Coupling effects of water and nitrogen on tree growth and soil water-nitrogen distribution in young Populus tomentosa plantations under surface drip irrigation

Soil water and nitrogen nutrients are key factors affecting forest growth, especially for young triploid Populus tomentosa. Water and fertilizer factors have synergistic effects, water deficit can inhibit the absorption of nutrients in forests, and excessive irrigation can easily cause nutrient leaching. Although excessive fertilization can improve the water and nitrogen use efficiency under certain conditions, the large amount of nitrogen remaining in the soil is bound to pollute the environment. At present, many scholars have simulated the soil moisture and nitrogen transport under drip irrigation through indoor soil column simulation test. The research on soil water and nitrogen transport in the field is mainly concentrated in the agricultural and horticultural field, and the key is to monitor nitrogen leaching and nitrogen balance. In the field of forestry, due to the complexity of field conditions and the simplification of evaluation indicators, most studies focus on the effects of different water-nitrogen coupling strategies on tree growth and physiological indicators, few researches were on the distribution of water and nitrogen in root zone. In order to explore the effects of water and nitrogen coupling treatments on growth of Populus tomentosa and water-nitrogen distribution in soil, a field experiment was conducted about 2-year-old triploid Populus tomentosa to investigate the effects of two irrigation levels (W20, W45) and three fertilizer levels (N80, N150, N220) coupling treatments on increment of diameter at breast height (DBH), dynamic distribution of soil water content (SWC) and mineral nitrogen content (Nmin) in 0-80 cm soil depth under drip emitter in growing season. Results showed that W20 treatment promoted the growth of DBH during April to July significantly (P<0.05), the interaction of irrigation and fertilizer factors was not significant (P>0.05). SWC was influenced greatly by irrigation treatment, which showed that the SWC of W20 treatment had reached 11.3% in dry season (from the end of April till the middle of June), 37.5% higher than blank control. There was no significant difference in different treatment (P>0.05）in rainy season (from the late June to early August), because SWC was influenced greatly by precipitation. Nmin was accumulated in 0 - 80 cm soil layer during dry season, especially in 0-20 cm soil layer, which showed positive correlation with fertilization. Nmin moved from topsoil towards deep soil in rainy season, which leached below 80 cm soil layer in W20 treatment and increased uniformly in 20-80 cm soil layers in W45. The average Nmin in 0-80 cm soil layer reached to 44.27 mg/kg in W45N150 treatment in rainy season, which was significant higher than other treatments (P<0.05). Nmin accumulated in 0-80 cm soil layer at the end of growing season. To sum up, N150 treatment can provide adequate nitrogen content in 0-80 cm soil layer in growing season. Combined with the growth of young trees, W20 treatment can promote the growth significantly during April to July (P<0.05), and W45 treatment can meet the water demand for growth of the tree after July.