Critical Zone Observatory Research Articles

Inferring Hillslope Groundwater Recharge Ratios From the Storage‐Discharge Relation

AbstractAccurate observation of hillslope groundwater storage and instantaneous recharge remains difficult due to limited monitoring and the complexity of mountainous landscapes. We introduce a novel storage‐discharge method to estimate hillslope recharge and the recharge ratio—the fraction of precipitation that recharges groundwater. The method, which relies on streamflow data, is corroborated by independent measurements of water storage dynamics inside the Rivendell experimental hillslope at the Eel River Critical Zone Observatory, California, USA. We find that along‐hillslope patterns in bedrock weathering and plant‐driven storage dynamics govern the seasonal evolution of recharge ratios. Thinner weathering profiles and smaller root‐zone storage deficits near‐channel are replenished before larger ridge‐top deficits. Consequently, precipitation progressively activates groundwater from channel to divide, with an attendant increase in recharge ratios throughout the wet season. Our novel approach and process observations offer valuable insights into controls on groundwater recharge, enhancing our understanding of a critical flux in the hydrologic cycle.

Geo-electrical anisotropy corrections derived from square array data to improve Earth resistivity models of the Shale Hills' critical zone

Geo-electric anisotropy in the shallow subsurface, often expressed as the “paradox of anisotropy”, is seldom accounted for in colinear array resistivity models of the critical zone. The paradox of anisotropy can be observed in areas of inclined bedding planes or fractures where longitudinal apparent resistivities are greater than transverse apparent resistivities, which is opposite of the true resistivities. In this study, we illustrate the paradox of anisotropy for fractured shale bedrock in Pennsylvania's Susquehanna Shale Hills Critical Zone Observatory (Shale Hills) using square and Wenner array data, and derive an anisotropy correction from the square array data, which is not influenced by the anisotropy. Square array measurements were made around a center point near the middle of a planar slope on the north-facing side of the Shale Hills watershed at four a-spacings (5, 10, 25, and 50 m), each rotated at 15° increments until a full rotation (0° to 165°) of measurements were collected. Coefficients of anisotropy, estimated from the square array data, were then applied to Wenner array data collected at the same location in the watershed to correct the anisotropic flow of current affecting the colinear (i.e., Wenner) array data. Correction of Wenner array data for profiles parallel and orthogonal to bedding strike yield favorable model results when compared to expected resistivity ranges with respect to bedding strike. We further demonstrate that application of the anisotropy corrections to several Wenner array profiles, followed by a pseudo 3D inversion of the profiles, yields a resistivity model of the subsurface providing new insights into groundwater flow through the Shale Hills' critical zone.

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.

Integrating decadal and century-scale root development with longer-term soil development to understand terrestrial nutrient cycling

Nearly 50 y ago, Walker and Syers hypothesized that sources of most terrestrial nutrients shift in dominance from mineral- to organic matter-derived over millennia as soils weather. We investigated how overlaying this soil development framework with vegetation dynamics that can feed back to soil development on relatively short timescales offers insight into ecosystem functioning. To test the hypothesis that forest nutrient economies mediate the nutritional importance of organic matter as mineral weathering proceeds, we paired litterfall decay experiments with soil mineralogical data from diverse forests across the Critical Zone (CZ) Observatory Network, USA. Our findings suggest that dominant sources of tree P may shift from organic matter-bound stocks to minerals as roots expand during the transition from mid to late stages of forest growth and encounter deeper soils that have experienced a lesser degree of weathering. Thus, plants may develop nutritional strategies that do not necessarily rely most heavily on the dominant P form present in an ecosystem, typically driven by stage of soil development, but rather on root proliferation over time, which governs the ability of plants to mine soil volumes at a diversity of depths. Ecosystem P nutrition therefore depends strongly on the interaction between dominant P form and root system growth, particularly as it reflects past land use for both plants and soils. We use these findings to produce a novel framework of vegetative nutrient economics that highlights how root system growth and land use change can influence nutrient transformations and bioavailability, and soil development, across Earth’s critical zones.

