Effects of phenolic acids on soil nitrogen mineralization over successive rotations in Chinese fir plantations

Phenolic acids are secondary metabolites of plants that significantly affect nutrient cycling processes. To investigate such effects, the soil available nitrogen (N) content, phenolic acid content, and net N mineraliza_x0002_tion rate in three successive rotations of Chinese fir plantations in subtropical China were investigated. Net N mineralization and nitrification rates in soils treated with phenolic acids were measured in an ex situ experiment. Compared with first-rotation plantations (FCP), the contents of total soil nitrogen and nitrate in second (SCP)- and third-rotation plantations (TCP) decreased, and that of soil ammonium increased. Soil net N mineralization rates in the second- and third-rotation plantations also increased by 17.8% and 39.9%, respectively. In contrast, soil net nitrification rates decreased by 18.0% and 25.0%, respectively. The concentrations of total phenolic acids in the FCP soils (123.22 ± 6.02 nmol g-1 ) were 3.0% and 17.9% higher than in the SCP (119.68 ± 11.69 nmol g-1) and TCP (104.51 ± 8.57 nmol g-1 , respectively). The total content of phenolic acids was significantly correlated with the rates of net soil N mineralization and net nitrifi-cation. The ex situ experiment showed that the net N mineralization rates in soils treated with high (HCPA, 0.07 mg N kg-1 day-1 ) and low (LCPA, 0.18 mg N kg-1 day-1 ) concentrations of phenolic acids significantly decreased by 78.6% and 42.6%, respectively, comparing with that in control (0.32 mg N kg-1 day-1 ). Soil net nitrification rates under HCPA and LCPA were significantly higher than that of the control. The results suggested that low contents of phenolic acids in soil over successive rotations increased soil net N mineralization rates and decreased net nitrification rates, leading to consequent reductions in the nitrate content and enhancement of the ammonium content, then resulting in enhancing the conservation of soil N of successive rotations in Chinese fir plantation.