Evaluating potential groundwater recharge in the unsteady state for deep-rooted afforestation in deep loess deposits

The changing nature of groundwater in the global water cycle

In recent decades, climate change and other anthropogenic activities have substantially affected groundwater systems worldwide. These impacts include changes in groundwater recharge, discharge, flow, storage, and distribution. Climate-induced shifts are evident in altered recharge rates, greater groundwater contribution to streamflow in glacierized catchments, and enhanced groundwater flow in permafrost areas. Direct anthropogenic changes include groundwater withdrawal and injection, regional flow regime modification, water table and storage alterations, and redistribution of embedded groundwater in foods globally. Notably, groundwater extraction contributes to sea level rise, increasing the risk of groundwater inundation in coastal areas. The role of groundwater in the global water cycle is becoming more dynamic and complex. Quantifying these changes is essential to ensure sustainable supply of fresh groundwater resources for people and ecosystems.

Unravelling the dynamics of soil microbial communities under the environmental selection and range shift process in afforestation ecosystems

The process of forest range shift not only affects the vegetation aboveground but also influences the dynamics of belowground microbial communities. To investigate the changes in soil under forest range shift, we examined the natural forest soil microbiome along with its corresponding physicochemical properties, as well as the afforestation of natural forest by seedlings and sowing. By utilizing natural forests and employing different afforestation methods, we simulated the three stages of forest range shift: the staging stage, regeneration, and colonization. We employed network analysis and phylogenetic assemblages to examine the structure of soil microbial communities during these three stages in a macro-environmental change context. Ordination and regression analyses were also used to explore the correlation between microorganisms, environmental factors, and changes in their niches. The findings revealed that different afforestation (range shift) types led to distinct microbial compositions. Seedling afforestation exhibited similarities to mature forests, suggesting a significant influence on below-ground microorganisms. In contrast, sowing-based afforestation resulted in small changes in soil microbes, indicating a legacy effect on grassland soils. The impact of the rhizosphere on microbial composition remained consistent across the three forest types. Overall, this study underscores the significance of forest range shift in shaping soil microbial communities and emphasizes the need to consider these dynamics in forest management and restoration endeavours.

Assessing groundwater recharge rates, water quality changes, and agricultural impacts of large-scale water recycling

The over-exploitation and insufficient replenishment of groundwater (GW) have resulted in a pressing need to conserve freshwater and reuse of treated wastewater. To address this issue, the Government of Karnataka launched a large-scale recycling (440 million liters/day) scheme to indirectly recharge GW using secondary treated municipal wastewater (STW) in drought-prone areas of Kolar district in southern India. This recycling employs soil aquifer treatment (SAT) technology, which involves filling surface run-off tanks with STW that intentionally infiltrate and recharge aquifers. This study quantifies the impact of STW recycling on GW recharge rates, levels, and quality in the crystalline aquifers of peninsular India. The study area is characterized by hard rock aquifers with fractured gneiss, granites, schists, and highly fractured weathered rocks. The agricultural impacts of the improved GW table are also quantified by comparing areas receiving STW to those not receiving it, and changes before and after STW recycling were measured. The AMBHAS_1D model was used to estimate the recharge rates and showed a tenfold increase in daily recharge rates, resulting in a significant increase in the GW levels. The results indicate that the surface water in the rejuvenated tanks meets the country's stringent water discharge standards for STW. The GW levels of the studied boreholes increased by 58-73 %, and the GW quality improved significantly, turning hard water into soft water. Land use land cover studies confirmed an increase in the number of water bodies, trees, and cultivated land. The availability of GW significantly improved agricultural productivity (11-42 %), milk productivity (33 %), and fish productivity (341 %). The study's outcomes are expected to serve as a role model for the rest of Indian metro cities and demonstrate the potential of reusing STW to achieve a circular economy and a water-resilient system.

Field variation of groundwater recharge and its uncertainty via multiple tracers' method in deep loess vadose zone

Accurate estimation of groundwater recharge is a precondition for assessing its spatial variation at different scales, especially field scale. In the field, the limitations and uncertainties of different methods are first evaluated based on site-specific conditions. In this study, we evaluated field variation in groundwater recharge via multiple tracers in the deep vadose zone on the Chinese Loess Plateau. Five deep soil profiles (approximately 20 m deep) were collected in the field. Soil water content and particle compositions were measured to analyse soil variation, and soil water isotope (³H, ¹⁸O, and ²H) and anion (NO₃^- and Cl^-) profiles were used to estimate recharge rates. Distinct peaks in soil water isotope and nitrate profiles indicated a vertical one-dimensional water flow in the vadose zone. Although the soil water content and particle composition were moderately variable, no significant differences were observed in recharge rates among the five sites (p > 0.05) owing to the identical climate and land use. The recharge rates did not show a significant difference (p > 0.05) between different tracers' methods. However, recharge estimates by the chloride mass balance method indicated higher variations (23.5 %) than those by the peak depth method (11.2 % to 18.7 %) among five sites. Moreover, if considering the contribution of immobile water in vadose zone, groundwater recharge would be overestimated (25.4 % to 37.8 %) using the peak depth method. This study provides a favourable reference for accurate groundwater recharge and its variation evaluated using different tracers' methods in deep vadose zone.