistic ecosystem model (ecosys) in a tundra hillslope site in the Seward Peninsula, Alaska. Modeled biomass of the dominant plant functional types agreed well with field measurements (R2 = 0.89) and accurately represented shrub expansion over the past 30 years inferred from satellite observations. In the well‐drained crest position, canopy water potential and plant nitrogen (N) uptake was modeled to be low from plant and microbial water stress. Intermediate soil water content in the mid‐slope position enhanced mineralization and plant N uptake, increasing shrub biomass. The deciduous shrub growth in the mid‐slope position was further enhanced by symbiotic N2 fixation primed by increased root carbon allocation. The gentle slope in the poorly drained lower‐slope position resulted in saturated soil conditions that reduced soil O2 concentrations, leading to lower root O2 uptake and lower nutrient uptake and plant biomass. A simulation that removed topographical interconnectivity between grid cells resulted in (1) a 28% underestimate of mean shrub biomass and (2) over or underestimated shrub productivity at the various hillslope positions. Our results indicate that land models need to account for hillslope‐scale coupled surface and subsurface hydrology to accurately predict current plant distributions and future trajectories in Arctic ecosystems.

Efeitos da dinâmica pluviométrica na interceptação e precipitação interna em uma encosta com Floresta Atlântica no Parque Nacional da Tijuca, RJ, Brasil.

Em vista à importância da cobertura vegetal para o balanço hídrico, o presente trabalho objetivou analisar os efeitos da dinâmica pluviométrica sobre a precipitação interna e o processo interceptação da água da chuva pela vegetação em uma encosta florestada do Parque Nacional da Tijuca. Foram discutidas as influências de um conjunto de variáveis independentes dadas pela dinâmica hidrometeorológica (volume e intensidade de precipitação inicial e estiagem antecedente) para variações temporais no volume e percentuais de interceptação, assim como no volume de precipitação interna, acoplada a uma análise espacial dada pela comparação entre diferentes posições na encosta (divisor de drenagem; alta encosta, baixa encosta; e fundo de vale) onde há heterogeneidade na estrutura da vegetação induzida pelo contexto geomorfológico. A interceptação da chuva em cada evento varia de forma diretamente proporcional ao tempo de estiagem antecedente, com o efeito complementar do volume de chuva nos 15 dias anteriores. Os percentuais interceptados variam em relações inversamente proporcionais ao volume de chuva inicial e suas intensidades de precipitação. É em relação à interceptação que mais significativamente se manifestam as variações espaciais, com destaque para menores valores no divisor de drenagem onde a abertura do dossel é maior. A precipitação interna varia no tempo sob forte influência – diretamente proporcional – dos volumes de precipitação inicial, sendo ainda melhor explicada quando considerada também a estiagem antecedente. As variações espaciais na precipitação interna são menos significantes do que em relação à interceptação, reduzindo a significância à medida que o volume de chuva inicial é maior. Effects of rainfall dynamics on interception and net precipitation on a Atlantic forested hillslope in Tijuca National Park, RJ, Brazil A B S T R A C TGiven the importance of vegetation cover for the hydric balance, the present paper aimed to analyze the pluviometric dynamics for the net precipitation and interception process, in the forested hillslope in the Tijuca National Park. The influence by a set of independents variables given by hydrometeorological dynamics (rainfall volume and intensity and drought context) was discussed face the temporal modulation for interception and net precipitation, coupled a spatial analyze given by comparisons between hillslope positions (summit; upper hillslope; lower hillslope; valley bottom), where there is a vegetation structure heterogeneity originated by geomorphological context. The volume intercepted at each event varies directly proportional to the antecedent drought time, with the complementary effect of the rainfall volume in the previous 15-days. The percentual interception varies inversely proportional to the gross precipitation and its intensity. It is about the interception wich best manifests the significance of the spatial variations, with highlights for the summit área, where the canopy openness is bigger. The net precipitation varies under the strong influence – directly proportional – by gross precipitation volumes, being better explained with the complementary effect by the previous drought. The spatial variations on the net precipitation are less significant in comparison to the interception, decreasing the significance in the time of increase of the gross precipitation volumes.Keywords: rainfall dynamics; interception; net precipitation.

