最新综述：深层土壤“活性氮动态反应器”新概念框架！

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Human activities have substantially increased global reactive nitrogen fluxes, contributing to environmental pollution and persistent uncertainties in regional and global nitrogen budgets. Emerging observations indicate substantial nitrate accumulation in the deep critical zone—the subsurface domain below the root zone and traditional solum—yet this domain has usually been treated as a passive sink. Here we synthesize disparate lines of evidence on subsurface nitrogen storage, transport, and transformation to develop a “dynamic nitrogen reactor” framework for the deep critical zone. We compare recent global estimates of deep nitrogen storage derived from process-based vadose-zone flux-budget approaches and machine-learning upscaling of deep soil profiles. One machine-learning extrapolation suggests that deep soils (>2 m) may store on the order of 101 Pg NO₃−–N globally, although this estimate remains highly uncertain because it is based on sparse deep-profile data and machine-learning extrapolation with strong biome and regional biases. These deep stores display pronounced spatial heterogeneity across arid, tropical, and intensively managed temperate regions. We then develop a process-based end-member framework for interpreting deep nitrogen storage and migration. The hydrologically limited, geochemically buffered, and hydraulically driven end-members describe the relative dominance of different controls rather than mutually exclusive system classes; individual critical zone profiles may express mixed or transitional behaviour across depths, flow paths, and time. Building on this framework, we synthesize the coupled hydrological, geochemical, and biogeochemical processes that govern deep nitrogen transport, retention, and transformation and examine how they generate multiscale dynamics: event-scale leaching pulses, interannual–decadal memory and management hysteresis, and centennial–millennial legacies. We then evaluate the environmental impacts of this deep dynamic nitrogen reactor, highlighting its roles in sustaining delayed groundwater and surface-water nitrate pollution, contributing to N₂O and CO₂ emissions via deep carbon–nitrogen feedbacks, and driving long-term changes in subsurface biogeochemistry and in the structure and functioning of the critical zone. Finally, we outline priorities for deep critical zone observatories, process-explicit critical zone system models, and vulnerability-based, legacy-aware nitrogen management.Human activities have substantially increased global reactive nitrogen fluxes, contributing to environmental pollution and persistent uncertainties in regional and global nitrogen budgets. Emerging observations indicate substantial nitrate accumulation in the deep critical zone—the subsurface domain below the root zone and traditional solum—yet this domain has usually been treated as a passive sink. Here we synthesize disparate lines of evidence on subsurface nitrogen storage, transport, and transformation to develop a “dynamic nitrogen reactor” framework for the deep critical zone. We compare recent global estimates of deep nitrogen storage derived from process-based vadose-zone flux-budget approaches and machine-learning upscaling of deep soil profiles. One machine-learning extrapolation suggests that deep soils (>2 m) may store on the order of 101 Pg NO₃−–N globally, although this estimate remains highly uncertain because it is based on sparse deep-profile data and machine-learning extrapolation with strong biome and regional biases. These deep stores display pronounced spatial heterogeneity across arid, tropical, and intensively managed temperate regions. We then develop a process-based end-member framework for interpreting deep nitrogen storage and migration. The hydrologically limited, geochemically buffered, and hydraulically driven end-members describe the relative dominance of different controls rather than mutually exclusive system classes; individual critical zone profiles may express mixed or transitional behaviour across depths, flow paths, and time. Building on this framework, we synthesize the coupled hydrological, geochemical, and biogeochemical processes that govern deep nitrogen transport, retention, and transformation and examine how they generate multiscale dynamics: event-scale leaching pulses, interannual–decadal memory and management hysteresis, and centennial–millennial legacies. We then evaluate the environmental impacts of this deep dynamic nitrogen reactor, highlighting its roles in sustaining delayed groundwater and surface-water nitrate pollution, contributing to N₂O and CO₂ emissions via deep carbon–nitrogen feedbacks, and driving long-term changes in subsurface biogeochemistry and in the structure and functioning of the critical zone. Finally, we outline priorities for deep critical zone observatories, process-explicit critical zone system models, and vulnerability-based, legacy-aware nitrogen management.