Low phosphorus and competition affect Chinese fir cutting growth and root organic acid content: does neighboring root activity aggravate P nutrient deficiency?

Abstract

In order to manage Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) plantations over the long term, it is critical to understand the species’ response to intraspecific competition at sites with low phosphorus (P). The objective of the present study was to investigate the effects of neighbor competition on the growth of Chinese fir under low P conditions, as well as on the morphology and organic acid content of the root tissues. We investigated the growth, dry matter accumulation, and organic acid content and type in root tips of Chinese fir cuttings in a greenhouse experiment, which was a design with three factors including competition intensity (single, mild, moderate, and severe), P supply level (0, 6, and 12 mg kg−1 KH2PO4), and competition time (50, 100, and 150 days). There were no significant interaction effects between all three or any two factors on shoot growth (height or root collar diameter), biomass accumulation, or root:shoot ratio. However, the interactive effects between competition time and competition intensity on all root morphological traits were significant, as was the interaction between P supply and competition (e.g., intensity, time) on root organic acid concentration. The number of organic acid types gradually decreased from the first 50 to 150 days under no P supply and low P supply treatment. Interestingly, capric acid was detected in severe and moderate competition treatments, but not detected under mild competition or in single plant controls at low P. Also, citric acid was detected in P supply treatments (6 and 12 mg kg−1), but not detected in the no P treatment. Mild early-stage intraspecific competition somewhat alleviated the suppressive effect of low P supply on the height of Chinese fir. Seedlings adjusted their shoot and root growth separately to accommodate stress from low P or adjacent competition. Low P supply and competition intensity additively accelerated root volume proliferation, but interactively enhanced organic acid concentration in root tips.