亚热带常绿阔叶林不同深度土壤胞外酶活性及化学计量比对长期氮添加的响应

    Responses of extracellular enzyme activities and soil stoichiometry to long-term nitrogen deposition at different depths in a subtropical evergreen broad-leaved forest

      摘要
    • 摘要: 氮沉降通过调控土壤胞外酶活性来影响土壤的碳、氮、磷养分的释放过程。尽管已有研究关注土壤胞外酶活性及其化学计量比对氮添加的响应，但长期（＞10年）氮添加的影响仍不清楚。依托11年的野外氮添加实验样地，以亚热带常绿阔叶林为研究对象，探究3种氮添加水平对照（CK，0 kg·hm−2）、低氮（LN，40 kg·hm−2）、高氮（HN，80 kg·hm−2）处理下不同土层深度（0～10、10～20、40～60、80～100 cm）的土壤胞外酶活性β-葡萄糖苷酶（BG）、N-乙酰-β-氨基葡萄糖苷（NAG）、酸性磷酸酶（AP）、亮氨酸氨基肽酶（LAP）及其化学计量比BG/AP、BG/（NAG+LAP）、（NAG+LAP）/AP的变化，揭示不同施氮水平下不同土层深度的微生物养分限制及其响应机制。结果表明：（1）土壤速效磷（aP）、硝态氮（NO3−-N）和矿质氮（AN）含量随土层加深而降低，而HN处理显著提高了NO3−-N和AN含量；（2）HN处理显著提高土壤BG、NAG和LAP活性，氮添加与土层的交互作用显著影响土壤AP活性，在0～10 cm土层中LN处理的土壤AP活性显著高于CK处理和HN处理，其余土层的氮添加水平下没有显著差异；（3）（NAG+LAP）/AP和BG/AP随着土层增加而增加且不受施氮水平的影响，而LN处理显著降低了BG/AP与（NAG+LAP）/AP；（4）土壤微生物碳限制随土层加深而增强，LN处理缓解土壤微生物碳限制。而3个氮添加水平下均存在微生物氮限制，LN处理显著降低0～10 cm土层下微生物的氮限制，但土层加深会加剧土壤微生物氮限制；（5）矢量角度（VA）与土壤碳氮比（Soil C/N）、碳磷比（Soil C/P）以及总碳（TC）、总氮（TN）、总磷（TP）、矿质氮（AN）、有效磷（aP）含量呈正相关，而矢量长度（VL）与BG/（NAG+LAP）、（NAG+LAP）/AP、BG/AP、土壤含水量（SWC）和pH呈显著正相关。长期氮添加提高了土壤的氮有效性，缓解了表层土壤微生物碳限制。但微生物氮限制一直存在，且随着土层深度增加，土壤养分限制更强。研究阐明了长期氮沉降下微生物养分限制的垂直分异特征及其驱动机制，为预测森林生态系统养分循环对全球变化的响应提供科学依据。

       

      Abstract: Nitrogen （N） deposition profoundly affect the release processes of carbon （C）, nitrogen （N）, and phosphorus （P） by regulating soil extracellular enzyme activities （EEAs）. This study utilized an 11-year field-based N addition experiment in a subtropical evergreen broad-leaved forest to investigate the changes in EEAs （β-1,4-glucosidase （BG） and β-1,4-N-acetyl-glucosaminidase （NAG）, leucine aminopeptidase （LAP）, acidic phosphatase （AP）） and their stoichiometric ratios （BG/AP, BG/（NAG+LAP）, （NAG+LAP）/AP） under three N addition levels （Control, CK, 0 kg·hm2, low nitrogen, LN, 40 kg·hm2, high nitrogen, HN,80 kg·hm2） and four soil depths （0～10 cm and 10～20 cm in the top layer, 40～60 cm in the middle layer, and 80～100 cm in the deep layer）. The aim was to reveal the response mechanisms of microbial nutrient limitations and their correlations with soil nutrient stoichiometric ratios under varying N inputs and soil depths. The results showed that: （1） HN significantly increased soil nitrate nitrogen（NO3−-N） and mineral nitrogen （AN） content; both soil nitrate nitrogen and mineral nitrogen decreased with increasing soil depth, as did soil available phosphorus （aP）; （2） HN significantly enhanced BG, NAG, and LAP activities. The interaction of N addition and depth significantly affected AP activity: in the 0～10 cm layer, LN increased AP activity compared to CK and HN, with no significant differences in deeper layers.（3） LN significantly reduced BG/AP and （NAG+LAP）/AP ratios, both of which were positively correlated with soil depth; （4） Microbial C limitation intensified with depth, while LN alleviated it. All treatments exhibited microbial N limitation, which was reduced by LN in the 0～10 cm layer but intensified with depth. （5） Vector angle （VA） was positively correlated with soil C/N, C/P, total C （TC）, total N （TN）, total P （TP）, AN and aP. Vector length （VL） showed positive correlations with BG/（NAG+LAP）, （NAG+LAP）/AP, BG/AP, soil water content （SWC） and pH. This research elucidates the vertical differentiation characteristics of microbial nutrient limitations under long-term nitrogen deposition and their driving mechanisms, providing a scientific basis for predicting the response of forest ecosystem nutrient cycling to global change.

       

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