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Accurate estimation of hydrogeological parameters is essential for groundwater modeling but largely depends on the quality of tracer data. Traditional tracers, including dyes and ions, are constrained by signal interference, a narrow selection of options, and poor detection sensitivity. This study investigated synthetic DNA fragments as high‐resolution tracers in replicated two‐dimensional sandbox experiments. Eight uniquely sequenced DNA tracers were co‐injected with dye tracers and assessed using two frameworks: tracer travel‐time inversion (TTTI) and a two‐step approach combining TTTI with hydraulic tomography. DNA tracers produced clearer breakthrough signals and achieved consistently higher accuracy (R2 values of 0.75 and 0.75 for twice TTTI; 0.79 and 0.76 for twice two‐step approach), whereas dye tracers yielded lower and more variable R2 values (0.64 and 0.59; 0.75 and 0.56). These findings demonstrate that DNA tracers, with high specificity and sensitive detection, enable a robust pathway for improving hydrological parameter inversion.

Current models on solute transport often fail to reproduce discharge‐dependent behavior of solute transport in stream reaches because they rely on the assumption of well‐mixed conditions and fail to account for the complex coupling between in‐stream and subsurface flow. StorAge Selection (SAS) functions describe outflow as a mixture of waters of different ages, providing a framework to overcome the well‐mixed assumption in “traditional” transport models. In this study, we applied SAS functions to model solute transport from 13 slug tracer experiments conducted under varying hydrological conditions in a headwater stream reach. Using SAS function parameters (expressed in units of volume) together with measurements of groundwater (GW) levels and streambed microtopography, we partitioned the total water storage within the study reach into distinct components: advective storage, in‐stream transient storage, tracer‐derived hyporheic storage, and GW level‐derived hyporheic storage. This partitioning assumes that transport processes and subsurface water flow in stream reaches are associated with different storage volumes. We found positive linear relationships between discharge and age‐ranked, advective, and tracer‐derived hyporheic storage. In‐stream transient storage increased with discharge up to 17 L s−1, corresponding to the discharge threshold above which streambed sediments became completely submerged, and declined at higher flows. This pattern likely reflects the contribution of eddies at lower discharge levels and highlights the importance of in‐stream transient storage for solute transport. Our results demonstrate that partitioning the total water storage in a reach–enabled only through applying SAS functions–is essential for understanding and modeling solute transport under varying hydrological conditions.

Penetrating fronts play an important role in the cross-shelf transport of terrestrial materials in the East China Sea (ECS). Using long-term satellite remote sensing data and numerical simulation data, the most likely period of occurrence and region of the penetrating fronts in the western ECS are analyzed in this study, and the evolutionary process and mechanism are also investigated. The statistical results of satellite-derived chlorophyll data from 1998 to 2022 (25 years) reveal that penetrating fronts occur most frequently near 27°N in winter, with the frequencies of occurrence in January, February and March being 47%, 65% and 64%, respectively. Backward Lagrangian tracer experiments demonstrate that the penetrating water near 27°N originate from three different regions. The core penetrating low-salinity water originates from the Zhejiang-Fujian coast, while the northern marginal water originates from the northern part of Taiwan Island, and the southern marginal water originates from the central Taiwan Strait. Mechanism analysis reveals that the core penetrating low-salinity water evolves in three successive stages. First, under the influence of the Zhejiang-Fujian Coastal Current driven by strong northeasterly winds, the low-salinity water along the Zhejiang-Fujian coast moves southwestward parallel to the coastline and accumulates in the western Taiwan Strait. Second, during the relaxation stage of the northeasterly wind, the northeastward Taiwan Strait Current strengthens, and the low-salinity water accumulated in the western Taiwan Strait moves to the northeast. Third, when the northeastward-moving lowsalinity water runs into the Western Kuroshio Branch off the northeastern Taiwan Island, the low-salinity water is rapidly stretched eastward, and a significant penetrating front eventually appears. Since the synoptic northeasterly wind and the Western Kuroshio Branch are dominant dynamic factors in this region during wintertime, we believe that the synoptic wintertime penetrating fronts near 27°N are common and important phenomena that play important roles in the cross-shelf transport of terrestrial materials in the ECS.

To investigate the hydrogeological characteristics of the karst trough zone in China, two groundwater systems (Yuquandong [YQD] and Migongquan [MGQ]) in Sixi Valley, western Hubei, were discussed. Seven groups of tracer experiments were conducted, with consideration of regional topography, geology and hydrogeology. By means of high-resolution continuous monitors, tracer breakthrough curves (BTC) of two systems were obtained and it was found that karst conduits of YQD system were more developed. From their hydrological and hydrochemical performance, two different systems were verified with relatively independent flow paths. Additionally, boundaries of the two systems were confirmed, sub-regions of recharge were delineated, and five conceptual models of groundwater recharge were determined, which are typical of the karst trough zone of China.

