模拟氮沉降对太岳山油松林土壤酶活性的影响

为研究土壤酶活性对氮沉降增加的响应,以山西太岳山油松人工林和天然林为研究对象,于2009年8月开始实施模拟氮沉实验,试验设置对照(CK,0 kg N hm-2a-1);低氮(LN,50 kg N hm-2a-1);中氮(MN,100 kg N hm-2a-1);高氮(HN,150 kg N hm-2a-1)4种氮处理,自2012年起每年5、7、9月在各处理样方采集表层0—20 cm土壤,测定土壤酶活性(过氧化物酶、多酚氧化酶、纤维素酶、蔗糖酶、脲酶、中性磷酸酶)。研究结果表明:施氮处理下的脲酶与中性磷酸酶活性均有所提高,而低氮处理下天然林中的多酚氧化酶与人工林中的蔗糖酶显著低于对照,中氮、高氮处理下过氧化物酶、多酚氧化酶、天然林中的纤维素酶以及人工林中的蔗糖酶显著降低。总的来说,人工模拟氮沉降促进了土壤中脲酶和中性磷酸酶的活性,抑制了过氧化物酶和多酚氧化酶的活性,并降低了天然林土壤中的纤维素酶活性和人工林中的蔗糖酶活性,但对天然林中蔗糖酶和人工林中的纤维素酶无影响。主导木质素降解的多酚氧化酶活性与纤维素酶、蔗糖酶活性显著相关,纤维素酶与蔗糖酶活性的下降可能是由木质素降解受到抑制,土壤微生物可利用碳源减少所引起。另外,受到天然林土壤含氮量较高的影响,与人工林相比,天然林的多酚氧化酶活性对模拟氮沉降更敏感。由于被抑制的酶均与土壤有机质降解密切相关,氮沉降增加将减缓山西油松林土壤有机质的降解,有利于有机质在土壤中的积累。

Effects of simulated nitrogen deposition on the soil enzyme activities in a Pinus tabulaeformis forest at the Taiyue Mountain

Human activities had dramatically increased the quantity of atmospheric nitrogen deposition since the industrial revolution. Increased nitrogen deposition in forest ecosystems would not only have a direct impact on tree growth, but also indirect effects of soil carbon storage influencing the microbial degradation of soil organic matter. Soil enzymes produced by microbes play a key role in the biochemical functioning of soil organic matter decomposition and nutrient cycling. Soil enzyme activity is one of the indicators of soil microbial community activity. In recent years, numerous studies have shown that increasing atmospheric nitrogen deposition had significant effects on soil enzyme activities. However, soil enzyme activities in different ecosystems always showed different responses to increasing nitrogen deposition. Thus, it was necessary to assess the impact of nitrogen deposition on forest soil enzyme activities in two different ecosystems. In this study, four treatments were installed in a plantation and a natural forest of Pinus tabulaeformis at the Taiyue Mountain of Shanxi Province China, starting in August 2009 and including four treatments: a) control (CK, 0 kg N hm^-2 a^-1), b) low nitrogen (LN, 50 kg N hm^-2 a^-1), c) medium nitrogen (MN, 100 kg N hm^-2 a^-1), and d) high nitrogen (HN, 150 kg N hm^-2 a^-1). Each treatment comprised three replicate plots of 2m×2m which were sprayed with CO(NH₂)₂ solutions at the beginning of each month. Soil samples were collected at a depth of 0-20 cm in May, July and September since 2012, and the activities of invertase, cellulase, peroxidase, polyphenol oxidase, urease and neutral phosphatase were determined. Soil invertase and cellulase were assessed using the 3,5-dinitrosalicylic acid colorimetric method, peroxidase and polyphenol oxidase by pyrogallol colorimetric method, urease by phenol-sodium hypochlorite colorimetric method and neutral phosphatase by di-sodium phenyl phosphate colorimetric method. Urease and neutral phosphatase activities increased with increasing nitrogen levels; polyphenol oxidase activity of the natural forest and invertase activity of the plantation decreased significantly in the low nitrogen treatment (P < 0.05); medium and high nitrogen treatments significantly inhibited peroxidase and polyphenol oxidase in both ecosystems, and cellulase in the natural forest and invertase in the plantation (P<0.05). In conclusion, simulated N deposition inhibited the activities of soil peroxidase and polyphenol oxidase, and stimulated urease and neutral phosphatase activities. Cellulase activity in the natural forest and invertase activity in the plantation were reduced by simulated N deposition, yet invertase activity in the natural forest and cellulase activity in the plantation had no significant variation. The polyphenol oxidase activity which plays a key role in the degradation of lignin significantly associated with activities of cellulase and invertase (P < 0.05) and the inhibition of lignin degradation would decrease soil dissolved organic matter. Therefore the inhibition of cellulase and invertase may be caused by the reduction of the carbon source which can be utilized by soil microbes. Furthermore, polyphenol oxidase activity was more sensitive in the natural forest than in the plantation because the natural forest had a higher soil nitrogen content. Because the enzymes which were inhibited were related to degradation of soil organic matter, the inhibition of soil activities under simulated nitrogen deposition could slow down the degradation of soil organic matter, leading to the increase of soil organic matter.