中国科学院水利部水土保持研究所机构知识库

自20世纪以来，陆地生态系统中大气氮沉积和施肥产生的大量氮磷输入已深刻影响了关键的生态系统过程，比如微生物特征、土壤碳氮磷等含量及组分分配、化学计量比等等，然而目前对氮磷添加对高寒草地的土壤系统的影响还存在较大争议。因此，本文以青藏高原高寒草地开展了9年的氮磷添加野外监测小区为研究对象，采用野外采样调查和室内分析的研究方法，研究了氮磷添加对土壤微生物多样性、土壤碳、氮、磷及其组分、化学计量特征的影响，阐明氮磷添加对微生物碳氮循环的影响机制，揭示土壤化学计量和微生物过程的相互关系，以期为未来全球变化下青藏高原高寒草地生态系统的土壤管理提供参考，主要取得的结论如下：
（1）氮磷添加影响了不同土层的土壤碳、氮磷含量和化学计量比，其中，NP处理提高了不同土层的土壤有机碳（SOC）含量，N处理和NP处理对各个土层的土壤全氮（TN）和硝态氮（NO3--N）含量有不同程度的提高。NP处理和P处理提高了除40-50 cm土层外各层土壤全磷（TP）含量。此外，N处理提高了不同土层的C:P、N:P，而NP处理降低了不同土层的N:P，说明NP合施可以缓解由于单独施氮造成的磷素的限制。
（2）氮磷添加影响了不同土层的有机碳组分和稳定性，不同活性的易氧化碳组分出现随土层的加深而下降的现象。氮磷添加降低了0-10 cm土层中不稳定的C2组分，并增加了稳定性C4组分从而累积了有机碳；N和NP处理分别增加了10-60 cm不稳定碳C1（101.5%~162.4%，102.2%~142.3%）、C2（100.7%~210.5%，101.9%~205.5%）和稳定性碳C4（102.3%~123.4%，102.6%~113.8%）的含量和CPMI（116.5%~288.5%，121.7%~185.6%），在提高地下土壤SOC的含量的同时改善了土壤质量。
（3）氮磷添加改变了土壤细菌-真菌群落的互作关系。其中，N处理、P处理和NP处理增加了细菌-真菌分子生态网络的节点数、连接数、平均连通度和网络密度，表明氮磷添加增加了微生物生态网络的复杂性和稳定性。氮磷添加增加了细菌内部以及细菌与真菌之间的正相关连接比例，降低了负相关连接比例，表明氮磷添加使微生物群落之间更为稳定，具有更高的抗干扰能力。
（4）氮磷添加改变了土壤表层的微生物群落组成，N处理和NP处理显著降低了MBC和MBN的含量，显著增加了β-葡萄糖苷酶的活性、执行反硝化的功能基因和降解纤维素、半纤维素、木质素和几丁质酶的基因丰度，单独的磷添加对调节土壤关键生物学过程的土壤微生物没有显著影响。进一步分析发现，编码降解半纤维素（R2=0.602，P<0.01），纤维素（R2=0.487，P<0.05），几丁质（R2=0.532，P<0.01）和木质素的酶（R2=0.422，P<0.05）的基因丰度与土壤C:N相关。为了维持化学计量的平衡，土壤微生物通过调节群落结构及酶活性来调节资源转化的速率和方向和获取缺乏的养分并释放多余的养分这两种方式来影响碳氮循环的变化。因此，土壤养分水平和土壤微生物群落需求之间的化学计量差异可以显著影响微生物群落组成、活性和碳氮循环。这些结果为氮磷输入对微生物碳、氮循环的影响机理提供了新的实验见解。

Since the 20th century, large amounts of nitrogen and phosphorus input from atmospheric nitrogen deposition and fertilization in terrestrial ecosystems have profoundly affected key ecosystem processes, such as microbial characteristics, soil carbon, nitrogen, and phosphorus content and distribution, stoichiometric ratios and so on. However, there is still considerable controversy about the effect of nitrogen and phosphorus addition on the soil system of alpine grasslands. Therefore, this article took the field monitoring community of nitrogen and phosphorus addition for 9 years in the subalpine meadow on the Tibetan Plateau as the research object. Adopting the research method of field sampling investigation and indoor analysis to study the nitrogen and phosphorus to add on soil microbial diversity, soil carbon, nitrogen, phosphorus and its components and stoichiometric characteristics, clarify the mechanism of nitrogen and phosphorus addition on the microbial carbon and nitrogen cycle, reveal the relationship between soil stoichiometry and microbial processes. In order to provide reference for the soil management of alpine meadow ecosystem in the Qinghai-Tibet plateau under the future global change, the main conclusions are as follows:
(1) Nitrogen and phosphorus addition affected soil carbon, nitrogen and phosphorus contents and stoichiometric ratios in different soil layers. Among them, NP treatment increased soil organic carbon content (SOC) in different soil layers, while N treatment and NP treatment increased soil total nitrogen content (TN) and nitrate nitrogen content (NO3--N) in each soil layer to different degrees.NP treatment and P treatment increased soil total phosphorus content (TP) in all soil layers except the 40-50 cm soil layer. In addition, the N treatment improved the C:P and N:P in different soil layers, while the NP treatment reduced the N:P in different soil layers, indicating that the combined application of N and P could alleviate the limitation of phosphorus caused by nitrogen application alone.
