长期氮添加对落叶松和水曲柳人工林土壤碳、氮、磷含量和胞外酶活性的影响

【目的】氮(N)沉降速率的持续增加影响着森林生态系统的物质循环。本研究旨在探讨长期N添加对两种人工林土壤微生物生物量和胞外酶活性的影响。【方法】基于黑龙江帽儿山森林生态系统国家野外科学观测研究站落叶松(Larix gmelinii)和水曲柳(Fraxinus mandshurica)人工林的长期(16年)N添加N添加速率为10 g/(m²·a)]试验,测定对照和N添加处理样地土壤的碳(C)、N、磷(P)各组分含量、微生物生物量以及C、N、P循环相关的胞外酶活性。【结果】长期N添加显著增加了两种人工林无机N含量,但降低了两种人工林微生物生物量C、N含量。N添加抑制了水曲柳人工林β-葡萄糖苷酶、过氧化物酶和酚氧化酶活性,但对落叶松人工林的这些C分解酶活性的抑制作用不显著。回归分析进一步发现微生物生物量、β-葡萄糖苷酶、过氧化物酶和酚氧化酶均随着土壤pH的降低而显著降低。几丁质酶活性在落叶松人工林中对N添加呈现负响应,却在水曲柳人工林中对N添加呈现正响应。然而,N添加均未显著改变两种人工林的磷酸酶活性、总有机C,以及酸水解法划分的C组分、全N、全P和有效P含量。【结论】长期N添加主要通过土壤酸化途径改变落叶松和水曲柳人工林土壤微生物生物量和酶活性,而N添加对胞外酶活性的影响因树种而异,这可能与两个树种的不同凋落物质量和菌根类型有关。N添加对土壤微生物和C分解酶活性的抑制作用并没有显著增加土壤C含量和改变C组分,其内在机制需要未来从土壤有机C的形成和稳定性方面进行揭示。

Effects of long-term nitrogen addition on soil carbon,nitrogen, phosphorus and extracellular enzymes in Larix gmelinii and Fraxinus mandshurica plantations

【Objective】Anthropogenic nitrogen (N) deposition is expected to continuously increase, which has significant effects on material circulation in forest ecosystems. The objectives of the current study were to explore the responses of microbial biomass and extracellular enzymes to long-term N addition and the regulation of tree species.【Method】A long-term (16-year) N addition experiment with a N addition rate of 10 g/(m²·a) in larch(Larix gmelinii) and ash(Fraxinus mandshurica) plantations was conducted at the Maoershan Forest Ecosystem Research Station, Heilongjiang Province, Northeast China. Soil C, N and P fractions, microbial biomass, and C-, N- and P-cycling related extracellular enzymes were investigated in control and N added plots.【Result】Results show that long-term N addition significantly increased the soil inorganic N but reduced the soil microbial biomass C and N in both plantations. N addition inhibited the activities of β-1,4-glucosidase, peroxidase and phenol oxidase in the F. mandshurica stand but had no effect on these C-degrading enzymes in the L. gmelinii stand. Regression analysis showed that microbial biomass and C-degrading enzymes (β-1,4-glucosidase, peroxidase and phenol oxidase) consistently decreased with decreasing soil pH. N-acetyl-β-glucosaminidase showed a negative response to N addition in the L. gmelinii stand but displayed a positive response to N addition in the F. mandshurica stand. However, N addition had no effect on the activity of acid phosphatase, total organic C, C fractions measured by an acid hydrolysis approach, total N, total P, and available P.【Conclusion】Long-term N addition has altered the soil microbial biomass and C-degrading enzymes in the L. gmelinii and the F. mandshurica plantation through soil acidification approach, while the N effects on extracellular enzymes are species-specific. This may have resulted from the differences in litter quality and mycorrhizal types between the L. gmelinii and the F. mandshurica. The inhibition of N addition on the microbial biomass and the C-degrading enzymes did not increase the soil organic C content and alter the C fractions. The mechanism for this phenomenon may be revealed by the formation and stabilization of soil organic C in a future study.