Soil and Plant Factors Affecting Changes in Forage Production Patterns on Mined Land 28 Yr After Reclamation

Coal surface mining in the northern Great Plains of the United States often produces mining spoils with physical and chemical barriers to successful revegetation, and this has resulted in experiments on reclamation with salvaged soil materials. There is a need to determine changes in soil properties and plant community and productivity decades after reclamation. Experiments were initiated in the mid-1970s by placing wedge-shaped masses of Haplustoll soils over leveled, saline-sodic mine spoils forming 2−5% hillslopes approximately 50 m long at two sites near Zap and Stanton, North Dakota. Seeding treatments at Zap included alfalfa (Medicago sativa L.), crested wheatgrass (CWG, Agropyron cristatum [L.] Gaertn.), smooth bromegrass (SBG, Bromus inermis Leyss.), and Russian wildrye (Psathyrostachy juncea [Fisch.] Nevski); at Stanton, native grasses (two Bouteloua spp.) replaced P. juncea. We examined soil, plant community, and productivity changes 3 decades after the start of the experiments. Leaching of soluble salts in subsoils and mine spoils improved soil quality (SQ). SQ improved more in lower slope areas because more rootzone was occupied by low SQ mine spoil. Initial forage yield patterns showed dependency on hillslope position, reflecting both soil depth effects in lower slope parts of hillslopes and apparent water redistribution effects in middle and upper slope areas. Evidence of SQ improvement over time was inferred by substantial decreases in yield dependency on hillslope position at both sites. The Zap site was grazed more heavily than Stanton, and species composition at Zap was 46% cool-season grasses (CWG greatest) and 40% broadleaves. Stanton had 87% cool-season grasses (SBG greatest) with 3% broadleaves. Leaching of salts was threefold greater at Zap and may have resulted from an abundance of deep-rooted broadleaves. Reclaiming mined land with salvaged soil and revegetation can improve SQ of mine spoil, which our results infer was driven by root growth and establishment of macroporosity.

Evaluation of soil erosion and sediment deposition rates by the 137Cs fingerprinting technique at different hillslope positions on a catchment.

Assessment of the variation of soil erosion and sediment mobilization at different hillslope positions using the 137Cs tracing technique has been carried out for the Dapotou closed watershed, a representative depression in the karst gabin basin in Southwest China. The results showed that the annual soil erosion rates in the shoulders, backslopes, and footslopes were 0.87, 0.35 and 0.49cmyear-1, respectively, while the soil sediment deposition rate in the depression bottom was 2.68cmyear-1. The average annual soil erosion modulus of the complete hillslope was 632tkm-2year-1, which confirmed the seriousness of erosion according to the gradation of the karst soil erosion standards. For the whole catchment, the sediment delivery ratio was estimated as 0.82. To identify which factor could play the most important role in influencing the estimates using 137Cs, a linear correlation and principal component analysis were conducted. The results showed that 137Cs concentrations at different soil depths of the different hillslope positions were significantly correlated with soil organic matter and total nitrogen (P < 0.05). As this watershed is a typical karst geomorphological type, these findings are expected to provide data support for larger watershed soil erosion management and ecological restoration in fragile karst ecosystems.

On the controls of preferential flow in soils of different hillslope position and lithological origin

AbstractSoils derived from different lithologies and their controls on preferential flow remain underexplored in forested landscapes. In the same lithology, the propensity for preferential flow occurrence at different hillslope positions also remains largely elusive. By utilizing a soil moisture response time method, we compared preferential flow occurrence between a shale site (Shale Hills, silt loam soils) and a sandstone site (Garner Run, sandy loam soils) at four hillslope positions: ridge‐top, North‐ and South‐facing mid‐slopes and toe slope, for over 2 years. The catchments are neighbouring and covered by temperate forest. For the four hillslope positions, Shale Hills had higher preferential flow frequencies compared to Garner Run. Between these two catchments, the South‐facing mid‐slope sites showed the highest contrasts in preferential flow frequency (33.5% of events at Shale Hills vs. 8.8% at Garner Run) while the ridge‐top sites showed the lowest contrasts (18.7 vs. 13.2%). Additionally, over the unfrozen period, for seven out of eight monitoring sites, drier antecedent conditions tended to be more favourable for preferential flows to occur, with significant (p &lt; .01) relationships at two sites. Except for the South‐facing mid‐slope sites, both Shale Hills and Garner Run had two preferential flow pathways. The characteristic preferential flow pathways at Shale Hills were the Bwand C horizons, and for Garner Run, preferential flow moved from the E/AE horizon to the Bwhorizon. This study shows that shale‐derived soils tended to have higher preferential flow occurrence than sandstone soils, but hillslope positions exhibit different levels of contrasts. More effort should be paid to study the impact of lithology on preferential flows in the context of land surface modelling and biogeochemical reactions to improve ecosystem services of headwater catchments.

