Root morphology and physiology responses of two subtropical tree species to NH4+-N and NO3--N deposition in phosphorus-barren soil

Both NH4+ and NO3- are capable of greatly influencing plant growth and biomass. However, the belowground responses of subtropical trees to NH4+ or NO3- deposition in phosphorus-barren soil remain poorly understood. Here, we discuss the effects of these two forms of N deposition on root development and experimentally analyze the impact on nitrogen and phosphorus absorption in two types of subtropical tree species (Schima superba; a broadleaved tree with a fibrous root system, and Pinus massoniana; a conifer with a taproot system). In a greenhouse in southern China, 1-year-old S. superba and P. massoniana seedlings grown on P-limited and P-normal soil were treated with NaNO3 and NH4Cl solutions of 0, 80, and 200 kg N ha(-1) year(-1), corresponding to the control, N80, and N200 groups, respectively. Root phenotype characteristics and metabolic ability were measured after 8 months of growth. The results showed that the root morphology and exudates variables differed significantly between the two species under different N and P treatments, and the root growth rate and P absorption were higher in P. massoniana. N addition differentially affected root growth and activity as follows: (1) N80 increased the root growth and activity of the two species, but NH4+-N80 led to thicker roots in S. superba compared with P. massoniana; (2) N200 had inhibitory effects on the roots of P. massoniana, for which NH4+-N200 led to thinner and longer roots and even the death of some roots; and (3) NH4+-N promoted phosphorus absorption efficiency and nitrogen absorption efficiency in the thicker roots (> 1.5 mm), and NO3--N stimulated activity in the thinner roots (0.5-1.5 mm) of S. superba. The opposite was observed for P. massoniana. P. massoniana had a higher root absorption efficiency; however, nitrogen deposition was found to be more beneficial to the root growth of S. superba.