Nitrate leaching characteristics of red soils from different parent materials in subtropical China

Globally, nitrate (NO₃^-) leaching from agroecosystems has been of major concern. There is evidence that NO₃^- leaching exhibits intense seasonal variation in subtropical regions. However, influencing factors to the seasonal dynamics remain unclear. In this study, a two-year field lysimeters experiment was conducted with three red soils derived from different parent materials (Quaternary red clay (QR), red sandstone (RS), and basalt (BA)). An N fertilizer (¹⁵N-enriched urea, 10 atom% excess) of 200 kg N ha^-1 yr^-1 was applied for maize. The effect of parent material on NO₃^- leaching characteristics was examined in surface (0-20 cm) and subsoil (20-100 cm) layers. The results showed due to the weakening of abundant drainage, there was no significant effect of parent materials on NO₃^- leaching characteristics in surface layers. Environmental factors (precipitation and temperature) and fertilization together led to obvious seasonal characteristics, i.e. abundant NO₃^- leaching during both crop growth and fallow periods. In subsoil layers, NO₃^- leaching characteristics were completely different among three soils. The concentrations and δ¹⁵N of NO₃^- in QR and RS soils showed a continuous increase after first year's fertilization, while those in BA soil remained relatively stable after reaching peak levels around harvest in first year. Meanwhile, the NO₃^- leaching amount in BA soil was significantly lower than in the other two soils. These might be explained by different NO₃^- adsorption capacities caused by the differences in mineral composition and free iron and aluminium contents. These elucidated in subsoil layers, NO₃^- leaching characteristics highly depended on parent materials. Meanwhile, adsorption capacity was limited and cannot slow NO₃^- leaching in the long run. Our results suggest that seasonal variation of NO₃^- leaching in surface layers and temporary retardant effect from NO₃^- adsorption capacity in subsoil layers should receive much attention when calculating and predicting NO₃^- leaching in subtropical regions.