Spatial Variability of Rainfed Wheat Production Under the Influence of Topography and Soil Properties in Loess-Derived Soils, Northern Iran

The wheat production variability is not well-understood in hilly region, especially in loess-derived soils of Golestan province in Iran with a sub-humid climate. Topography can greatly influence the production of agricultural crops by affecting soil quality. A study area located in Golestan province was selected in order to assess the spatial variability of wheat production and to develop regression models between the crop, soil properties, and topography attributes. The samples of wheat and soil were randomly taken from 100 points at different hillslope positions (i.e., shoulder, back-, foot-, and toe-slope). The soil physicochemical analysis and the measurement of wheat yield components were conducted. The digital elevation model (DEM; 10 m resolution) was used, and the topographic attributes (i.e., elevation, slope, wetness index, stream power index, curvature, erosivity factor, and watershed specific area) were calculated. The results showed that the greatest total yield and the highest grain yield were estimated to be 14.53 and 4.41 ton ha−1, respectively, in areas with a slope of less than 10%, which were significantly higher than those in the steep areas (slope classes of 10–30% and > 30%). The highest and the lowest total yields, with average values of 15.82 and 5.68 ton ha−1, were observed in the toeslope and shoulder slope positions, respectively. The greatest grain yields were obtained from the foot- and toeslope positions with the average values of 4.61 and 4.66 ton ha−1, respectively. The topographic curvature and wetness index had a significant correlation with the yield of wheat. According to the regression equations, topographic indexes can well justify the spatial variability of wheat yield, indicating the importance of these factors by influencing the distribution of moisture during the process of wheat production in the study region. The enhancements of wheat yield components in the lower slope positions could be attributed to an increase in soil depth and plant available water as well as to the accumulation of further soil organic matter and nutrient elements, including nitrogen and potassium, in such positions as a result of soil redistribution. Moreover, the results illustrated that by using easy accessable, cheap, and none destructive data (DEM derivatives and soil properties); it is possible to predict the production yield of wheat with a reliable estimation. We concluded that for better farming management and productivity in hilly regions, topographic attributes should be considered for plantation. Therefore, this study introduces the most suitable slope positions and topographic attributes for crop production with the least soil degradation. Shoulder and backslope positions are the most unsuitable slopes possibly better for orchards while toeslopes and footslopes could be used for intensive crop production.

Spatial Variability of Preferential Flow and Infiltration Redistribution along a Rocky-Mountain Hillslope, Northern China

Rock fragments in soil strongly increase the complexity of hydrological processes. Spatial variability of preferential flow and infiltration characteristics, especially along a rocky-mountain hillslope are poorly understood. In this study, five rainfall–dye tracer experiments were performed in the rocky Taihang Mountains, northern China, to investigate the spatial variability of preferential flow and infiltration redistribution on different hillslope positions. Tracers were used to distinguish macropore flow and actual water flow patterns, and preferential flow indices and spatial non–uniformity of the infiltration redistribution were calculated using image analysis. Results showed increasing trends in the dye coverage, maximum infiltration depth, and steady infiltration rate with increased hillslope position, with a preferential flow fraction of 0.10, 0.11, 0.15, 0.29, and 0.26 for the bottom–, down–, mid–, upper–, and top–slope positions, respectively. With increased hillslope position, the spatial non–uniformity of the infiltration redistribution gradually increased in orthogonal and parallel directions to the stained section, and was supported by the fractal dimensions. Positive (gravel mass ratio, saturated water content, altitude, hydraulic conductivity and roots) and negative (bulk density and clay content) impacts on preferential flow and infiltration redistribution were quantitatively emphasized. The characteristic and mechanism of infiltration process were further identified along a rocky-mountain hillslope.

