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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]

High nitrate (NO 3 --N) accumulation has been observed globally in vadose zones due to the overuse of synthetic fertilizer, but its impacts on nitrogen (N) biological processes remain poorly understood, especially after land use conversion from farmlands to deep-rooted apple orchards. In this study, we collected paired soil samples to determine available N contents, NO 3 --N isotopes, and abundances of N functional genes (NFGs) in 10-m loess profiles under cultivated farmlands and adjacent apple orchards (n = 300). The average NO 3 --N accumulation under orchards was 4773 kg N ha−1 (10 times higher than farmlands), with the NO 3 --N profiles resembling a parabolic curve within 0 – 4 m below the surface. Regardless of excessive fertilizer application and well-developed root systems, the deep-rooted orchards had minor effects on N sources and biological processes in deep loess deposits, as indicated by our estimates of isotopes and NFGs abundances. We hypothesized that the limited and inactive microorganisms in the subsoil are more resistant to environmental disturbances, and that carbon source and ammonium substrate dominated the gene expression and occurrence of NFGs-indicated biological processes. The limited root biomass per unit depth and the inhibition effects of a dry soil horizon on root turnover and exudate release likely contributed to the less perturbed N biological processes under orchards. As the first study to explore N transformation processes in 10-m loess deposits, this study provided insights for complicated N biogeochemical cycles in arid regions with deep unsaturated soils and intensive agricultural land use. [Display omitted] • The contents, isotopes, and gene abundances were determined in 10-m loess profiles. • The nitrate accumulation under orchards were 10 times higher than farmlands. • Deep-rooted orchards had minor effects on nitrogen biological processes. • Carbon source and ammonium substrate dominated nitrogen biological processes. [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]

• Stable and radioactive isotopes were combined for sources and ages of soil water. • Each component of soil water balance was partitioned. • Soil water above 8 m originated from precipitation over the past ~55 years. • Fruit trees increased transpiration but decreased evaporation relative to farmland. Deep-rooted fruit trees mine more water from deep soils than their shallow-rooted counterparts. Understanding how deep soil water (DSW) is replenished and subsequently depleted by deep-rooted fruit trees, therefore, are important for informing sustainable water resources management particularly in arid regions. In this study, we collected soil samples from the surface down to 20 m under four land use types (farmland, 8-year apple orchard, 12-year peach orchard, and 25-year apple orchard) in China's Loess Plateau. We then measured the soil water content, stable (δ2H and δ18O) and radioactive (3H) isotopic compositions. The radioactive isotope was used to constrain the age of soil water while the stable isotopes were used to determine the types of storms that would have contributed to recharging the DSW. We then implemented a soil water balance model to identify the mechanisms underlying the changes in DSW. Mechanistically, our results show that water movement in these soils was predominantly via piston flow. The age of DSW below 8 m was determined to be older than 55 years. Altogether, these results support an interpretation that DSW may have only been recharged by high-intensity, low-frequency rainfall events during the wet season (July to September), but that the magnitude of DSW recharge was likely to be influenced by subsequent water mining by deep-rooted fruit trees. The deep-rooted fruit trees consumed more DSW than farmland vegetation, substantially limiting the magnitude of DSW recharge under the orchards. Our simple soil water balance model, informed by water stable isotopes and supplemented with information from tritium, provides a technique for partitioning soil water balance (SWB) and insights into the long-term effects of land use change on water resources in arid regions. [ABSTRACT FROM AUTHOR]