Carbon (C) allocation strongly influences plant and soil processes. Short-term C allocation dynamics in ecosystems and their responses to environmental changes are still poorly understood. Using in situ 13CO2 pulse labeling, we studied the effects of 1 wk of shading on the transfer of recent photoassimilates between sugars and starch of above- and belowground plant organs and to soil microbial communities of a mountain meadow. C allocation to roots and microbial communities was rapid. Shading strongly reduced sucrose and starch concentrations in shoots, but not roots, and affected tracer dynamics in sucrose and starch of shoots, but not roots: recent C was slowly incorporated into root starch irrespective of the shading treatment. Shading reduced leaf respiration more strongly than root respiration. It caused no reduction in the amount of 13C incorporated into fungi and Gram-negative bacteria, but increased its residence time. These findings suggest that, under interrupted C supply, belowground C allocation (as reflected by the amount of tracer allocated to root starch, soil microbial communities and belowground respiration) was maintained at the expense of aboveground C status, and that C source strength may affect the turnover of recent plant-derived C in soil microbial communities.

In agro-urban environments, the water resource conveyed by rural channels is susceptible to a gradual impoverishment due to the continuous combined sewer overflow release, constituting a pending and urgent issue for water management companies and the entire community. Reliable one-dimensional longitudinal dispersion coefficients D are required to model and study the hydrodynamics and water quality patterns at the scale of rural channel networks. Empirical formulas are usually adopted to estimate D but the accuracy in the prediction could be questionable. In order to identify which are the most suitable formulas to determine D in rural channels, field tracer measurements were carried out in three rural channels with typical geometry and configuration. The obtained D values were then compared with the most commonly used predicting formulas that the literature provides. The accuracy of the predictors was further checked by simulating different flow rates inside the tested channels by using a one-dimensional hydraulic model. Starting from the obtained results, indications and guidelines to choose the most suitable formulas to predict D in rural channels were provided. These indications should be followed when developing realistic quality models in the agro-urban environments, especially in those cases where direct measurements of the longitudinal dispersion coefficient D are not available.Article highlightsLongitudinal dispersion coefficient measurements by means of field tracer experiments in different rural channels.Field data prediction by using different formulas present in the literature.Definition of guidelines on the most reliable formulas to be used in the rural channels context.

Heterogeneous flow pathways through the soil determine the transport of dissolved and particle-bound nutritional elements like phosphorus (P) to ground and surface waters. This study was designed to understand the spatial patterns of P in agriculturally used soils and the mechanisms causing P accumulation and depletion at the centimetre scale. We conducted dye tracer experiments using Brilliant Blue on a loamy Stagnosol in North-Eastern-Germany. The plant-available P was analysed using double lactate extraction (DL-P). The plant-available P content of the topsoil was significantly higher than that of the subsoil in all three replicates ( p  < 0.001). The topsoil’s stained areas showed significantly higher P contents than unstained areas ( p  < 0.05), while the opposite was found for the subsoil. The P content varied enormously across all observed soil profiles (4 to 112 mg P kg −1  soil) and different categories of flow patterns (matrix flow, flow fingers, macropore flow, and no visible transport pathways). The P contents of these transport pathways differed significantly and followed the order: P matrix flow  > P finger flow  > P no visible transport pathways  > P macropore flow . We conclude that P tends to accumulate along flow pathways in the topsoil in the observed fertilized and tilled mineral soil. In contrast, in the subsoil at a generally lower P level, P is depleted from the prominent macroporous flow domains.

