Summer fallow drives leaching of legacy nitrogen in cereal land converted from old orchard

Understanding the fate of high nitrate accumulated in soil profiles (legacy nitrogen; LN) of cereal land converted from intensive orchards (OC) is important for N management and environmental protection. This study has quantified changes in water and nitrate contents in soil profiles of OC fields during summer fallow on the southern Loess Plateau. HYDRUS-1D model was used to simulate the changes of water and N in soil profiles. The results show that summer fallow significantly increased the total water storage in the 0 – 500 cm soil profile of the OC treatment. Water recovery reached to the depth of 440 cm after the fallow. The scenario analysis of different precipitation patterns revealed a significant correlation between LN leaching and precipitation patterns. The leaching of LN in the soil layers at 200 – 300, 300 – 400, and 400 – 500 cm was estimated to be 390, 297, and 113 kg N ha −1 , respectively. The model predicted that recovery depths of soil water content at the average precipitation scenario during the summer fallow after 10 years of OC reaches up to a depth of 15 m; the peak depth of LN after 15 years of OC can reach up to 10 m. It highlights that while converting old orchards to cereal land improves soil water recovery during summer fallow, it simultaneously increases N leaching into deeper soil profile, which diminishes crop-available nitrogen. Balancing the trade-off between soil water recovery and legacy nitrate leaching into deeper soil is a challenge. • Water/nitrate in orchard-converted cereal land (OC) during fallow was studied. • OC increased soil water storage during summer fallow. • Summer fallow increased nitrate leaching from 0 – 2 m into 2 – 5 m soil profiles. • Balancing water recovery and N leaching into deeper layers remains a challenge.