关键词: 杨树, 经营模式, 土壤, 氮含量, 酶活性, 防护林, 江苏宜兴

Abstract:

【Objective】This study investigated the effects of three forest management modes on soil nitrogen content and related enzyme activities associated with the construction of shelterbelts surrounding the shore of Taihu Lake.【Method】Poplar plantations that have been standardized and aged, with three management modes, i.e., Populus euramericana cv. 35 (poplar) pure plantation (PPP), Populus euramericana cv. 35-Ligustrum lucidum(poplar-ligustrum) mixed plantation (LMP), and Populus euramericana cv. 35-Photinia serrulata (poplar-photinia) mixed plantation (PMP), were selected in Zhoutie Town of Yixing City in Jiangsu Province, China. Soil samples were collected from a one meter-deep soil profile to measure the soil nitrogen content, the activities of related enzymes in spring, and the relationship among them. The physical and chemical properties of the soil were also analyzed.【Result】① The levels of total nitrogen (TN) content were about 0.17-1.35 g/kg and were significantly (df=2, F=102.820, P<0.05) affected by the applied forest management modes. Compared with the content of TN in PPP, those in LMP and PMP increased by 21.8% and 69.7%, respectively. Along the soil profiles, the TN content decreased with increasing soil depth in the two mixed plantations and was significantly different among layers (df=3, F=108.289, P<0.05). The levels of nitrate nitrogen (N {\mathrm{O}}_3^{-} -N) and ammonium nitrogen (N {\mathrm{H}}_4^{+} -N) contents were 5.67 to 9.79 mg/kg and 3.22 to 12.43 mg/kg, respectively. They were also significantly affected by management modes (df=2, F=18.764, P<0.05; df=2, F=9.655, P<0.05). The content of N {\mathrm{O}}_3^{-} -N in LMP and PMP decreased by 11.8% and 27.3%, respectively, compared with that in PPP; while the content of N {\mathrm{H}}_4^{+} -N was significantly increased in the layers of ≥20-40, ≥40-60, and ≥60-80 cm when compared with those in PPP (df=3, F=106.230, P<0.05; df=3, F=119.794, P<0.05). Along the soil profiles, N {\mathrm{O}}_3^{-} -N and N {\mathrm{H}}_4^{+} -N contents generally decreased with increasing soil depth in the three forest management modes. The level of microbial biomass nitrogen (MBN) was about 6.04-9.52 mg/kg and the mean value of MBN in the LMP was 7.5% higher than that in PPP. ② The activities of soil urease (UE), nitrate reductase (NR), and nitrite reductase (NiR) were 3.43-9.16, 0.16-1.04, and 0.15-0.25 mg/(g·d), respectively. Compared with PPP, the activity of the UE in LMP was significantly increased in all soil layers (df=2, F=19.600, P<0.05), while the activity of NR in topsoil was significantly decreased (df=3, F=43.637, P<0.05). In PMP, however, the activity of UE was significantly decreased (df=3, F=17.825, P<0.05), but the activity the of NR was increased in comparison to those in the PPP. Moreover, there was no significant difference in soil NiR activity among the three forest management modes.These results indicate that UE and NR are more sensitive to management modes and could serve as indicators of soil nitrogen dynamics. In addition, soil nitrogen content and enzyme activities were closely correlated with water content and pH value.【Conclusion】The LMP and PMP management modes can improve the soil available nutrients, efficiently absorbing and decreasing N {\mathrm{O}}_3^{-} -N in the soil, when compared with the PPP. In this way, the risks of nitrate leaching into the deep soil layer and that of polluting shallow groundwater are reduced. At the same time, the change in microbial biomass nitrogen and enzyme activity in the soil accelerated the soil nitrogen transformation and nutrient cycling. This suggests that multi-layer mixed forest management modes should be introduced and applied in the process of constructing shelterbelt forests surrounding the shore the Taihu Lake.

Key words: poplar, management mode, soil, nitrogen content, enzymatic activity, shelterbelts, Yixing City,Jiangsu Province