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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]

[Display omitted] • Water-nitrate coupling is explored under farmland and apple orchards • Large water deficit and nitrate accumulation are observed under apple orchards • Water deficit and nitrate accumulation are spatiotemporally asynchronous • Soil properties significantly affect water-nitrate coupling in deep soils Understanding the relationship between water and nitrate in soils is critical to agricultural production and aquifer vulnerability assessments, especially in regions with intensive agricultural activities and thick unsaturated zones. Deep soils usually record long-term variations in coupling water and nitrate processes, influenced by multiple environmental factors. As such, this study aims to investigate water and nitrate-nitrogen (NO 3 –−N) contents and their relationships in soil profiles of 13–23 m deep under farmland and neighboring apple orchards with different ages. Soil properties were considered to explore the individual effect of a single factor or combined effects of multiple factors by wavelet analysis and partial least squares path models. Compared with farmland, apple orchards had large water deficit and NO 3 –−N accumulation due to root water uptake and synthetic fertilizer inputs, with high spatiotemporal asynchrony in deep unsaturated zones. Specifically, water deficit and NO 3 –−N accumulation were respectively observed in relatively stable (5–15 m) and active layers (0–5 m, spatial asynchrony), with higher water deficit rates under young apple trees (e.g., 12 and 22 years old) but larger NO 3 –−N accumulation rates under old apple trees (e.g., 23–32 years old, temporal asynchrony), respectively. Under farmland and orchard, soil texture-organic carbon regulated soil water movement, while soil water-organic carbon affected nitrate transport and transformation processes, which further altered soil pH and electric conductivity conditions in deep vadose zones. The growth of apple trees leads to asynchronous water depletion and fertilizer accumulation in soils, and the > 20 years apple trees are limited by insufficient water despite the excessive fertilizer. The dried soil layers not only reduce potential groundwater recharge, but also prolong the residence time of nitrate bombs in thick unsaturated zones. This study provides essential information for sustainable management of vegetation and water resources, and benefits the complicated hydrological and biogeochemical cycles in thick vadose zones. [ABSTRACT FROM AUTHOR]

Understanding the behavior of nitrate in the unsaturated zone is critical for predicting groundwater nitrate pollution. Although this issue has been intensively investigated for shallow soil profiles, it remains poorly understood in deep soils. We collected soil samples up to 20 m depth under farmland and apple orchards with trees that were 16 and 22 years old (A16 and A22). We then determined soil-water content, stable isotopes (δ2H and δ18O) and nitrate content under two land use types. The stable isotopes of nitrate under two apple orchards were measured to a depth of 16 m. The mean nitrate contents in the shallow soils (0–6 m) under apple orchards (78.8 ± 87.8 mg kg−1 for A16, 180 ± 155 mg kg−1 for A22) were significantly higher than that of farmland (2.40 ± 1.56 mg kg−1) (p < 0.01), whereas the nitrate contents in the deep soils (6–20 m) were almost similar between apple orchards and farmland. Ammonium or urea-based nitrogen (N) fertilizers (30–61%) were the predominant contributors to nitrate in the shallow soil layers, followed by soil organic N and manure & sewage, whereas the soil organic N (mean 34%) was the primary source of nitrate in deep soils. Isotopic data revealed that the N transformation was controlled by nitrification, and denitrification was not apparent throughout the soil profiles. The downward transport of nitrate lagged behind soil-water. Nitrate was leached by piston flow in the vadose zone, and the nitrate from N application since the 1980 s has only infiltrated to 6 m depth. The difficulty in recharging deep soils mitigates the transport of nitrate to lower depths. The combination of water-nitrate stable isotopes provides new insights for identifying nitrate sources, transformation, and transport in the thick unsaturated zone. • Stable isotopes of water and nitrate were used to explore nitrate transport. • The nitrate hardly further transported to the deep soils (> 6 m). • The transport downward of nitrate lagged behind that of soil water. • Nitrate was pushed down by the piston flow in the vadose zones. [ABSTRACT FROM AUTHOR]

