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Given the important applications of oriental arborvitae (Platycladus orientalis /L./ Franco), we examined the interaction effects of soil moisture content (SMC) and urea fertilizer on survival, growth and some physiological characteristics of seedlings of this species. In this study, 270 uniform oriental arborvitae seedlings were transplanted in 2 kg containers filled with the top soil of the natural stand in Soorkesh forest reserve. After 2 months, 6 levels of SMC (100%, 75%, 50%, 25%, 15% and 10%) and 3 levels of urea [control: 0, low nitrogen (LN): 75 mg·kg^–1, and high nitrogen (HN): 150 mg·kg^–1] were used from June till the end of the first growing season in 3 replications and 5 seedlings per replication as subsamples. The results showed that all seedlings survived under SMC above 25% in control and urea fertilizer treatments. However, under 15% and 10% SMC, the seedling survival decreased significantly in control treatment, but adding urea fertilizer significantly increased the survival, especially in HN treatment. Growth and morphological properties of seedlings, including root and shoot length, number of first-order lateral roots and total biomass, showed a significant decrease at 75%. Seedling quality index gradually decreased with SMC reduction, but the addition of urea to the soil significantly increased it under all SMC treatments. The results also indicated that a decrease in SMC significantly decreased relative water content of leaves and total chlorophyll content, and urea fertilizer increased total chlorophyll under all SMC treatments, but it increased relative water content only at 100% to 25%. Moreover, water use efficiency of P. orientalis seedlings significantly increased under low soil moisture treatments, and urea significantly increased it, especially under HN treatment. In conclusion, urea fertilizer, especially in the higher amount (150 mg·kg^–1), improved growth and quality of seedlings under different soil moisture conditions, and increased their survival under severe drought.

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Poplar (Populus deltoides Bartr. ex Marsh.) produce a large amount of biomass per unit area and is important fast-growing species in different planting systems. However, the appropriate space between poplar trees is essential to high-performance productivity in diverse regions. The present study monitored the effect of different spacing configurations (A: 4 × 3 m; B: 4 × 3 m (pure poplar); C: 6 × 3 m; D: 8 × 3 m; E: 10 × 3 m and F: pure crop) of poplar-based agroforestry with two canola (Brassica napus L.) and wheat (Triticum aestivum L.) cropping system on biomass production and carbon storage. Over the eight years, the total poplar biomass production was significantly different (P ≤ 0.05) in poplar-based configurations and ranged from 9.1 to 13.4 mg ha–1. The highest carbon storage of 6.5 mg ha–1 was observed in configuration E. Crop production of canola and wheat in configurations B, C, D, and E, did not show a significant difference with pure crop cultivation, while configuration A was significantly lower. Our result indicated that configuration E, with the highest total biomass production but no significant difference in crop production, is the optimum system of poplar-based agroforestry in regions with similar temperate climate conditions of Northern Iran. Finally, poplar-based agroforestry provides high efficiency of carbon sequestration in trees which can conserve all market and non-market benefits. Keywords: aboveground biomass, carbon concentration, crop yield, productivity, Populus deltoides, Northern Iran