Mapping depth to the argillic horizon on historically farmed soil currently under forests

The Piedmont region of the southeastern United States experienced a period of accelerated erosion in the 1800s. Clear-cutting of the forests coupled with soil tilling and inadequate erosion control practices led to substantial soil redistribution and loss. This redistribution exposed the subsoil clay (argillic) horizon in many locations and altered hydrologic processes (i.e., infiltration or interflow) across the landscape. Describing the current hydrologic and biogeochemical processes in this landscape requires a spatially explicit mapping of soil re-distribution and the current depth to the argillic horizon. To achieve this mapping, we measured the depth to the argillic horizon in highly eroded (historically farmed) and undisturbed hillslopes (reference areas) by direct measures—soil augur and tile push probe—and by electromagnetic induction (EMI) combined with ancillary data (i.e., terrain features). Direct measures indicated that the depth to the argillic horizon in the historically farmed watersheds ranged from 26 ± 2 to 87 ± 12 cm in ridge-top and toe-slope, respectively, while in the reference hillslopes depths from ridge-top to toe-slope only ranged from 30 ± 3 to 48 ± 12 cm. Using remotely-sensed data, the best predictive models of the depth to the argillic horizon were based on measurements of the site topographic characteristics (percent slope and hillslope position), along with the following EMI data products: the 2 m horizontal coplanar (HCP) EMI measurement and some of its derivatives (i.e., 2 m HCP – 2 m perpendicular, 2 m HCP – 1 m HCP). Our data suggest that in this previously farmed and reforested landscape, geophysical sensing is an efficient means of predicting depth-to-clay when combined with landscape feature characteristics (R2 = 0.69).

Slope Position Rather Than Thinning Intensity Affects Arbuscular Mycorrhizal Fungi (AMF) Community in Chinese Fir Plantations

Background and Objectives: Arbuscular mycorrhizal fungi (AMF) play a crucial role in individual plant capability and whole ecosystem sustainability. Chinese fir, one of the most widely planted tree species in southern China, forms associations with AMF. However, it is still unclear what impacts thinning management applied to Chinese fir plantations has on the structure and diversity of soil AMF communities. This research attempts to bridge this knowledge gap. Materials and Methods: A thinning experiment was designed on different slope positions in Chinese fir plantations to examine the impacts of slope position and thinning intensity on colonization, diversity, and community composition of AMF. Results: Our research showed that the altitudinal slope position had significant effects on colonization, diversity, and community composition of AMF in Chinese fir plantations. In addition, the interaction between slope position and thinning intensity had significant effects on AMF diversity. Colonization by AMF on the lower slope position was significantly higher than on the upper slope position, while AMF diversity on the upper slope position was higher than on the middle and lower slope positions. Glomus was the most abundant genus in all slope positions, especially on the middle and lower slope positions. The relative abundance of Diversispora was significantly different among slope positions with absolute dominance on the upper slope position. Scutellospora was uniquely found on the upper slope position. Furthermore, soil Mg and Mn contents and soil temperature positively affected AMF community composition at the operational taxonomic unit (OTU) level. Conclusions: These findings suggested that slope position should be considered in the management of Chinese fir plantations. Furthermore, both chemical fertilization and AMF augmentation should be undertaken on upper hill slope positions as part of sustainable management practices for Chinese fir plantations.

Terrain attributes and forage productivity predict catchment-scale soil organic carbon stocks