The cultivation of deep-rooted vegetation greatly alters the tradeoffs between water and carbon dynamics, yet their impact on microbial carbon sequestration (MCS) remains inadequately documented, particularly in thick vadose zones. This information is crucial for global climate mitigation and ecosystem service enhancement. As such, we collected triplicate samples for each land use type, exploring the abundance and structure of cbbL -harboring microbial communities within 10-m deep profiles under cultivated farmland and two apple orchards of varying ages (n = 72). Our findings indicated a shift from water-dominated (improved water retention at the expense of reduced carbon sequestration) to carbon-dominated benefits (enhanced carbon sequestration at the expense of decreased water retention) in deep soil layers (2–10 m) after the transformation from farmland to apple orchards. Despite the lack of significant differences among the three land use types, the gene abundances and microbial diversity indices reduced with soil depths because of variations in substrates, resources, and microenvironments. The cbbL -harboring microorganisms consisted of 5 phyla, 7 classes, 19 orders, 30 families, and 55 genera, with Proteobacteria as the predominant bacterial phylum. The relative contributions of different genera varied with soil depths, reflecting their distinct survival strategies. The microbial network was positively connected in various soil layers under farmland and apple orchards. Furthermore, the results of correlation, redundancy analysis, and structural equation modeling jointly indicated that plant roots and soil properties, including total nitrogen and organic carbon, governed MCS processes in deep unsaturated zones. Specifically, the low microbial variation between farmland and apple orchards can be attributed to the predominant effects of climatic and pedogenic factors on soil properties, the restricted contribution of plant roots to soil nutrient levels per unit depth, and the adverse effects of plant-induced ecohydrological consequences on root systems and microbial populations. This study offers new insights within thick vadose zones containing deep-rooted plants, thereby contributing to a better understanding of global terrestrial carbon cycle processes. [Display omitted] • Microbial carbon sequestration processes were explored in 10-m deep profiles. • The farmland-to-apple orchards conversion led to water-carbon tradeoffs. • Microbial carbon sequestration differed with depths rather than land use types. • Depth-dependent plant roots and soil nutrients affects carbon sequestration. [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]

• 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]

• Estimated potential recharge vary with methods and vegetation types. • Recharge estimated by CMB and CP is problematic under deep-rooted plants. • CMB performs the best in estimating recharge under shallow-rooted plants. • CWMB present the best results for recharge estimation under deep-rooted plants. • Uncertainties in methods originate from the requirements of methodological theory. Chloride (Cl)-based tracing methods are popular for deep drainage (or potential groundwater recharge) estimation in the unsaturated zone, but uncertainties in various such methods have rarely been investigated. We collected 1827 samples from 24 soil profiles under plants with shallow or deep root systems in the thick unsaturated loess deposits of China, and measured soil water, Cl, and tritium contents. With the drainage estimated with the tritium peak method as reference, we evaluated the applicability of several Cl-based approaches for drainage estimation under shallow- and deep-rooted plants, including the chloride mass balance method (CMB), chloride storage method (CS), chloride peak method (CP) and chloride-water mass balance method (CWMB). The estimated deep drainages varied with methods, and the uncertainties were large with a coefficient of variation of 0.71. Compared to the drainage obtained from tritium-based methods (33.6 ± 11.2 and 9.6 ± 14.5 mm year−1 under shallow- and deep-rooted plants, respectively), CMB and CS (38.0 ± 16.6 and 36.7 ± 17.6 mm year−1) performed better than CP (23.2 ± 17.8 mm year−1) under shallow-rooted plants, while CWMB (7.6 ± 13.9 mm year−1) performed better under deep-rooted plants. The large discrepancy in estimated drainage originates from methodological theory such as the time scales of representation and sensitivity to exogenous Cl inputs. The root water uptake-induced soil water depletion can enrich soil Cl concentration and greatly influence CMB and CP. Our findings provide insights for how to estimate deep drainage through appropriate methods. [ABSTRACT FROM AUTHOR]

