Above- and below-ground biomass distribution and morphological characteristics respond to nitrogen addition in Pinus tabuliformis

Background: With the continuing deposition of atmospheric nitrogen (N), N has become a major factor affecting ecosystem carbon (C) balance and N cycles. However, it is still unclear how increasing N deposition alters above-and below-ground biomass allocation, especially among different order roots. Pinus tabuliformis Carriere is an important native species in the Loess Plateau of China, as well as the preferred species for artificial afforestation in this area. Methods: One-year-old P. tabuliformis seedlings were treated with six different N concentrations (0, 0.02, 0.04, 0.08, 0. 16, and 0.32 g N kg(-1) soil year(-1)) for 3 years. Individual P. tabuliformis seedlings were separated into leaves, shoots, and roots in May 2011. Variations in leaf morphology, root characteristics, and their biomasses were investigated. Results: (1) The highest N fertilizer level (0.32 g N kg(-1) soil year(-1)) significantly decreased leaf area, root length, and specific root area. (2) Lower levels of N significantly reduced root length, root diameter, and specific root area in the first-and second-order roots. (3) Leaf biomass increased significantly in the higher N treatments (0.02-0.08 g N kg (-1) soil year(-1)). However, total seedling biomass under the highest treatment decreased significantly due to excessive N. Biomass of the roots and leaves in the 0.04 g N kg(-1) soil year(-1) treatment increased compared with that in the control. Conclusions: (1) The lower-order roots (and leaves) were more sensitive than the higher-order roots (and stems) to N application. (2) The specific root area to specific leaf area ratio might be more suitable than area or biomass indices in explaining the utilisation of N by plants. (3) The functional balance hypothesis did not adequately explain the distribution of photosynthetic products among P. tabuliformis organs, but the source-sink relationship hypothesis did. This indicates that roots were the main sink and received a greater distribution of photosynthetic products.