Investigating Near-Surface Hydrologic Connectivity in a Grass-Covered Inter-Row Area of a Hillslope Vineyard Using Field Monitoring and Numerical Simulations

The interplay of surface and shallow subsurface fluxes plays a critical role in controlling water movement in hillslope agroecosystems and impacting soil and plant health during prolonged dry periods, demonstrating a need for in-field monitoring. This study was conducted for two years (2021–2022) by combining field monitoring of the grass-covered inter-row area (passive wick lysimeter, surface runoff, and meteorological data), laboratory determination of soil hydraulic properties (SHPs), and numerical modeling with the aim to explore near-surface fluxes at the SUPREHILL Critical Zone Observatory (CZO) located on a hillslope vineyard. Additionally, sensitivity analysis for basic root water uptake (RWU) parameters was conducted. The model was evaluated (R2, RMSE, and NSE) with lysimeter (hillslope) and runoff (footslope) data, producing good agreement, but only after the inverse optimization of laboratory estimated hydraulic conductivity was conducted, demonstrating that adequate parameterization is required to capture the hydropedological response of erosion-affected soil systems. Results exhibit the dependence of runoff generation on hydraulic conductivity, rainfall, and soil moisture conditions. The data suggest different soil-rewetting scenarios based on temporal rainfall variability. Sensitivity analysis demonstrated that Leaf Area Index (LAI) was the most responsive parameter determining the RWU. The study offers an approach for the investigation of fluxes in the topsoil for similar sites and/or crops (and covers), presenting the methodology of self-constructed soil–water collection instruments.

Physical and biogeochemical drivers of solute mobilization and flux through the critical zone after wildfire

A nine-year time series of nutrient cation and anion concentration and efflux from three forested catchments in the Jemez River Basin Critical Zone Observatory (JRB-CZO) in northern New Mexico was used to quantify the pulse of chemical denudation resulting from varying levels stand-replacing wildfire intensity in May-June of 2013. The 3 years of pre-fire and 6 years of postfire data were also probed to shed light on the mechanisms underlying the pulsed release and the subsequent recovery. The initial large solute pulse released to the streams—K+, Ca2+, Mg2+, SO42-, Cl−, dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and total dissolved nitrogen (TDN)—was caused by leaching of hillslope ash deposits during the first monsoon storms post-fire. Debris flow following the wildfire likely redistributed much of the ash-containing sediments along streams and valley bottoms. Sustained elevated solute concentrations observed in the surface waters throughout the post-fire period relative to pre-fire baselines is consistent with these soluble materials being periodically flushed from the soils during wet seasons, i.e., snowmelt and summer monsoons. As microbial mediated reactions and biotic uptake—due to plant regrowth—recover after fire, nutrient ion export (e.g., NO3-, Cl− and SO42-) steadily decreased toward the end of the post fire period, but remained above pre-fire levels, particularly for NO3- and SO42-. Surface water concentrations of polyvalent cations (e.g., Al and Fe) decreased significantly after the fire. Our observations suggest that changes in organic matter composition after fire (e.g., increased humification index—HIX) and the presence of pyrogenic carbon may not favor organo-metal complexation and transport. Finally, differences in burn severity among the three watersheds presented in this study, provide insights of the relative impact of solute exports and resilience. The catchments that experienced high burn severity exhibited greater solute fluxes than the less severely burn. Moreover, despite these differences, toward the end of the post-fire period these surface waters presented low and similar solute effluxes, indicating system recovery. Nonetheless, magnitudes and rebounds were solute and process specific. The results of this study highlight the importance of surface and near surface physical and biogeochemical processes on the long-lasting geochemical denudation of forested catchments following wildfires of varying intensities.

Monitoring of Thick Vadose Zone Water Dynamics Under Irrigation Using a 48 m Deep Caisson at the Luancheng Critical Zone Observatory

AbstractUnderstanding soil water dynamics in the deep vadose zone (DVZ, below the root zone) of irrigated farmland is critical for estimating groundwater recharge and assessing the impacts of agricultural activities on groundwater quality. However, the state and movement of soil water in the DVZ remain poorly understood owing to the lack of in‐situ observations. Here, based on a 48‐m deep caisson (Critical Zone Observatory, CZO) established in a typical irrigated farmland in the piedmont region of the North China Plain, we extended the monitoring of soil water content and matric potential across the entire vadose zone profile. We found that a high soil water content and matric potential (close to or slightly higher than the field capacity) was maintained in the DVZ of typical irrigated cropland. The response of soil water in the DVZ to the water inputs at the ground surface was predominantly sequentially delayed with an increase in depth in the DVZ. As a result, the wetting front propagated into the soil layer to a depth of 38 m within 228 days. Moreover, high soil moisture contents and matric potentials in the DVZ resulted in a dramatic but explicable difference between the estimated average pore water velocity (0.73 m year−1) and the wetting front propagation rate (0.16 m day−1). This study demonstrates the unreported rapid response of soil water in the DVZ to water input at the ground surface under intensively irrigated cropland and could promote a better understanding of groundwater recharge processes with a thick unsaturated zone.

