Effects of simulated nitrogen deposition on root exudates and phosphorus efficiency in Pinus massoniana families under low phosphorus stress

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Aims Atmospheric nitrogen (N) deposition largely increased in recent years, resulting in an increased N availability and N:P ratio in forest soils. The objective of this study was to determine the effects of simulated N deposition on P efficiency and root secreted acid phosphatase and organic acids in Pinus massoniana under low P stress. Methods Treatments included two P conditions, i.e. homogeneous low P availability vs. heterogeneous low P availability among soil layers, in combination with two N deposition levels in a two-year pot experiment. Four full-sib progenies collected from the second-generation breeding population of P. massoniana were used. Important findings Under both the homogeneous and heterogeneous low P conditions, N:P stoichiometric ratio in P. massoniana seedlings was significantly increased by simulated nitrogen deposition, which stimulated the amount of root acid phosphatase and organic acid secretion. The amount of root exudates was higher under the homogeneous low P condition than under the heterogeneous low P condition. The level of root secreted organic acids was significantly correlated with the growth in P. massoniana seedlings. Under the homogeneous low P condition, P acquisition efficiency and biomass in P. massoniana seedlings were not affected by simulated N deposition, mainly due to the high N:P ratios and low sensitivity to N addition; whereas under the heterogeneous low P condition, greater growth response to N addition was observed due to the higher N:P ratio. An increased root secretion of organic acids contributed to enhancement of P acquisition efficiency and growth under the high N deposition level. Significant variations among families in growth response to the simulated N deposition were observed. Under the homogeneous low P condition, seedlings in the family 71 × 20 were found to respond to the simulated N deposition with increased root organic acids and biomass. Under the heterogeneous low P condition, seedlings in the families of 36 × 29, 71 × 20, and 73 × 23 responded to the simulated N addition with increased 28 植物生态学报 Chinese Journal of Plant Ecology 2014, 38 (1): 27–35 www.plant-ecology.com biomass and root secretion.