Long-term legacy impacts of nitrogen fertilization on crop yield, nitrate accumulation, and nitrogen recovery efficiency

Abstract

Inefficient nitrogen (N) utilizing cropping systems add substantial amount of unconsumed N fertilizer in the soil profile as residual N. High nitrate accumulation in the soil profile is detrimental to soil health and significantly contributes to groundwater pollution. This study investigated the long-term fate of residual N in the deep soil profile, including its contributions to crop yield and nitrogen recovery efficiency. A field experiment was carried out for seven years comprising winter wheat and summer maize as an annual rotation. For the initial two years (2015–2017) N fertilizer was applied at two rates (N340 including 160 kg N ha 1 for wheat and 180 kg N ha 1 for maize, N500 including 220 kg N ha 1 for wheat and 280 kg N ha 1 for maize) with and without nitrification inhibitor (dicyandiamide, DCD). While for the next five years (2017–2022) the crops were grown without N fertilization. N fertilization increased grain yield compared to control, and DCD addition caused additional increases in yield mainly at 500 kg N ha 1 . The yield did not decrease in the first year without fertilization but started to decrease considerably in later years. N fertilization increased grain N uptake by more than 100 % in all treatments in fertilized years and the first unfertilized year compared to control but decreased strongly in the second year without fertilization. The cumulative fertilizer N recovery did not differ among N fertilized treatments during fertilized years but increased strongly when N uptake under unfertilized years were incorporated. The cumulative fertilizer N recovery in N340 + DCD increased from 37 %–44 % in fertilized years to 74 %–80 % in unfertilized years. A substantial residual nitrate accumulation (237–489 kg N ha 1 ) accumulated in the soil profile (0–200 cm) after two years of fertilization. Crops grown during unfertilized years fulfilled the majority of their N requirements from residual nitrate as indicated through considerable decrease in residual N accumulation (31 %–41 %) in the first unfertilized year. Soil N accumulation became less than 60 kg ha 1 after growing crops without fertilization for four years with no difference among treatments including control. This study highlights that residual N is an important component of crop N uptake and should therefore be used to correct N application rates to avoid overuse of N fertilizers.

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