秦岭地区华北落叶松人工林地土壤养分和酶活性变化

以秦岭地区不同林龄(5年生、10年生、20年生、30年生和40年生)华北落叶松人工林为研究对象,采用野外调查采样和室内分析相结合的方法,研究了不同林龄华北落叶松人工林地土壤剖面p H值、有机质养分和酶活性的变化。结果表明:土壤p H值随着林龄有降低趋势,随着土层深度的增加有升高趋势。土壤有机质和土壤速效氮、速效磷和速效钾在近熟龄期显著高于幼龄期。土壤中的磷酸酶、脲酶、蔗糖酶和过氧化氢酶活性随着林龄都呈"高—低—高"的趋势,幼龄期的土壤蔗糖酶活性显著高于近熟龄的,而磷酸酶、脲酶和过氧化氢酶活性是近熟龄期的显著高于中幼龄。土壤速效养分和酶活性都随着土层深度的增加有显著的降低。相关分析表明磷酸酶活性与有机质、速效氮、速效磷、速效钾和脲酶活性呈极显著的正相关性(P0.01),脲酶与有机质、速效氮和速效钾呈极显著正相关。蔗糖酶与过氧化氢酶活性显著负相关(P0.05),与p H值有一定的正相关性。秦岭地区华北落叶松人工林进入近熟林之后土壤肥力有一定的恢复,而在中幼龄阶段土壤养分比较缺乏,尤其是氮磷。

Soil nutrients and enzyme activities in Larix principis-rupprechtii plantations in the Qinling Mountains,China

Larix principis-rupprechtii was introduced in Shaanxi Province more than 50 years ago, and most of these trees were planted in the Qinling Mountains. In order to study the fertility of soil in different-aged Larix principis-rupprechtii plantations in this area, we chose young- (5 and 10 years old), middle- (20 years old), and near-mature- (30 and 40 years old) aged Larix principis-rupprechtii plantations in the Nantan Nursery Garden of Taibai and Huoditang of Ning-shan in Shannxi Province as the experimental sites, and established three 20 × 20-m plots in each age stand plantation in 2012. In each plot, soil samples were collected from three depths (0-20 cm, 20-40 cm, and 40-60 cm) at five randomly located points. Using these samples, we examined the soil chemical properties (pH, organic matter, available N, available P, and available K) and enzyme activities (phosphatase, urease, invertase, and catalase) in the different-aged plantations. Soil pH in the 0-20-cm layer decreased from 6.71 in the 10-year-old plantation to 5.72 in the 40-year-old plantation; however, with an increase in soil depth there was a slight increase in pH. Soil organic matter and available nutrients in near-mature plantations were significantly higher than those in the young plantations (P<0.05). The variation ranges of these soil properties in the 0-20cm layer were as follows: organic matter, 10.12-23.12 mg/kg; available N, 27.82-142.82 mg/kg; available P, 1.91-2.53 mg/kg; and available K, 104.33-264.67 mg/kg. There were significant reductions in soil organic matter and available nutrients as the soil depth increased (P<0.05). With an increase in plantation age, the activities of soil phosphatase, urease, invertase, and catalase showed a "high-low-high" tendency. In the 0-20cm soil layer, the variation ranges for these activities were as follows: phosphatase, 28.06-53.93 mg/100 g; urease, 0.31-0.65 mg/g; invertase, 6.04-68.42 mg/g; and catalase 1.39-4.61 ml. The activities of these four enzymes conspicuously decreased in the 20-60cm layer (P<0.05). The activity of invertase in young plantations was significantly higher than that in the near-mature plantations, whereas for the other three enzyme activities, the converse was true. Correlation analysis indicated that soil pH was significantly negatively related to soil organic matter, available N, available P, and catalase activity. This suggested that the efficacy of soil nutrients increased as soil pH decreased, and that catalase promoted this effect. Phosphatase activity was highly significantly positively related to soil organic carbon and available N, P, and K (P<0.01), indicating that soil phosphatase increased the contents of soil available N and K, whereas it accelerated the decomposition of organic P. There were highly significantly positive correlations between urease activity and organic matter and available N, P, and K. In this respect, urease had the same effect as phosphatase. This study indicated that because of the lack of soil available N and available P in this area and the rapid growth of plants, the fertility of plantations in the Qinling Mountains until Larix principis-rupprechtii reaches an age of 20 years. With a further increase in plantation age, however, soil enzyme activities increased as organic matter, which provides zymolyte for soil enzymes, accumulated. This provided more available nutrients for Larix principis-rupprechtii and improved soil fertility. Accordingly, on the basis of these findings, we suggest that fertilizer should be applied to soil until Larix principis-rupprechtii plantations reach 20 years of age in order to avoid nutrient limitation for plant growth.