氮添加对中国陆地生态系统植物-土壤碳动态的影响

Effects of nitrogen addition on plant-soil carbon dynamics in terrestrial ecosystems of China

近年来,中国大气氮沉降水平不断增加,过量的活性氮输入深刻影响了我国陆地生态系统碳循环。虽然已有大量的研究报道了模拟氮添加实验对我国陆地生态系统碳动态的影响,但是由于复杂的地理条件和不同的施氮措施,关于植物和土壤碳库对氮添加的一般响应特征和机制仍存在广泛争议。因此,采用整合分析方法,收集整理了172篇已发表的中国野外氮添加试验结果,在全国尺度上探究氮添加对我国陆地生态系统植物和土壤碳动态的影响及其潜在机制。结果表明,氮添加显著促进了植物的碳储存,地上和地下生物量均显著增加,且地上生物量比地下生物量增加得多。同时,氮添加显著增加了凋落物质量,但对细根生物量没有显著影响。氮添加显著降低了植物叶片、凋落物和细根的碳氮比。总体上,氮添加显著增加了土壤有机碳含量并降低了土壤pH值,但对可溶性有机碳、微生物生物量碳和土壤呼吸的影响并不显著。在不同的地理条件下,土壤有机碳含量对氮添加的响应呈现增加、减少或不变的不同趋势。回归分析表明,地上生物量与土壤有机碳含量之间,以及微生物生物量碳与土壤有机碳含量之间呈负相关关系。虽然氮添加通过增加凋落物质量显著促进了植物碳输入,但同时也会通过刺激微生物降解来增加土壤碳的分解,这可能是由于增加的氮输入显著降低了凋落物和细根的碳氮比来提供高质量的分解底物。土壤有机碳积累对氮添加的响应取决于植物碳输入和土壤碳输出之间的权衡。综上所述,氮添加会促进中国陆地生态系统的植物和土壤碳储存,但其响应程度取决于不同的生态系统类型和施氮措施。在氮沉降背景下,地上碳输入与土壤有机碳含量之间的负相关关系可能会影响对陆地土壤碳收支的预测,在未来的生态系统模型中应该充分考虑植物-土壤碳动态之间的平衡。

In recent years,the rate of atmospheric nitrogen(N)deposition in China has been increasing.Excessive N input has a strong impact on the carbon(C)cycle of terrestrial ecosystems in China.Although a large number of studies have reported the effects of simulated N deposition experiments on the C dynamics in terrestrial ecosystems in China,due to the complex geographical conditions and different fertilization regimes,the general response and the mechanism of plant and soil C pools to N addition are still widely debated.Here,we collected and compiled the available data from 172 published field N-addition experiments in China to assess the effects of N addition on plant and soil C dynamics in terrestrial ecosystems by meta-analysis,and to explore its underlying mechanism at the national scale.The results showed that N addition stimulated plant C fixation,both aboveground and belowground biomass were significantly increased,and the response of aboveground biomass was higher than that of belowground biomass.Meanwhile,N addition significantly increased litter mass,but it did not significantly affect fine root biomass.The ratios of C:N in plant leaf,litter,and fine root were significantly decreased by N addition.Generally,N addition increased soil organic C(SOC)concentration and decreased soil pH across all the selected studies,while it had minor effects on dissolved organic C(DOC),microbial biomass C(MBC),and soil respiration(SR).We found that the responses of SOC concentration to N addition were increased,decreased,or unchanged under different geographical variation.Regression analysis showed that there were negative correlations between aboveground biomass vs.SOC,and MBC vs.SOC.Despite N addition significantly increased plant C inputs into the soil via increased litter mass,it also might increase the soil C decomposition by stimulating microbial degradation,since elevated N input would provide high-quality substrates via significantly decreasing the C:N ratio in litter and fine root.The response of SOC accumulation to N addition depends on the trade-off between plant C input and soil C output.In conclusion,we suggest that N addition increases plant C sequestration and soil C storage in terrestrial ecosystems in China,and the effect magnitude was dependent on the ecosystem types and fertilization regimes.Our findings highlight that the negative correlation between aboveground C input and SOC under N deposition may affect the prediction of terrestrial soil C budget,and this relationship should be given full consideration in further ecosystem models.