[Effects of short-term combined application of ammonium nitrogen and nitrate nitrogen on the growth and leaf traits of Machilus pauhoi seedlings].

Trees are characterized with selective absorption of different forms of nitrogen. Ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3--N) are the main forms of nitrogen for plant absorption. We examined the differences of absorption between NH4+-N and NO3--N for 1-year-old Machilus pauhoi seedlings planted in local hilly red soil in a pot experiment. A controlled experiment with 7 different NH4+-N/NO3--N treatments was conducted, to study the effects of nitrogen forms and different NH4+-N/NO3--N ratios on the growth and leaf traits of M. pauhoi seedlings. The results showed that there were no significant differences in the relative growth rate of ground diameter (GD), plant height (TH), and biomass (RGR) of M. pauhoi seedlings with different NH4+-N/NO3--N ratios for four months, but these parameters were relatively high under the treatment of NH4+-N:NO3--N=5:5. The seedlings of M. pauhoi didn't show obvious preference for NH4+-N and NO3--N in short term. The extremely low NH4+-N/NO3--N ratio application was unsuitable for their growth. Different NH4+-N/NO3--N application had significant effects on leaf area (LA), specific leaf area (SLA), leaf dry matter content (LDMC), leaf relative water content (LRWC), net photosynthetic rate (Pn), intercellular CO2 concentration (Ci), water use efficiency (WUE), and photosynthetic nitrogen use efficiency (PNUE). M. pauhoi seedlings under the treatment of NH4+-N:NO3--N=1:9 had the highest LA, SLA, Pn, WUE and PNUE. However, the seedlings under the treatment of NH4+-N:NO3--N=9:1 had the lowest LDMC, leaf tissue density (LTD), LRWC and Ci. Different NH4+-N/NO3--N combined application did not affect leaf nitrogen content (LN) and leaf phosphorus content (LP), which were highest under the treatment of NH4+-N:NO3--N=5:5. Across different NH4+-N/NO3--N combined treatments, GD, TH, and RGR were significantly negatively correlated with SLA, while both GD and RGR were significantly negatively correlated with PNUE. Our results could provide theoretical basis for precise nutrient management and high-efficiency cultivation techniques during the seedling stage of the M. pauhoi.