Optimizing N applications increases maize yield and reduces environmental costs in a 12-year wheat-maize system

Context
Maize plays a crucial role in global food security, while extensive use of nitrogen (N) fertilizers in maize production has posed severe environmental risks. The challenge of optimizing N fertilizer applications to obtain high maize yield, high NUE and low N losses has received considerable attention but lacks evidence from long-term field experiments.
Objective
This study aims to quantify the influences of long-term continuous N applications on soil mineral N (SMN) and their subsequent effects on root growth, aboveground biomass accumulation, yield formation, and environmental benefits of maize, and determine the optimal N rate that ensures sustained high maize yield and environmental sustainability over the long-term.
Methods
A winter wheat-summer maize double cropping system was established in the North China Plain (NCP) in 2010. This included five continuous N fertilizer treatments during the maize season, comprising 0, 75, 150, 225 and 300 kg N ha−1, which were denoted as N0, N75, N150, N225 and N300 in the study.
Results
Increasing N rates resulted in higher N surpluses and SMN, leading to much higher aboveground biomass and maize yield, but also caused reduced NUE over the 12-year period. Specifically, the mean maize yield was 6.7, 8.8, 9.9, 10.4 and 10.1 Mg ha−1 for the N0–N300 treatments during 2011–2022, respectively. The low yield in the N0 treatment was mainly because long-term zero N inputs led to low SMN thus restricting roots growth, aboveground biomass accumulation and yield formation. In contrast, high SMN inhibited roots distribution, which subsequently negatively influenced post-silking dry matter remobilization and yield and thus caused a much lower harvest index (HI) in the N300 treatment. Effects of N fertilizer on maize yield intensified along with increasing experimental duration, thus requiring more N to achieve high yields in the latter years. The optimal N rates showed a strong positive correlation with the annual maximum yields over the experimental period, averaging 153 kg N ha−1 in achieving high yield, high NUE and low surplus.
Conclusions
Our findings demonstrated that the prolonged low N input could result in soil depletion, limiting maize growth and thereby compromising yield sustainability. Conversely, excessive N application led to SMN accumulation and higher N loss risks. The optimal N rate is 153 kg N ha−1 that can obtain long-term high maize yield stability while minimizing environmental costs in the NCP.
Implications
Our long-term experimental results provide robust evidence for optimizing N fertilizer applications in achieving high yield and high NUE with low N surplus in maize production in the wheat-maize double cropping in the NCP and similar cropping systems worldwide.