Characterization of low-N responses in maize (Zea mays L.) cultivars with contrasting nitrogen use efficiency in the North China Plain oleh Xiang-ling LI, Li-guo GUO, Bao-yuan ZHOU, Xiang-ming TANG, Cong-cong CHEN, Lei ZHANG, Shao-yun ZHANG, Chong-feng LI, Kai XIAO, Wei-xin DONG, Bao-zhong YIN, Yue-chen ZHANG