Deep percolation paradox: How cropland cropping reactivates nitrate transport in thick vadose zones of converted orchards

Land use change can greatly alter hydrological processes such as rainfall infiltration, evapotranspiration and solute (e.g. nitrate) transport in vadose zone and hence ecosystem service. In northern China, plenty of apple orchards, where accumulate substantial nitrate due to overdue fertilization and soil desiccation, are converted to cropland such as maize in recent years, mainly driven by local government’s agricultural policy. However, little is known how this kind of land use change influence spatiotemporal distribution of soil water availability and nitrate especially in deep layers. To this end, we collected soil samples from apple orchards of three different ages (10, 20, 30-year-old) and cropland (maize) that were converted from apple orchards of the above corresponding ages on the Loess Plateau in northern China. We found that after 6 years of converting orchards of different ages to cropland, soil water deficit was greatly mitigated with the cumulative amount of soil water replenishment reaching 193.42 mm, 224.25 mm, and 267.71 mm, respectively; and the deepest recharge depth reached 8.6 m. Concurrently, over 70 % of nitrate was leached down to the same depth, creating a “nitrate time bomb”. The ages of orchard and subsequent grain crop cultivation significantly influence nitrate accumulation post-conversion. This finding suggests that great attention should be paid to land-use transitions from orchards to cropland and necessary measurements should be taken to prevent the potential risk of nitrate contamination to groundwater by rapid nitrogen leaching. • The orchard-to-crop conversion eases soil water deficits on the Loess Plateau. • Maximum soil water recharge reached 8.6 m depth six years after land use conversion. • Over 70 % of nitrate leached into deep soil, raising concerns for groundwater safety. • Age of orchards and crops affects nitrate buildup and water recovery post-conversion.