Deep nitrate accumulation in typical black soil critical zones of Northeast China.

Deep nitrate accumulation below 1 m has been observed in various soil regions, yet remains undocumented in the black soil (mainly Phaeozems and Chernozems) region. Climatic and edaphic factors likely influence deep nitrate accumulation on a large scale, although existing studies primarily focus on individual sites. In order to evaluate the distribution and controlling factors of deep nitrate in the black soil region, inorganic nitrogen forms and regolith properties of nine boreholes spanning humid, semi-humid, and semi-arid areas in Fujin, Hailun, and Lindian in northeast China were analyzed down to a depth of 10 m. The results revealed significant nitrate accumulation in Lindian, peaking at 11.03 mg N kg-1 at a depth of 3 m underground. Nitrate storage from the land surface to a depth of 10 m in Lindian ranged from 459.65 kg N ha-1 to 1072.88 kg N ha-1, with over 70 % of nitrate stored below 1 m. Nitrate accounted for 97.74 % of the total N stock in Lindian. Ammonium accumulation has been observed at a deeper depth in Hailun, with no nitrate accumulation detected in Hainlun and Fujin. Regolith properties such as clay, silt, sand, and pH playing a crucial role in reshaping the vertical pattern of nitrate. The presence of nitrate pools at greater depths in intensively managed black soil regions should be taken into account for the sustainable utilization of soil resources and the mitigation of groundwater pollution risks.

Genome-Resolved Metagenomics and Denitrifying Strain Isolation Reveal New Insights into Microbial Denitrification in the Deep Vadose Zone.

The heavy use of nitrogen fertilizer in intensive agricultural areas often leads to nitrate accumulation in subsurface soil and nitrate contamination in groundwater, which poses a serious risk to public health. Denitrifying microorganisms in the subsoil convert nitrate to gaseous forms of nitrogen, thereby mitigating the leaching of nitrate into groundwater. Here, we investigated denitrifying microorganisms in the deep vadose zone of a typical intensive agricultural area in China through microcosm enrichment, genome-resolved metagenomic analysis, and denitrifying bacteria isolation. A total of 1000 metagenome-assembled genomes (MAGs) were reconstructed, resulting in 98 high-quality, dereplicated MAGs that contained denitrification genes. Among them, 32 MAGs could not be taxonomically classified at the genus or species level, indicating that a broader spectrum of taxonomic groups is involved in subsoil denitrification than previously recognized. A denitrifier isolate library was constructed by using a strategy combining high-throughput and conventional cultivation techniques. Assessment of the denitrification characteristics of both the MAGs and isolates demonstrated the dominance of truncated denitrification. Functional screening revealed the highest denitrification activity in two complete denitrifiers belonging to the genus Pseudomonas. These findings greatly expand the current knowledge of the composition and function of denitrifying microorganisms in subsoils. The constructed isolate library provided the first pool of subsoil-denitrifying microorganisms that could facilitate the development of microbe-based technologies for nitrate attenuation in groundwater.

Tracing the Sources and Fate of NO3- in the Vadose Zone-Groundwater System of a Thousand-Year-Cultivated Region.

Excess nitrate (NO3-) loading in terrestrial and aquatic ecosystems can result in critical environmental and health issues. NO3--rich groundwater has been recorded in the Guanzhong Plain in the Yellow River Basin of China for over 1000 years. To assess the sources and fate of NO3- in the vadose zone and groundwater, numerous samples were collected via borehole drilling and field surveys, followed by analysis and stable NO3- isotope quantification. The results demonstrated that the NO3- concentration in 38% of the groundwater samples exceeded the limit set by the World Health Organization. The total NO3- stock in the 0-10 m soil profile of the orchards was 3.7 times higher than that of the croplands, suggesting that the cropland-to-orchard transition aggravated NO3- accumulation in the deep vadose zone. Based on a Bayesian mixing model applied to stable NO3- isotopes (δ15N and δ18O), NO3- accumulation in the vadose zone was predominantly from manure and sewage N (MN, 27-54%), soil N (SN, 0-64%), and chemical N fertilizer (FN, 4-46%). MN was, by far, the greatest contributor to groundwater NO3- (58-82%). The results also indicated that groundwater NO3- was mainly associated with the soil and hydrogeochemical characteristics, whereas no relationship with modern agricultural activities was observed, likely due to the time delay in the thick vadose zone. The estimated residence time of NO3- in the vadose zone varied from decades to centuries; however, NO3- might reach the aquifer in the near future in areas with recent FN loading, especially those under cropland-to-orchard transition or where the vadose zone is relatively thin. This study suggests that future agricultural land-use transitions from croplands to orchards should be promoted with caution in areas with shallow vadose zones and coarse soil texture.

Evaluating nitrate transport and accumulation in the deep vadose zone of the intensive agricultural region, North China Plain.

