Quantifying the role of peanut root and root nodule in nitrogen absorption and fixation under four forms of N fertilizers

Peanut (Arachis hypogaea L.) is an important oilseed and leguminous crop. The effect of various forms of nitrogen (N) fertilizer on peanut N acquisition needs to be evaluated by clarifying the N sources from root system absorption and from root nodule fixation. In this study, four kinds of N fertilizers (CONH2–N, NH4+-N, NO3−-N, NH4++NO3−-N, labeled by 15N isotope) were used to quantify N absorption and N fixation of peanut, as well as, their relationships with peanut root morphological characteristics and root nodule properties. Results demonstrated that peanut of CONH2–N treatment held the highest N absorption and N fixation amounts, followed by NH4+-N, NH4++NO3−-N and NO3−-N treatments. Detailly, CONH2–N treatment significantly increased total root length/surface area (especially for fine root of 0–1.0 mm) and N absorption/fixation amounts, suggested that significant correlations were exist between root length/surface area and N absorption, as well as, between root nodule number/weight and N fixation. In addition, with the increase of total root length of 100 cm/plant and root surface area of 10 cm2/plant, N absorption amount of peanut was separately increased by 10.0 and 8.1 mg/plant. With the root nodule number and weight increased by 1.0 unit/plant and 10 mg/plant, N fixation amount of peanut was increased by 13.0 and 17.2 mg/plant during root nodule development. In general, CONH2–N is the optimal N fertilizer for peanut growth and development with the highest root N absorption and root nodule N fixation, and was suitable for the rational N management of peanut planting.