Thermo-hydrological modeling of temperature variations in the spring waters of a headwater catchment (Strengbach critical zone Observatory–France)

This study is geared towards evaluating the hydrological information that can be extracted from spring water temperature variations in shallow and thin aquifers of headwater catchments. A series of temperature variations (from 2013 to 2017) in four spring water sources at the Strengbach Critical Zone Observatory (CZO) was analyzed and interpreted by relying upon a coupled two-dimensional unsaturated–saturated flow and heat transfer model specifically designed for the study. Temperature variations of spring waters at the Strengbach catchment obey a seasonal non-phased and attenuated cyclicity compared with temperature inputs from the air or very shallow waters in soils. Simulation results show that, within the shallow subsurface horizons, heat transfer is mainly controlled by thermal conduction and not by fluid flow. The results also emphasize that the depth and temperature value of the thermal invariance zone are key parameters to surface temperature attenuation patterns in the first meters beneath the surface. The average maximum porosity (or saturated water content) is another important parameter impacting heat transfers in the shallow subsurface. Stronger attenuations and delays of the thermal signal occur as the porosity increases. These different results indicate that temperature variations in the spring waters, especially the attenuation and phase shift of the signals compared with incoming air-surface water temperatures, do not bring information on hydrological transfers. In very shallow systems, characteristics, such as flow velocities and water residence times, cannot be inferred from temperature variations. Nevertheless, this study suggests that spring water temperature variations could become a way of identifying the usually poorly known saturated water content and its spatial variability at the catchment scale.

Estimation of Precipitation Fraction in the Soil Water of the Hillslope Vineyard Using Stable Isotopes of Water

This paper presents research related to the estimation of the precipitation fraction in the soil water of a sloped vineyard at the SUPREHILL Critical Zone Observatory (CZO) in Zagreb, Croatia. Numerous investigations have shown that exploration of hillslope soils can be very challenging due to the existence of heterogeneity and different soil properties, as well as due to anthropogenically induced processes, which can affect precipitation infiltration and soil water flow. Within this research, physicochemical soil properties, soil water content (SWC), and isotopic composition of soil water and precipitation (δ2H and δ18O) have been examined. The isotopic signature of soil water was monitored in 24 points, at 4 depths, throughout the hillslope vineyard. Soil water isotopic composition from all monitoring points coincided with the Local Meteoric Water Line (LMWL), with almost no variability at 100 cm depth, which was consistent with the smallest variation of SWC at 80 cm depth and indicated that most of water mixing takes place in the shallower part of the hillslope. Results suggested the existence of heterogeneity, uneven erosion processes in the footslope of the observed vineyard, and different infiltration patterns. Fractions of precipitation varied significantly depending on the depth and position in the vineyard, from approximately 1% up to 98%, where more precipitation fraction has been determined in the surface and subsurface runoff. Additionally, statistical analysis and a more detailed evaluation of precipitation fractions at the 40 cm depth, where wick lysimeters are installed, have shown that Corg content is related to the silt fraction, while the first results indicate that the infiltration patterns were dependent on the common influence of all observed physicochemical properties.

Seasonal dynamics of marly badlands illustrated by field records of hillslope regolith properties, Draix–Bléone Critical Zone Observatory, South‐East France

