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[Display omitted] • Multiple isotopes revealed nitrate transport and transformation processes. • Old apple orchards had large water deficit and nitrate accumulation. • High transpiration effects resulted in no water and nitrate fluxes in deep layers. • The fertilizer-based nitrate bombs would reach the aquifers after hundreds of years. The deep-rooted economic trees decrease water storage but increase nitrate-nitrogen (NO 3 –N) accumulation in the soil due to the root water uptake and excessive use of fertilizers. Knowing how the NO 3 accumulates in the soil and to what extent it can be flushed out or transformed can provide insights on the processes controlling N flux to waterways. We sampled cultivated farmland and orchards with varying ages of apple and peach trees in the loess deposits to > 15 m depth to measure the contents and isotopes of water and NO 3 in soils. The novelty of this study is that the water isotopes are used to analyze water and NO 3 transport, while the NO 3 isotopes are further employed for NO 3 source and transformation analysis. The parabolic NO 3 –N profiles (258–3579 kg N ha−1) were mainly observed within 0–5 m under different land use types. The surface-aggregated NO 3 reservoirs under old apple orchard were about six times of farmland due to the overuse of chemical fertilizers, while those of peach orchard were only half of farmland because of less fertilizer inputs and possible root absorption effects. Piston flow resulted in the overall slow water movement and low NO 3 flux under different land use types in deep loess deposits. Despite the soil water recharge by precipitation and low surface evaporation, the strong transpiration effects further resulted in no water and NO 3 fluxes in the deep layers under old apple orchard. Nitrate reservoirs were mainly contributed by soil organic N (44 % ± 26 %) and ammonium N fertilizer (36 % ± 20 %), but their transformation was dominated by nitrification under old apple orchard and by mineralization in deep layers. The risks of NO 3 bombs are temporarily weakened by lower dried soil layers under deep rooted plants; however, with lower water movement rates and limited denitrification potential, NO 3 would eventually reach the aquifers in a few hundred years or more. This study provides new insights for clarifying NO 3 transport and transformation processes by combing multiple isotopes, and the NO 3 bombs would chronically endanger the sustainable development of terrestrial ecosystems and the environment in arid regions. [ABSTRACT FROM AUTHOR]

The legacy nitrate–nitrogen (NO3−−N) in the soils is a long‐term threat to groundwater in regions with thick unsaturated zones, and it is thus important to investigate the amounts and sources of NO3−−N for effective environmental management. Given the substantial conversions of arable farmlands to non‐fertilizer land uses for the restoration of degraded land, the legacy effects of NO3−−N in thick loessial unsaturated zones in China remain uncertain. We collected soil samples from loess profiles >13‐m deep under arable farmland and grassland, apricot, pine, peashrub, willow, and poplar converted from arable farmland over 15–35 years. We determined their nitrate content and isotope compositions to quantify the accumulation and sources of the former and its potential threat to groundwater. The seven NO3−−N profiles exhibited a parabolic shape with peak depths at 2.6–9.2 m. Greater peak depths corresponded to greater NO3−−N values, and the peak nitrate level varied from 0 to 10 m. The total NO3−−N accumulation ranged 3,181–9,018 kg N ha−1, and the non‐fertilizer profiles accounted for 35–73% of the arable farmland. The nitrate sources varied with depths. According to the isotope mass balance, the nitrate above, near, and below the peak depths mainly originated from atmospheric NO3− (25–34%), synthetic N fertilizer (49–68%), and soil organic N and manure (50–73%). The estimated residence time of nitrate in the soil ranged from 270 to 620 years. As such, the nitrate legacy effects should be given considerable attention for the protection of the soil and groundwater environment. [ABSTRACT FROM AUTHOR]

• Combined effects of multiple factors were explored for soil water and nitrate. • Land use is the key factor regulating deep soil water and nitrate reservoirs. • Soil water was influenced by magnetic susceptibility and sand. • Nitrate was affected by soil water, pH, and soil organic carbon. Identifying the environmental factors controlling deep soil water and nitrate is important for the sustainability of vegetation and ecosystems. However, the combined effects of multiple factors at varying scales have been poorly understood, especially in those regions with thick vadose zones and deep-rooted plants. The aim of this study is to identify the multivariate controls of water and nitrate in seven > 13 m deep boreholes under different land use types on China's Loess Plateau. After measuring soil water and nitrate contents, the combined effects of climatic factors (precipitation, temperature, and potential evapotranspiration) and soil properties (soil texture, magnetic susceptibility, pH, EC, and soil organic carbon) were explored by wavelet analysis. Land use is the key factor regulating deep soil water and nitrate reservoirs, with high water deficit and low nitrate accumulation within 0–10 m under non-fertilized forestlands and shrublands. The positive and negative standardized ratios respectively suggested the synergistic and antagonistic relationships of water and nitrate under shallow- and deep-rooted vegetation. As the important agent for nitrate leaching, soil water is influenced by magnetic susceptibility and sand. Furthermore, soil water, pH, EC, and soil organic carbon are individually or simultaneously responsible for nitrate transport and transformation, especially at large scales (> 7.5 m). This study provides novel insights for vegetation and environmental management, and benefits the parameterizing of process-based hydrological and biogeochemical models at regional or global scales, especially in the deep unsaturated zones. [ABSTRACT FROM AUTHOR]

• Stable and radioactive isotopes were combined to reveal sources of deep soil water. • Extreme wet events in rainy seasons recharge deep soil water through piston flow. • Deep-rooted plants require rainfall with higher intensities than farmland. • Deep-rooted plants prevent rainfall from replenishing deep soil water. Deep soil water (DSW) is of utmost importance for plant growth and groundwater recharge, but the depletion of DSW has been largely reported due to land use change, especially because of the conversion from shallow- to deep-rooted plants. To answer the question whether DSW can be replenished, it is important to reveal the recharge mechanism of DSW under different vegetation types. China's Loess Plateau has experienced drastic land use change resulting in soil water depletion in deep vadose zones, which provides a good platform to explore these issues. We collected 380 soil samples from five boreholes up to 18 m deep covered by five plants (grassland, apricot, pine, poplar, and peashrub), and determined the contents, stable and radioactive isotopes (δ2H, δ18O, and 3H) of soil water. DSW, defined as soil water below 3 m in this study, was mainly recharged by the wet events in rainy season, especially those with intensities ≥ 50 mm/day, through piston flow. But they were significantly influenced by land use types. Specifically, the forestland and shrubland had lower evaporation effects and higher precipitation offset, and their soil water was recharged by wet events with higher intensities than those under grassland. Nevertheless, the deep-rooted plants substantially reduced recharge rates due to excessive root water uptake, and thus prevented rainfall from replenishing DSW. This study has implications to groundwater sustainability in those regions with limited water resources, drastic land use change, and thick unsaturated zones. [ABSTRACT FROM AUTHOR]