Effects Of Nitrogen And Phosphorus Addition On Soil Microbes And Stoichiometric Characteristic Of Alpine Meadow In Qinghai-Tibet Plateau

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 0-10cm 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?-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.