AbstractSparsely vegetated badlands are loci of intense erosion that is sufficiently rapid to have observable effects on human timescales. Characterizing and understanding the physical weathering processes in these settings are key to predicting the temporal variability of regolith production and sediment flux, as well as their evolution under changing climate conditions. Here, we study intra‐annual changes of hillslope properties and explore the relationship between sediment production and transport in steep marly badland catchments of the Draix–Bléone Critical Zone Observatory (SE France), where decades‐long monitoring records show rapid morphological changes. There is evidence for seasonal dynamics of these badlands, but characterization and quantification of physical weathering processes have been lacking up to now. We explore this gap by monitoring key regolith parameters including grain‐size distribution (characterized by D50), surface resistance and water content in the regolith layer (surface to ∼10 cm depth) at different locations, through repeated field surveys over a 2.5‐year period. While water content appears to be directly controlled by the last previous rainfall event, the cyclic annual pattern in observed D50 suggests that loose and finely fragmented regolith is mainly produced and accumulates during the winter season, whereas sediment transport is dominant during spring–summer. This dynamic reduces regolith thickness and induces coarsening of hillslope surface material between early spring and autumn. Principal component analysis (PCA) highlights the strong correlation between resistance and D50. We therefore suggest that D50 provides the best proxy of regolith weathering in these marls. The spatial variability of the regolith was analysed through a geophysical profile, highlighting distinct behaviour depending on slope aspect. However, the distribution of slope angles is independent of aspect at the catchment scale. These results corroborate the strong annual dynamics of these catchments, where hillslopes and gullies are drained during spring and early summer high‐intensity precipitation events, inducing high sediment yields.

Investigation of Hillslope Vineyard Soil Water Dynamics Using Field Measurements and Numerical Modeling

Soil heterogeneities can impact hillslope hydropedological processes (e.g., portioning between infiltration and runoff), creating a need for in-depth knowledge of processes governing water dynamics and redistribution. The presented study was conducted at the SUPREHILL Critical Zone Observatory (CZO) (hillslope vineyard) in 2021. A combination of field investigation (soil sampling and monitoring campaign) and numerical modeling with hydrological simulator HYDRUS-1D was used to explore the water dynamics in conjunction with data from a sensor network (soil water content (SWC) and soil-water potential (SWP) sensors), along the hillslope (hilltop, backslope, and footslope). Soil hydraulic properties (SHP) were estimated based on (i) pedotransfer functions (PTFs), (ii) undisturbed soil cores, and (iii) sensor network data, and tested in HYDRUS. Additionally, a model ensemble mean from HYDRUS simulations was calculated with PTFs. The highest agreement of simulated with observed SWC for 40 cm soil depth was found with the combination of laboratory and field data, with the lowest average MAE, RMSE and MAPE (0.02, 0.02, and 5.34%, respectively), and highest average R2 (0.93), while at 80 cm soil depth, PTF model ensemble performed better (MAE = 0.03, RMSE = 0.03, MAPE = 7.55%, R2 = 0.81) than other datasets. Field observations indicated that heterogeneity and spatial variability regarding soil parameters were present at the site. Over the hillslope, SWC acted in a heterogeneous manner, which was most pronounced during soil rewetting. Model results suggested that the incorporation of field data expands model performance and that the PTF model ensemble is a feasible option in the absence of laboratory data.

Leached Copper Correlation with Dissolved Organic Carbon in Sloped Vineyard Soil

The solubility and mobility of copper (Cu) in soil is strongly influenced by the presence of dissolved organic carbon (DOC); however, the interactions between Cu and DOC are complex and not yet fully understood. In this study, Cu and DOC concentrations were measured monthly for two years in leachates from self-constructed lysimeters installed at inter- and intra-row vineyard hilltop, backslope, and footslope areas at the SUPREHILL Critical Zone Observatory, Croatia. The aim was to quantify Cu and DOC leaching from the hilltop towards the backslope and the footslope. The assumed strong relationship between Cu and DOC in the leachates was statistically analyzed and explained using chemical equilibrium software. Leachates were analyzed for pH, EC, DOC, Cu, and major ion concentrations. The highest Cu concentrations found in leachates from the intra-row footslope suggested Cu downhill transport. Although not strong, a significant positive correlation between Cu and DOC in footslope leachates confirmed the relevance of Cu complexation by DOC. Speciation confirmed that more than 99.9% of total Cu in leachates was found as a Cu-DOC complex. Data implied the role of soil water flow pathways in explaining Cu downhill transport. Critical timing for applying Cu fungicides at sloped vineyards was highlighted.