Evaluation of the nitrate transport process in the deep vadose zone (DVZ) is important for groundwater quality management, especially in intensive agricultural regions, such as the North China Plain (NCP). The NCP produces ~20% of the total food grain in China, owing to timely groundwater irrigation and excessive chemical N fertilizer applications, and faces severe groundwater environmental degradation. This study evaluated the potential impacts of intensive agriculture on groundwater quality by investigating nitrate accumulation and transport in the DVZ of wheat-maize double-cropping field based on sediment sampling (maximum depth of 45.2 m) over three sub-regions of the NCP. The results showed that legacy nitrate­nitrogen (NO3--N) accumulated in the DVZ ranged from 118.5 to 6302.8 kg N ha-1 across the NCP; it increased with depth at an average rate of ~157 kg ha-1 m-1. Nitrate transport and accumulation in the DVZ were spatially varied and mainly controlled by the DVZ sediment textures, in addition to water and nitrogen inputs from the ground surface. Coarse sediments retained lower soil water content, resulting in less nitrogen storage; however, they provided greater nitrate transport velocity. Higher transport velocities observed in the alluvial-proluvial fan allowed chemical N fertilizer to reach the water table. However, in other regions, nitrate transport velocities were lower than the water table decline rates, implying that groundwater quality may not have been impaired by chemical N fertilizer. Furthermore, a reductive environment was identified in some areas with fine sediments, indicating a favorable environment for denitrification in the DVZ. The findings of the current study could provide an important foundation for groundwater quality management in agricultural areas, such as the NCP and similar regions.

Potential groundwater recharge from deep drainage of irrigation water.

Knowledge of soil water dynamics in the deep vadose zone provides valuable information on the temporal and spatial variability of groundwater recharge. However, semi-arid climate can complicate how the input of water, such as irrigation, can contribute to potential groundwater recharge. This study assessed the recharge rates and their timing under irrigated cropland from a semi-arid region of northern Iran. A deep drainage (10 m) experiment was performed and in situ soil water content was measured to analyze the soil water dynamics and model hydraulic parameters using HYSDRUS-1D. The best parameters selected from inverse parameter optimization were used to calibrate model and estimate the long-term (20-year) average groundwater recharge and the influence of the root zone, unsaturated zone and the time scale on the recharge processes. The simulated annual flux ranged from 24 mm to 268 mm (mean of 110 mm) at 2-m depth and ranged between 26 mm to 207 mm (mean of 95 mm) at the 10-m depth. High fluxes, observed between December and April, may be the result of greater precipitation combined with the irrigation return flow. The May-October period showed a gradual decrease in flux at the depth of 2 m. At the depth of 10 m, the flux showed some continuity (base flux) during the long-term recharge simulation. In total, 12.7% of the input water contributed to the recharge of the groundwater. The annual soil water fluxes were almost similar irrespective of depth below the root zone and the flux rates did not show any clear relation between the different components of the water budget at any depth. This approach improved our understanding of the recharge process in the deep vadose zone in a semiarid region and can help for the development of effective management of groundwater resources.

Direct observations of rock moisture, a hidden component of the hydrologic cycle.

Recent theory and field observations suggest that a systematically varying weathering zone, that can be tens of meters thick, commonly develops in the bedrock underlying hillslopes. Weathering turns otherwise poorly conductive bedrock into a dynamic water storage reservoir. Infiltrating precipitation typically will pass through unsaturated weathered bedrock before reaching groundwater and running off to streams. This invisible and difficult to access unsaturated zone is virtually unexplored compared with the surface soil mantle. We have proposed the term "rock moisture" to describe the exchangeable water stored in the unsaturated zone in weathered bedrock, purposely choosing a term parallel to, but distinct from, soil moisture, because weathered bedrock is a distinctly different material that is distributed across landscapes independently of soil thickness. Here, we report a multiyear intensive campaign of quantifying rock moisture across a hillslope underlain by a thick weathered bedrock zone using repeat neutron probe measurements in a suite of boreholes. Rock moisture storage accumulates in the wet season, reaches a characteristic upper value, and rapidly passes any additional rainfall downward to groundwater. Hence, rock moisture storage mediates the initiation and magnitude of recharge and runoff. In the dry season, rock moisture storage is gradually depleted by trees for transpiration, leading to a common lower value at the end of the dry season. Up to 27% of the annual rainfall is seasonally stored as rock moisture. Significant rock moisture storage is likely common, and yet it is missing from hydrologic and land-surface models used to predict regional and global climate.

Model fit to experimental data for foam-assisted deep vadose zone remediation.

This study investigates how a foam model, developed in Roostapour and Kam [1], can be applied to make a fit to a set of existing laboratory flow experiments in an application relevant to deep vadose zone remediation. This study reveals a few important insights regarding foam-assisted deep vadose zone remediation: (i) the mathematical framework established for foam modeling can fit typical flow experiments matching wave velocities, saturation history, and pressure responses; (ii) the set of input parameters may not be unique for the fit, and therefore conducting experiments to measure basic model parameters related to relative permeability, initial and residual saturations, surfactant adsorption and so on should not be overlooked; and (iii) gas compressibility plays an important role for data analysis, thus should be handled carefully in laboratory flow experiments. Foam kinetics, causing foam texture to reach its steady-state value slowly, may impose additional complications.