(2) Nitrogen and phosphorus addition affected the organic carbon fractions and soil carbon stability in different soil layers. The oxidizable carbon fractions with different labilities and oxidizabilities appeared to decrease with the deepening of the soil layer. Nitrogen and phosphorus addition reduced the labile C2 fraction in the 0-10 cm soil layer, and increased recalcitrant C4 fraction to accumulate organic carbon; N treatment and NP treatment increased the carbon pool management index and the contents of very labile C1 (101.5%~162.4%，102.2%~142.3%), labile C2 (100.7%~210.5%，101.9%~205.5%), and recalcitrant C4 fraction (102.3%~123.4%，102.6%~113.8%) in the 10-60 cm soil layer, which promoted the accumulation of SOC and improved the stability of SOC in the underground soil while improving the soil quality.
(3) Nitrogen and phosphorus addition changed the interaction between soil bacterial-fungal communities. Among them, N treatment, P treatment and NP treatment increased the number of nodes, connections, average connectivity and network density of the bacterial-fungal molecular ecological network, indicating that nitrogen and phosphorus additions increased the complexity and stability of the microbial ecological network. The addition of nitrogen and phosphorus increased the positive correlation connection ratio within bacteria and between bacteria and fungi, and decreased the negative correlation connection ratio within bacteria and between bacteria and fungi, indicating that the addition of nitrogen and phosphorus makes microbial communities more stable and has higher anti-interference ability.
(4) Nitrogen and phosphorus addition changed the composition of the microbial community on the surface of the soil. N treatment and NP treatment significantly reduced the content of MBC and MBN, significantly increased the activity of β-glucosidase, denitrification genes and the relative abundances of the genes encoding enzymes degrading cellulose, hemicellulose, lignin, and chitin, and phosphorus addition alone had no significant effect on soil microorganisms that regulated key biological processes in the soil. Further analysis revealed that the relative abundances of the genes encoding enzymes degrading hemicellulose (R2= 0.602, P< 0.01), cellulose (R2= 0.487, P< 0.05), chitin (R2= 0.532, P< 0.01) and lignin (R2= 0.422, P< 0.05) was correlated with soil C:N. In order to maintain the balance of stoichiometry, soil microorganisms affect the changes of the carbon and nitrogen cycle by adjusting the community structure and enzyme activity to adjust the rate and direction of resource conversion and obtain the lack of nutrients and release excess nutrients. Thus, stoichiometric differences between soil nutrient levels and soil microbial community requirements can significantly influence microbial community composition, activity, and carbon and nitrogen cycling. These results provide new experimental insights into the effect of nitrogen and phosphorus input on microbial carbon and nitrogen cycles.