Low rates of rock organic carbon oxidation and anthropogenic cycling of rhenium in a slowly denuding landscape

AbstractThe oxidation of petrogenic organic carbon (OCpetro) is a source of carbon dioxide to the atmosphere over geological timescales. The rates of OCpetro oxidation in locations that experience low rates of denudation remain poorly constrained, despite these landscapes dominating Earth's continental surface area. Here, we track OCpetro oxidation using radiocarbon and the trace element rhenium (Re) in the deep weathering profiles, soils and stream waters of the Susquehanna Shale Hills Critical Zone Observatory (PA, USA). In a ridge‐top borehole, radiocarbon measurements reveal the presence of a broad OCpetro weathering front, with a first‐order assessment of ~40% loss occurring over ~6 m. However, the low OCpetro concentration (< 0.05 wt%) and inputs of radiocarbon throughout the deepest parts of the profile complicate the assessment of OCpetro loss. The OCpetro weathering front coincides with a zone of Re depletion (~90% loss), and we estimate that > 80% of Re in the rock is associated with OCpetro, based on Re/Na and Re/S ratios. Using estimates of long‐term denudation rates, the observed OCpetro loss and the Re proxy are equivalent to a low OCpetro oxidation yield of < 1.7 × 10−2 tC km−2 yr−1. This is consistent with the low OCpetro concentrations and low denudation rates at this location. In addition, we find the surface cycle of Re is decoupled from that of deep weathering, with an enrichment of Re in surface soils and elevated Re concentrations in stream water, precipitation, and shallow groundwater. A mass balance model shows that this can be explained by a historical anthropogenic contribution of Re through atmospheric deposition. We estimate that the topsoil Re pool could take decades to centuries to deplete and call for a renewed focus on anthropogenic perturbation of the surface Re cycle in low denudation rate settings.

Quantification of Intra- vs. Inter-Row Leaching of Major Plant Nutrients in Sloping Vineyard Soils

Nutrient leaching from agricultural soils presents an economic loss for farmers and can degrade the quality of the surrounding environment. Thus, leachates from 18 in situ wick lysimeters, installed at 40 cm soil depth at the vineyard hilltop, backslope, and footslope intra- and inter-row area (SUPREHILL Critical Zone Observatory, Croatia) were collected monthly over two years and analyzed for major plant nutrient ions. Our objectives were to quantify nutrient losses via leaching from the hilltop towards the backslope and to the footslope, and to compare leaching from vine plant rows (intra-row) with grassed areas between vine rows (inter-row). We found that the concentrations of nitrate, orthophosphate, and potassium were significantly higher in leachates collected at the footslope as compared to the hilltop and backslope only at intra- and not at inter-row positions, while ammonium was independent of the slope and row positions. The vineyard intra-row is identified as the probable spatial origin of nutrient leaching along the slope, thus confirming spatially different contributions of overall hillslope to major plant nutrients leaching. The experimental field scheme used in this study, which separately analyses vineyard intra- and inter-row, was confirmed to be an adequate approach for optimizing vineyard management practices.

Alterations of ecosystem nitrogen status following agricultural land abandonment in the Karst Critical Zone Observatory (KCZO), Southwest China.

Secondary succession after agricultural land abandonment generally affects nitrogen (N) cycle processes and ecosystem N status. However, changes in soil N availability and NO3 - loss potential following secondary succession are not well understood in karst ecosystems. In the Karst Critical Zone Observatory (KCZO) of Southwest China, croplands, shrub-grass lands, and secondary forest lands were selected to represent the three stages of secondary succession after agricultural land abandonment by using a space-for-time substitution approach. The contents and 15N natural abundance (δ 15N) of leaves, soils, and different-sized aggregates at the three stages of secondary succession were analyzed. The δ 15N compositions of soil organic nitrogen (SON) in aggregates and soil to plant 15N enrichment factor (EF = δ 15Nleaf -δ 15Nsoil), combined with soil inorganic N contents and δ 15N compositions were used to indicate the alterations of soil N availability and NO3 -loss potential following secondary succession. Leaf N content and SON content significantly increased following secondary succession, indicating N accumulation in the soil and plant. The δ 15N values of SON also significantly decreased, mainly affected by plant δ 15N composition and N mineralization. SON content in macro-aggregates and soil NH4 + content significantly increased while δ 15N values of NH4 + decreased, implying increases in SON stabilization and improved soil N availability following secondary succession. Leaf δ 15N values, the EF values, and the (NO3 --N)/(NH4 +-N) ratio gradually decreased, indicating reduced NO3 - loss following secondary succession. Soil N availability improves and NO3 - leaching loss reduces following secondary succession after agricultural land abandonment in the KCZO.