Afforestation is beneficial to improving soil carbon pools. However, due to the lack of deep databases, the variations in soil carbon and the combined effects of multiple factors after afforestation have yet to be adequately explored in >1 m deep soils, especially in areas with deep-rooted plants and thick vadose zones. This study examined the multivariate controls of soil organic carbon (SOC) and inorganic carbon (SIC) in 0–18 m deep under farmland, grassland, willow, and poplar in loess deposits. The novelty of this study is that the factors concurrently affecting deep soil carbon were investigated by multiwavelet coherence and structural equation models. On average, the SOC density (53.1 ± 5.0 kg m−2) was only 12% of SIC density (425.4 ± 13.8 kg m−2), with depth-dependent variations under different land use types. In the soil profiles, the variations in SOC were more obvious in the 0–6 m layer, while SIC variations were mainly observed in the 6–12 m layer. Compared with farmland (SOC: 17.0 kg m−2; SIC: 122.9 kg m−2), the plantation of deciduous poplar (SOC: 28.5 kg m−2; SIC: 144.2 kg m−2) increased the SOC and SIC density within the 0–6 m layer (p < 0.05), but grassland and evergreen willow impacted SOC and SIC density insignificantly. The wavelet coherence analysis showed that, at the large scale (>4 m), SOC and SIC intensities were affected by total nitrogen-magnetic susceptibility and magnetic susceptibility-water content, respectively. The structural equation model further identified that SOC density was directly controlled by total nitrogen (path coefficient = 0.64) and indirectly affected by magnetic susceptibility (path coefficient = 0.36). Further, SOC stimulated the SIC deposition by improving water conservation and electrical conductivity. This study provides new insights into afforestation-induced deep carbon cycles, which have crucial implications for forest management and enhancing ecosystem sustainability in arid regions. [Display omitted] • Soil organic and inorganic carbon varied with land use types and soil depths. • Combined effects of multiple factors on soil carbon were studied at various scales. • Magnetic susceptibility increased the coherence between SOC and total nitrogen. • Water content increased the coherence between SIC and magnetic susceptibility. • We revealed the internal relationship between soil organic and inorganic carbon. [ABSTRACT FROM AUTHOR]

Water quality is an increasing concern in the dry regions of the world as it affects and reduces the quantity of available water. Our objective was to investigate the sources, drivers, spatiotemporal patterns of nitrate‑nitrogen (NO 3 –N) transport in the streamwater and groundwater in a dry and a wet season in seven large rivers located in the Loess Plateau of China (640,000 km2, 100 million population), which is a region with marked influence of human activities on streamflow and groundwater. We collected 510 streamwater and groundwater samples and found that NO 3 –N was significantly lower in the dry season (< 5.0 mg L−1) than the wet season (> 5.0 mg L−1). In the wet season, NO 3 –N was lower in the streamwater than groundwater; however, the spatial variation in the NO 3 –N was greater in streamwater, with higher concentrations in two rivers (Wei and Fen). The source characterization using stable isotopes of NO 3 from the Wei River showed that chemical N fertilizers and soil organic N contributed ~ 75% of NO 3 to streamwater and that soil organic N was the greatest contributor of NO 3 to groundwater (~ 60%) than streamwater (< 40%). The spatial pattern of NO 3 –N was dominated by fertilizer application and varied seasonally with rainfall-runoff and streamflow-groundwater connectivity. Our results showed the complicated patterns and sources of NO 3 pollution in streamwater and groundwater and highlight that more emphasis should be placed to prevent and restore the degraded water quality in the dry regions. Unlabelled Image • Wet-season nitrate contents in waters were higher than the dry season. • The spatial pattern of nitrate transport was controlled by land use structure. • Seasonality of hydrologic cycle influenced temporal variation in nitrate loss. • Major sources of nitrate in waters were chemical fertilizer and soil organic N. • Limited water quantity in the dry region was further reduced by water pollution. [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]

The water and organic carbon stored within unsaturated zones are crucial in terrestrial ecosystems; however, the coupling effects of deep soil water and organic carbon are still poorly documented. To explore the water-carbon coupling effects within soils under tree plantations, we investigated the water and organic carbon content to 20 m deep under cultivated farmland and orchards with apple trees of varying ages (5, 10, 15, 20, and 24 years) in the loess-covered region of China. A conditional process model was used to explore the impacts of vegetation and edaphic factors on the water-carbon coupling processes. Compared with farmland, young apple orchards (0–10 years) showed lower water deficit, but the soil water storage reduced by 21%− 32% under old apple orchards (15–24 years). The organic carbon content exhibited negligible variation among apple trees of different ages. However, the water-carbon coupling effects varied with apple tree ages. A positive correlation existed between water storage and organic carbon density under young apple orchards (p < 0.05), but a negative correlation was found between water deficit and organic carbon sequestration under old apple orchards (p < 0.01). The conditional process model suggested that the water-carbon coupling relationship under young apple orchards was primarily controlled by clay content due to lower root biomass, while that under old apple orchards was concurrently shaped by silt content and coarse roots because of the developed root system of apple trees. This study offers novel insights into the sustainable management of agriculture and tree plantations in regions with deep-rooted vegetation and thick unsaturated zones. [Display omitted] • We analyzed water and carbon coupling effects within 0–20 m under apple orchards. • Apple trees triggered large water deficit but little organic carbon variation. • Soil texture led to positive water and carbon effects under young apple orchards. • Plant root caused negative water and carbon effects under old apple orchards. [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]