Effects of long-term nitrogen application and irrigation on soil organic nitrogen fractions and soluble nitrogen in Populus tomentosa plantations in the North China Plain

• Runzhe ZhangBeijing Forestry University
• Lishui NieBeijing Forestry University
• Mengyao HuangBeijing Forestry University
• Jiang WangBeijing Forestry University
• Nie HaoliangBeijing Forestry University
• Jialei ZhuShaoguan University
• Huijuan BoShanxi Agricultural University
• Yuanxin LiuBeijing Institute of Technology
• Yonglin ZhengBeijing Forestry University