Accurate assessments of soil organic carbon (SOC) stocks are needed at multiple scales given their importance to both local soil health and global C cycles. Rangelands cover 54% of California, representing a large stock of SOC, but existing SOC estimates are uncertain. To improve understanding of fine-resolution SOC stocks in complex terrain and provide guidance to rangeland SOC inventories, we grid-sampled 105 locations (21-m grid cells) at two depths (0–10 and 10–30 cm) in a 10-ha annual grassland catchment in California’s Central Coast Range. Soils were analyzed for bulk density, coarse fragments, SOC and texture. Monthly aerial imagery was acquired by an unmanned aerial vehicle to compare surface reflectance during two contrasting years (wet vs. dry) to SOC stocks. We found that the 0–30 cm soil thickness held 3.64 ± 0.71 kg SOC m−2 (mean ± SD) with a range of 1.97–5.49 kg SOC m−2. The 0–10 cm soil thickness stored 47% of the 0–30 cm SOC stock with SOC concentrations twice as high in the 0–10 cm layer (1.40 ± 0.38%) as in the 10–30 cm layer (0.71 ± 0.15% SOC). Multiple linear regression (MLR) models explained 50–57% of SOC variability at 0–30 and 10–30 cm, but only 25% of variability at 0–10 cm. Based on cross-validation tests, MLR outperformed spatial interpolation methods and Random Forest models, best explaining SOC stocks with five environmental covariates: wet-year greenness, mean curvature, elevation, insolation, and slope. Lower hillslope positions, concave landforms, and enhanced wet-year greenness were associated with more SOC, and explained 11%, 24%, and 31% of variability in 0–30 cm SOC stocks, respectively. This study demonstrates that the accuracy of regional-scale SOC mapping of California rangelands benefits from considering microclimatic and topographic controls at the catchment-scale, in addition to broader scale mineralogical and macroclimatic controls identified in previous SOC studies.

Using stable isotopes to identify major flow pathways in a permafrost influenced alpine meadow hillslope during summer rainfall period

AbstractGlobal warming has leaded to permafrost degradation, with potential impacts on the runoff generation processes of permafrost influenced alpine meadow hillslope. Stable isotopes have the potential to trace the complex runoff generation processes. In this study, precipitation, hillslope surface and subsurface runoff, stream water, and mobile soil water (MSW) at different hillslope positions and depths were collected during the summer rainfall period to analyse the major flow pathway based on stable isotopic signatures. The results indicated that (a) compared with precipitation, the δ2H values of MSW showed little temporal variation but strong heterogeneity with enriched isotopic ratios at lower hillslope positions and in deeper soil layers. (b) The δ2H values of middle‐slope surface runoff and shallow subsurface flow were similar to those of precipitation and MSW of the same soil layer, respectively. (c) Middle‐slope shallow subsurface flow was the major flow pathway of the permafrost influenced alpine meadow hillslope, which turned into surface runoff at the riparian zone before contributing to the streamflow. (d) The slight variation of δ2H values in stream water was shown to be related to mixing processes of new water (precipitation, 2%) and old water (middle‐slope shallow subsurface flow, 98%) in the highly transmissive shallow thawed soil layers. It was inferred that supra‐permafrost water levels would be lowered to a less conductive, deeper soil layer under further warming and thawing permafrost, which would result in a declined streamflow and delayed runoff peak. This study explained the “rapid mobilization of old water” paradox in permafrost influenced alpine meadow hillslope and improved our understanding of permafrost hillslope hydrology in alpine regions.

Effect of hillslope aspect on landform characteristics and erosion rates.

Understanding the effect of aspect on landform characteristics and erosion rates is an important prerequisite for soil and water conservation in hilly areas. In a cultivated area of the Chinese Loess Plateau, hillslope length, gradient and aspect (east, west, south, and north) were measured on two typical Mao (round loess hill), and net soil loss and location (upper, middle and lower positions) were studied using the 137Cs tracing loss ratio. Hillslope length on different aspects was in the order, north > west > east >south, but gradient changes were inconsistent and more complicated. Southern slopes were shorter and steeper, while on northern slopes, it was the opposite. Erosion rate on hillslopes with different aspects ranged from 1440 to 2631 t/km2 · a, and on northern slopes they were c.24-81% larger than on southern slopes. Upper and middle hillslope positions usually had higher erosion rates than lower positions. The greatest erosion rates were at upper positions on northern slopes, and upper positions on south slopes had relatively lower erosion rates. For hillslope positions influenced by wind erosion in winter and spring, the 137Cs loss ratio could be > 80%, while for the same positions on south slopes without wind erosion, it was < 80%. Our findings demonstrate that aspect is a key driver of landform characteristics and erosion rates on hillslopes, and they could be usefully employed for the prevention and control of soil erosion in this region.