Nitrogen dynamics in the Critical Zones of China

Nitrogen dynamics at ecosystem levels profoundly impact the Earth’s surface system due to their environmental and ecological significance. Exploring the sources and transformation of nitrogen in various Critical Zones is vital to understanding biogeochemical cycles and sustainable development. This study summarized nitrogen characteristics in soil profiles and nitrogen dynamics in diverse terrestrial ecosystems based on data from typical Critical Zones of China. The results indicated that nitrogen accumulates in the deep soils of cropland ecosystems due to intensive fertilizer applications, which potentially harms soil functions and water quality. Therefore, it is necessary and meaningful to take adequate measures to alleviate nitrogen accumulation in deep soils. Additionally, surplus nitrogen transported into groundwater and riverine systems from soil has emerged as an important issue for environmental management. There are serious nitrogen pollution issues in many river water and groundwater areas, which could be addressed by reducing the fast leaching and considerable nitrogen accumulation in the vadose zone. Systematic and long-term observational studies are needed to achieve the ultimate goal of ecological conservation and high-quality development. Therefore, future research should consider monitoring and evaluating ecosystems based on the long-term Critical Zone Observatories networks to advance appropriate environmental management strategies that adapt to nature’s rules and strengthen the ecosystem service function for sustainable development.

Hydrogeochemical modeling of the spatiotemporal variations in 87Sr/86Sr isotope ratios and Sr concentrations of spring waters in a headwater catchment (Strengbach CZO–France)

The variations in strontium (Sr) isotope ratios in river waters at a regional scale are usually discussed in terms of lithological diversity constituting the watershed bedrock. To discuss the origin of the spatiotemporal variations in Sr concentrations and isotopic compositions of spring waters at the headwater watershed scale, we analyzed the 87Sr/86Sr and 143Nd/144Nd isotopic ratios of the main constitutive minerals of the granitic bedrock of the Strengbach Critical Zone Observatory (CZO) and developed coupled hydrogeochemical modeling integrating the Sr concentration and isotopic composition. The simulation results show that the long-term (1990–2010) decrease in Sr concentrations in the CS springs of the Strengbach catchment can be explained by the response of mineral weathering along the water pathway to the pH increase that affected the incoming solutions to the weathering system during the same period. This pH increase reduces the water path proportion marked by a high flux of apatite and biotite dissolution. Thus, it reduces the Sr concentration of the outgoing spring waters. The different responses of the various CS springs to the incoming solutions' pH increase can be explained by the more or less long residence time of the water in the bedrock for the different springs. The analytical data also confirm that the 87Sr/86Sr isotope ratios of the CS springs varied little or not at all on average from 1990 to 2010. In contrast, they varied significantly during a hydrological cycle and from one spring to another. The constancy of the average 87Sr/86Sr isotope ratios over several years is explained by a granitic bedrock water path sufficiently long for soil solution pH variations to no longer influence the emergence of the spring. The simulation results also show that the isotopic variations observed in the CS springs during a hydrological cycle can be explained by differences in the transit time of the water in the substratum between high and low waters and the modifications that they entail on the degree of saturation of the water with respect to biotite and apatite. These differences do not need to be explained by differences in the mineralogy of the rocks drained by waters between high and low discharge periods. Similarly, the different average isotope ratios from one spring to another can be explained by the more or less long transit times in the bedrock of the waters feeding these different springs. Again, it is not necessary to explain them by local lithological differences from one spring to another. These different results highlight that at the scale of an upstream basin such as the Strengbach, the Sr isotopic variations observed in the CS springs should not systematically indicate lithological differences in the rock. The development of coupled isotope modeling approaches should help to disentangle the part of this process relating to hydrological processes in the isotopic variability of the spring waters.

