固氮树种对桉树人工林土壤团聚体酶活性及其化学计量比的影响

Effects of nitrogen-fixing tree species on soil aggregate-associated enzyme activities and ecoenzymatic stoichiometric ratios in Eucalyptus plantations

关键词:  土壤酶活性, 酶化学计量, 桉树人工林, 固氮树种, 土壤团聚体

Key words:  soil enzyme activity, ecoenzymatic stoichiometry, Eucalyptus plantations, nitrogen-fixing tree species, soil aggregate

摘要:

土壤酶活性及其化学计量比是反映土壤养分有效性和微生物生长代谢养分需求的重要指标,但固氮树种对桉树人工林土壤团聚体酶活性及其化学计量特征的影响尚不明确。为探究桉树人工林土壤团聚体酶活性及其化学计量比对引入固氮树种的响应及其主要影响因子,该文通过选择11年生巨尾桉纯林(PP)和巨尾桉/降香黄檀(固氮树种)(MP)混交林作为研究对象,采集0～10 cm的原状土样,采用改良干筛法将土壤分成>2 mm、1～2 mm、0.25～1 mm和<0.25 mm 4个径级,测定各径级土壤团聚体的理化性质和与土壤碳(C)、氮(N)、磷(P)循环相关的水解酶活性 [β-葡萄糖苷酶(BG)、N-乙酰-葡萄糖苷酶(NAG)、亮氨酸氨基肽酶(LAP)、 酸性磷酸酶(ACP)]。结果表明:(1)PP和MP土壤团聚体均以>2 mm为主,其含量高于其他粒径团聚体。与PP相比,MP中各粒径土壤团聚体的有机碳(SOC)、总氮(TN)、 硝态氮(NO₃^--N)、有效磷(AP)、pH、BG、NAG和ACP均显著提高(P<0.05),但LAP活性的变化不显著。(2)pH、TN、AP、 NO₃^--N和铵态氮(NH₄⁺-N)均与酶化学计量比呈不同程度的显著相关关系,而冗余分析(RDA)发现,NO₃^--N、SOC和AP的含量是驱动土壤团聚体水解酶活性和酶化学计量比产生差异的最主要环境因子(P<0.05)。(3)该研究区域土壤C:N:P的酶化学计量比分别是1:0.86:0.74,与全球生态系统C:N:P酶化学计量比1:1:1相偏离,表明该区域的微生物生长代谢容易受到C源限制; MP中大多径级的土壤团聚体C:N和C:P酶化学计量比均低于PP,且在<0.25 mm径级中差异显著(P<0.05)。综上认为,固氮树种的引入,不仅能缓解桉树人工林土壤微生物生长代谢的C源限制,而且在一定程度上能缓解其土壤的N、P限制,为固氮树种在提高桉树人工林的土壤质量和生产力等方面的应用提供了科学依据。

Abstract:

Soil enzyme activity and its stoichiometric ratio are important indicators reflecting soil nutrient availability and microbial growth and metabolic nutrient requirements. However, the effects of nitrogen-fixing tree species on soil aggregate-associated enzyme activities and ecoenzymatic stoichiometric characteristics in Eucalyptus plantations are still unclear. In this study, an 11-year-old pure E. urophylla plantation(PP)and an 11-year-old mixed E. urophylla and Dalbergia odorifera plantation(MP)in the Shaoping Experimental Field of the Tropical Forestry Experimental Centre of Chinese Academy of Forestry were selected as the research objects. Soil aggregates which were collected from 0-10 cm soil layer were classified into >2 mm, 1-2 mm, 0.25-1 mm, and <0.25 mm fractions by the improved dry-sieving procedure. And then the soil physicochemical properties and the activities of hydrolase [β -glucosidase(BG), N-acetyl-glucosidase(NAG), leucine aminopeptidase(LAP)and acid phosphatase(ACP)] related to soil carbon(C), nitrogen(N)and phosphorus(P)cycling in soil aggregates were measured and the effects of mixing nitrogen-fixing tree species with Eucalyptus on soil enzyme activities and its stoichiometry were examined. The results were as follows:(1)The dominant soil aggregates were >2 mm fractions which were higher than other aggregates. The contents of SOC, TN, NO₃^--N, AP, pH and activities of BG, NAG, ACP were significantly increased on each aggregate fractions except for LAP, respectively, in MP than in PP.(2)pH, TN, AP, NO₃^--N, NH₄⁺-N had significant correlation with the ecoenzymatic stoichiometry. The redundancy analysis(RDA)found that the contents of NO₃^--N, SOC, and AP were key factors affecting the soil hydrolase enzyme activities and its stoichiometry.(3)The ecoenzymatic stoichiometry of C, N, P in this area was 1:0.86:0.74 [deviated from the global pattern(1:1:1)] which indicated that the soil microorganisms in this area were easily limited by C source. The C:N and C:P ratio of enzyme stoichiometric in most of the soil aggregates in MP were lower than those in PP, and significant differences were found in <0.25 mm fractions(P<0.05). Our findings suggest that the introduction of nitrogen-fixing tree species into eucalyptus plantations can not only alleviate the C source limitation of soil microbial growth and metabolism, but also effectively alleviate the N and P limitation of soil in this region. Findings from this study provide a scientific basis for the application of nitrogen-fixing tree species in improving soil quality and productivity of eucalyptus plantations.