森林生态系统具有碳源和碳汇双重功能,调控森林中碳输入方式对于实现我国“碳中和”目标具有重要意义。作为森林土壤有机碳(SOC)的主要来源,不同碳(C)输入方式(如地上凋落物、地下植物根系等)对森林生态系统土壤氮(N)循环的影响一直是相关学者的研究重点。笔者综述了目前国内外不同C输入方式对土壤活性N库、土壤N矿化、硝化过程及氧化亚氮(N₂O)排放的影响研究现状,分析了森林土壤活性N库及N转化过程对不同C输入变化的响应,发现:① 地上C排除可以降低土壤有效态氮(主要包括 $NH_{4}^{+}$-N和 $NO_{3}^{-}$-N)的含量,但地下C排除却增加了土壤 $NH_{4}^{+}$-N含量。C输入方式的改变对土壤微生物生物量氮(MBN)含量影响具有不确定性,这可能与生态系统类型、树种组成、时间尺度等因素相关。此外,地下C排除对土壤可溶性氮(DON)含量的影响较地上C排除的大,地下根系可能是影响土壤DON含量的主要贡献者。② 地上C输入对土壤N矿化及硝化速率的影响在短期内较大,而长期影响较小。其主要是通过间接改变土壤微生物活性从而影响了土壤N矿化及硝化过程,地下C排除增加了土壤N矿化速率,且随着时间尺度的增加表现更加明显。③ 地上C输入通过改变硝化和反硝化微生物的可利用C源而间接影响了N₂O的排放,且受到树种影响显著,而地下C输入对N₂O的影响因根系质量等的差异而发生改变。综上可知,森林土壤活性N库及N循环过程对不同C输入具有不同的响应机制,且受生态系统类型、物种、时间等因素影响较大。目前关于两种乃至多物种不同C的输入对森林土壤N影响的研究较少,且定性研究较为普遍;对优化森林生态系统地上地下C输入动态模型和精准预算不足,尚未建立完整的碳减排生态补偿机制。今后的研究亟须定量了解不同森林生态系统不同的C输入及其两者或者多物种之间的交互影响对土壤N的影响机制,且需更多地考虑在时间尺度上的长期变化过程;需要提升核算与预测森林地上地下碳中和能力,加快森林碳中和技术研发,为提前实现“碳中和”战略目标提供科技支撑。

The forest ecosystem has the dual functions of carbon source and carbon sink, and regulating the carbon input mode in forest is of great significance for realizing the goal of “carbon neutrality” in China. The effects of changes in carbon (C) inputs (such as above-ground litter and under-ground roots) on soil nitrogen (N) cycling have long been a research focus in forest ecosystems. Based on previous studies worldwide, this paper reviews the research progress on the effects of different C inputs on the responses of the forest soil active N pool and N transformation processes, including soil active N pools, soil N mineralization, nitrification processes, and nitrous oxide (N₂O) emissions. We found: (1) Above-ground C removal reduced soil available nitrogen contents (mainly $NH_{4}^{+}$-N and $NO_{3}^{-}$-N). However, under-ground C removal increased the soil $NH_{4}^{+}$-N content. Changes in different C inputs showed inconsistent effects on soil microbial biomass nitrogen (MBN), which may be related to the ecosystem type, tree species composition, timescale, and other factors. In addition, the influence of under-ground C removal on soil soluble nitrogen (DON) was greater than that of above-ground C removal, and the under-ground root system may be the main contributor to soil DON. (2) The effects of above-ground C inputs on soil N mineralization and nitrification rates were greater in the short but less in the long term. Soil N mineralization and nitrification processes were affected indirectly by changes in soil microbial activity. The under-ground C removal increased soil N mineralization rates, and the effect was more obvious with an increase in a time scale. (3) Above-ground C inputs indirectly affected N₂O emissions by changing the available C sources of nitrifying and denitrifying microorganisms and was significantly affected by tree species. However, the effect of the under-ground C inputs on N₂O varied with the root quality. Current research mainly focuses on the impacts of a certain type of above-and under-ground C inputs and its removal on soil N pool components and N transformation. In conclusion, soil active N pools and N cycling differed in the response mechanisms to different C inputs in forests and were greatly affected by the ecosystem type, species, time, and other factors. The dynamic model and accurate budget for optimizing the above- and under-ground carbon inputs of forest ecosystems are insufficient, and a complete ecological compensation mechanism for carbon emission reduction has not been established. Future research needs to quantitatively understand the mechanisms of soil N affected by different C inputs and interactions between two or more species in different forest ecosystems, and more consideration should be given to the long-term soil N dynamics. Therefore, it is necessary to improve the ability of accounting and forecasting forest carbon neutralization above-and under-ground, accelerate the development of C neutralization technology in forest ecosystems in future research, which provides a scientific and technological support for achieving the “C neutrality” strategic goal in advance.