Middle Risle River Critical Zone Observatory: monitoring karst-process evolution and its impact on surface water and on aquatic ecosystems

Abstract Rivers in karstic environments are known to be greatly influenced by surface water–groundwater interactions, with significant localized inflows during floods from springs, or with losses that can dry up rivers. The Middle Risle River is frequently affected by the development of sinkholes in a chalk karst area (Normandy, France). In the 2010s, two new major sinkholes in the riverbed caused a complete loss of water into the underlying phreatic aquifer, causing the river to dry up over several kilometres. The resulting changes in hydrogeological processes and surface water–groundwater interaction greatly affected water quality, water use and water-dependent ecosystems, causing a political crisis in this river-dependent touristic valley. To understand these phenomena and improve crisis management, the Middle Risle Critical Zone Observatory was set up to enhance monitoring, surveying and/or modelling of groundwater and river levels, river and spring flow, water temperature and conductivity, and ecosystem characteristics (fish, macro-invertebrates and vegetation). The results showed notable impacts on fish, macro-invertebrates and vegetation, some plants proving to be reliable indicators of surface-water–groundwater interaction. The dynamics of local hydrogeological processes were assessed and linked to the measured effects on ecosystems and water supply. Inverse modelling based on an analytical solution of the diffusive wave equation assessed lateral flow during floods, quantifying the spatial–temporal variability of surface-water and groundwater exchanges. It also highlighted the important role of karst zones in both storage and flood-peak attenuation processes, thereby protecting downstream villages against floods.

Bioavailability of phosphorus to loblolly pine and red maple in clay and saprolite from the southeastern Piedmont, USA

AbstractSoil P is present in multiple forms through the profile with plants potentially accessing different P pools at different depths through distinct pathways. Understanding P pool distribution and dynamics is necessary for sustainable forest management. Our research investigates soil P dynamics in clay (at 60–100 cm depth) and saprolite (at 450–500 cm depth) of the Calhoun Critical Zone Observatory (CCZO) in South Carolina through a 1‐yr greenhouse planting of loblolly pine (Pinus taeda L.) and red maple (Acer rubrum L.). We compared total plant P uptake to changes in soil P pools defined by Mehlich 3 (MIII) P, Hedley P, or total P fractions and to indices of potential iron (Fe) reduction and phosphatase activity as potential pathways of P release. We hypothesized that (a) the soil content of MIII P would be insufficient to meet plant demand; (b) ectomycorrhizal (EM) pine to differ from arbuscular mycorrhizal (AM) maple in accessing organic P pools; and (c) Fe reduction to correlate with declines in NaOH P (associated with Fe‐bound P) with greater decreases in the Fe‐oxide‐rich clay layer. Our results indicated MIII extractable P is plant available but present in insufficient quantities to meet total demand. In Hedley P pools, resin Pi and NaOH Pi supplied the most P for plant growth, and stable P pools (concentrated HCl P and residual P) buffered losses from these P pools. Plant‐available P in these soils seems sufficient to meet long‐term plant demand particularly if deeply rooted trees grow into the saprolite that had higher available soil P supply.

Estimating solute travel times from time series of nitrate concentration in groundwater: Application to a small agricultural catchment in Brittany, France

Stream flow and concentration dynamics are usually supposed to integrate the diverse hydrological and biogeochemical processes occurring within catchments and their variability in space and time. Proxies like transit times and young water fractions are often used to understand this variability. However, this procedure might not be suitable in catchments with ephemeral streams. The objective of this paper is to test the hypothesis that in catchments where groundwater flow is the main source of water output, solute travel times can be inferred from the monitored groundwater levels and solute concentration time series of piezometer networks. We used data from a well-studied small agricultural catchment in Brittany, belonging to the French network of Critical Zone Observatories. The data set includes long-term nitrate concentration time-series measured both at the stream outlet and the network of piezometers. We applied a parsimonious, conceptual dual-permeability mixing modelto calibrate our dataset and found that the nitrate travel times estimated in mid to upslope piezometers were consistently higher than the one at the stream outlet. We further observed an asynchronicity in seasonal concentration-discharge dynamics between the piezometers and the stream. We hypothesized that either due to riparian denitrification and uptake by vegetation, a pronounced seasonality is being generated which is making our model underestimate the nitrate travel times at the stream, or the piezometers are acting as proxies to the slower conceptual store of our mixing model, and shallow subsurface flow or some hydrologically significant heterogeneities are contributing to the faster route which is not tapped into by the piezometer network. Latter hypothesis is further supported by the fact that nitrate travel velocity along two different transects are in agreement with the slower conceptual store of our model. Nevertheless, we established that to specifically estimate the groundwater travel time of a catchment, a piezometer free from riparian influence alongside a stream is very beneficial.