The specification of inlet boundary conditions plays a critical role in computational fluid dynamics (CFD) simulations of pollutant dispersion from nuclear facilities, particularly in regions characterized by uneven terrain. Previous studies have often simplified such terrain by approximating it as a flat surface to reduce computational complexity. However, this approach fails to adequately capture the realistic atmospheric boundary layer dynamics inherent to uneven topographies. To address this limitation, this study conducted atmospheric dispersion tracer experiments specifically designed for nuclear facilities situated on non-uniform terrain. A novel inlet boundary condition, termed the Atmospheric Boundary Layer of Uneven Terrain (ABLUT), was developed by modifying the existing atmBoundaryLayer model in OpenFOAM. Numerical simulations were performed using both the default and the proposed ABLUT boundary conditions, incorporating different turbulence models and examining the influence of turbulent Schmidt numbers across a range of 0.3 to 1.3. The results demonstrate that the ABLUT boundary condition, particularly when coupled with a turbulent Schmidt number of 0.7 and the SST k−ω turbulence model, yields the closest agreement with experimental tracer dispersion data. Notably, comparative analyses between the default and improved models revealed significant discrepancies in near-surface wind speed profiles, with deviations becoming increasingly pronounced at higher elevations. Numerical simulations were conducted to assess the ground-level distribution of Total Effective Dose Equivalent (TEDE) for four typical radionuclides (3H, 14C, 85Kr and 129I) emitted from nuclear facilities under both higher and lower wind speed conditions. Results demonstrate that the TEDE maxima across all scenarios remain orders of magnitude below regulatory annual limits. These findings provide critical insights for enhancing the accuracy of wind field simulations in the vicinity of nuclear facilities located on uneven terrain, thereby contributing to improved risk assessment and environmental impact evaluations.

Nitrification in the rhizosphere is a crucial process in controlling nitrogen‐use efficiency (NUE) in flooded paddy soils. To understand the relationship between the nitrification ability of the rhizosphere and NUE, pot experiments using 15N tracer technique were conducted to investigate the impacts of the rhizosphere soil net nitrification rate on NUE and denitrification losses at different rice growth stages in two paddy soils, which were sampled from Jurong (JR) and Yancheng (YC) in Jiangsu Province in China. The results showed that the nitrification rate in JR rhizosphere soil was lower than in YC rhizosphere soil at all rice growth stages. The abundance of ammonia‐oxidizing bacteria (AOB), ammonia‐oxidizing archaea (AOA), and pH in YC rhizosphere soils were always higher than in JR rhizosphere soils. Rice yield, biomass, NUE, leaf glutamine synthetase (GS) activity, and nitrate reductase (NR) activity were higher in JR soils with low nitrification rates than in YC soils with high nitrification rates (p < 0.05). In contrast, denitrification loss from JR soil (12.69%–23.41%) was lower than that from YC soil (26.83%–40.98%; p < 0.05) for each rice growth stage. The biomass and NUE decreased significantly as the net nitrification rate, the abundance of AOA and AOB of both the JR and YC rhizosphere soils increased (p < 0.05), and the denitrification loss was enhanced as the rhizosphere nitrification rate increased in the JR and YC soils (p < 0.05) during the rice growth period. The rhizosphere‐dominant AOB community Nitrosospira is the key factor affecting the nitrification rate and then decreasing rice NUE. In general, the rhizosphere nitrification rate in paddy soils is a primary factor controlling the rice NUE and denitrification loss. The impacts of the rhizosphere soil nitrification rate on NUE and denitrification losses at different rice growth stages in two paddy soils were investigated. Rhizosphere nitrification played a dominant role in increasing denitrification and decreasing NUE. Rhizosphere nitrification rate was affected by soil pH and nitrifying microbial community composition.

Afforestation is globally increasing to produce timber and pulp wood, but also to enhance ecosystem services such as carbon sequestration, nutrient retention or groundwater recharge. In China, large areas have been and will be afforested in order to compensate for the negative impacts of former clear‐cuttings and to make use of the ecosystem services associated with afforestation. In order to further optimize these services with regard to balanced nutrient (particularly nitrogen) cycles, it is important to know whether the use of mixtures of native tree species in afforestation projects promotes the acquisition and retention of nitrogen compared with the currently established large‐scale monocultures. To test the effect of species richness on system N retention and tree sapling N uptake, we conducted a ¹⁵N tracer experiment in a young tree plantation. To this end, saplings of four abundant early successional tree species were planted in monocultures, in two‐ and four‐species mixtures and as single trees. Nitrogen retention increased with higher species richness due to enhanced N pools in sapling biomass. These species richness effects strengthened over time. Species‐specific differences in ¹⁵N recoveries over time revealed below‐ground niche differentiation with regard to N uptake, which is likely to result in complementary resource use among coexisting species. Synthesis and applications. This study provides evidence that mixed afforestation promotes N retention from the sapling stage. To further improve ecosystem services associated with afforestation, we strongly suggest the use of mixtures of native tree species instead of monocultures. Mixtures of four species may reduce system N losses and thus may lessen groundwater contamination due to N leaching. We encourage further investigations to find optimal species combinations that promote a wide range of ecosystem services related to more closed nutrient cycles and minimized soil erosion. In our study, the plantations' capability to retain N could be optimized by means of both increasing tree species richness and by choosing the optimal species combinations.