Neighboring tree species alter uptake of NH4+ and NO3− by Chinese fir

Most of the plantations were planted with single species for economic reasons. Continuous planting of such single tree species over a longer period has led to a series of ecological problems including soil degradation, low stand productivity and ecosystem stability. Studies have suggested that mixed-species plantations have greater levels of ecosystem functions and services such as higher nutrient utilization and productivity than do monocultures. Chinese fir (Cunninghamia lanceolata), as an important tree species of fast-growing timber, has been widely planted in subtropical areas of China over 4 decades. The growth of Chinese fir is largely modified by soil nutrient dynamics. To better manage Chinese fir plantation and provide suitable tree species selection for mixed Chinese fir, the study aims to explore the effect of different neighboring species on nitrogen (N) uptake of Chinese fir because N is a major nutrient limiting plant growth. Three typical species (Schima superba, Michelia macclurei, and Pinus massoniana) were selected to plant with Chinese firs. Similar size of 1-year-old seedlings was planted in pots, each containing one C. lanceolata and one other tree species, with two C. lanceolata seedlings in one pot as the control. After 6 months, the soil was labeled with 15NH4+ or 15NO3− at 0–5 cm and 5–15 cm depths. The results showed that the neighbor species identity caused the changes of Chinese firs biomass. The above- and below-ground biomass of Chinese firs in monoculture was 8.7 g and 4.2 g, respectively, while P. massoniana increased it by 60% and 40%, respectively. M. macclurei did not change the biomass of Chinese firs, while S. superba decreased the above- and below-ground biomass by 28% and 33%, respectively. Neighboring tree species increased N uptake rates of Chinese firs from the 5- to 15-cm-deep layer, but not from the 0- to 5-cm-deep layer. Chinese firs preferred NO3− in the 5- to 15-cm-deep layer, while in the 0- to 5-cm-deep layer, it did not show preference between the two N forms. These findings demonstrated, under the pot-specific vertical distribution patterns of roots, species interaction between certain neighboring species promoted the growth and N uptake of Chinese firs as they had flexible uptake strategies to use inorganic N in different soil layers. This pot study provided insights into forest management to enhance ecosystem services through the selection of appropriate combinations of tree species.