Legacy nitrate in the deep loess deposits after conversion of arable farmland to non‐fertilized land uses for degraded land restoration

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 0–10 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 270–620 yr. As such, the nitrate legacy effects should be given considerable attention for the protection of the soil and groundwater environment.