Water use and nitrate nitrogen changes in intensive farmlands following introduction of poplar (Populus x euramericana) in a semi-arid region

Nitrate pollution from agricultural activities has become a common problem in the North China Plain due to overapplication of N fertilizer. Agroforestry is considered a potentially effective land-use practice for mitigating nitrate leaching. In this study, an agroforestry system of 1-2-year-old poplar (Populus x euramericana cl. '74/76') trees intercropped with winter wheat (Triticum aestivum) and maize (Zea mays) were rotated annually and compared with a sole-crop system in order to quantitatively investigate changes in NO ₃-N accumulation and water use following introduction of the trees into the farmland. The results revealed that evapotranspiration (ET) decreased by 6.1% and 3.7% during the wheat season and increased by 11.5% and 12.1% during the maize season, for fertilized and nonfertilized treatments intercropped with young poplar trees, respectively. NO₃-N concentration decreased significantly in the intercropped system. NO₃-N accumulation in the soil after the winter wheat and maize harvests decreased by 29.7 kg N ha ^-1 and 47.9 kg N ha^-1 in the fertilized intercropped system, respectively, compared to the fertilized sole-crop system, while the grain yield of intercropped maize decreased by 31.3% and 33.7% for the fertilized and nonfertilized treatments, respectively. Water use efficiency (WUE) decreased by 38.5% and 40.7%, respectively. These results indicate that poplar tree roots play a significant role in alleviating NO₃-N accumulation in the soil profile due to more NO₃-N uptake in the intercropped system. However, they severely influenced maize grain yield and WUE. Thus, young poplar trees can be introduced into wheat fields. However, it is impractical to introduce them into a wheat and maize rotation system, in view of sustainable agriculture and environmental protection.