Seedling growth and nutrition responses of two subtropical tree species to NH4+-N and NO3−-N deposition

Abstract

Deposition rates of NH4+ and NO3−, two naturally occurring forms of nitrogen (N), vary worldwide. Here, we discuss the combined effect of deposition of N forms on plant growth in low-phosphorus (P) soil and analyze the patterns that trees utilize to adapt to specific N forms in low-P and high-P soils. One-year-old Schima superba and Pinus massoniana seedlings grown in soil with 1.1 mg kg−1 available P (low-P) and 25 mg kg−1 available P (high-P) were treated with NaNO3 or NH4Cl solutions: N0 = 0 (control), N80 = 80 kg N ha−1 year−1, and N200 = 200 kg N ha−1 year−1. Soil N, P, seedling traits,and leaf N and P contents were measured. Leaf N/P ratios were calculated to determine plant nutrient status. Pinus massoniana had a larger suitable P range and thus showed better growth in low-P soil than S. superba. Although S. superba preferred NH4+ compared with P. massoniana, the addition of NH4+ aggravated the plant growth disadvantage in low-P soil, and high concentrations resulted in the death of P. massoniana plants. The moderate addition of NO3− promoted the growth of the two species. However, the addition of a high dose of NO3− inhibited P. massoniana growth in both low-P and high-P soil. The addition of NH4+-N and NO3−-N resulted in an increase and decrease in the leaf N/P ratios and made plants prone to P and N limitation, respectively. The addition of NH4+ and NO3−-N resulted in different species responses. The responses of tree species to N deposition may vary depending on the forms and rates of N deposition, and species-dependent N-requirements, which in turn are based on the species-specific P-requirements and soil P content. We conclude that plant biomass is related to soil NH4+/NO3− ratios in high-P soil.