Study On N₂O Emission And Its Influencing Factors Of Rain Feed Artificial Grassland In The North Foot Of Yinshan Mountain

Artificial grassland not only plays an important role in the production and development of agriculture and animal husbandry,but also has extremely important significance in environmental protection and environmental industry.Among greenhouse gases,N₂O has become the focus of many experts and scholars in recent years because of its strong warming potential and importance.This study takes the artificial grassland in the agro pastoral ecotone at the northern foot of Yinshan Mountain as the object.In this study,three different types of artificial grasslands:Gramineae monoculture（Elymus sibirica）,Leguminosae monoculture（Medicago sativa）and Gramineous-legume mixture（Elymus sibirica+Medicago sativa）were established from 2019 to 2021.The static box method was used to measure the annual N₂O flux in the field,and the soil physical and chemical properties such as soil temperature,water content,pH,electrical conductivity,total carbon,total nitrogen,ammonium nitrogen,nitrate nitrogen and organic matter were sampled and analyzed during the test period.The abundance of functional genes of soil ammonia oxidizing bacteria was detected by real-time quantitative PCR,and the influence of planting mode on N₂O flux and its regulatory factors were analyzed.In order to provide the basis for reasonable utilization of artificial grassland.The main research results are as follows:1.After three years of planting,the gramineae monoculture,leguminosae monoculture and gramineous-legume mixture grassland are N₂O sources in the plant growth season,and N₂O sinks in the winter soil permafrost period,because the N₂O emitted in the growing season is much higher than that absorbed by the soil during the permafrost period,the rain fed artificial grassland at the north foot of Yinshan Mountain is the source of N₂O.The highest emission flux was leguminosae monoculture,followed by the gramineae monoculture,and the gramineous-legume mixed sowing was the lowest（P<0.05）.The N₂O emission flux was the highest in the year of planting,and gradually decreased with the increase of planting years,and the peak emission occurred in the alternating periods of freezing and thawing in autumn and winter in the year of planting and in the alternating periods of freezing and thawing in spring in the second and third years.Compared with the two single-sowing grasslands,the mixed-sowing grassland significantly increased the forage yield,and also significantly decreased the cumulative soil N₂O emission and the emission per unit of forage yield（P<0.05）.2.Soil temperature,soil water content,and ammonium nitrogen in the artificial grassland with gramineae monoculture and leguminosae monoculture were higher than those in the gramineous-legume mixture grassland,and the content of soil nitrate was lower than that in the mixed grassland.There was a significant positive correlation between soil N₂O flux and soil temperature,soil water content,ammonium nitrogen and nitrate nitrogen（P<0.05）.Soil N₂O flux of legume uniseeding grassland was significantly positively correlated with soil water content,soil pH,ammonium nitrogen and nitrate nitrogen（P<0.05）,and negatively correlated with soil temperature（P<0.05）.There was a significant positive correlation between soil N₂O flux and soil temperature,soil water content,ammonium nitrogen and nitrate nitrogen（P<0.05）.3.The abundance of soil ammonia oxidizing archaea（AOA）had no significant change in all kinds of grassland in the establishment year and the second year（P>0.05）,but the AOA gene abundance of alfalfa monoculture grassland in the third year was significantly higher than that of the same year of Elymus sibiricus monoculture and mixture of alfalfa and Elymus sibiricus（P<0.05）.The abundance of ammonia-oxidizing bacteria（AOB）was the highest in alfalfa monoculture grassland,followed by Elymus sibiricus monoculture grassland,and the lowest in monoculture and mixture of alfalfa and Elymus sibiricus grassland in the three years after planting.There were significant differences in the abundance of AOB gene among the three grasslands（P<0.05）.The abundance of AOA gene in alfalfa monoculture grassland showed a significant linear positive and negative correlation with N₂O flux,and the abundance of AOA gene in monoculture grassland and mixed grassland showed a significant linear positive and negative correlation（P<0.01）.The gene abundance of AOB in the three grasslands was significantly positively correlated with N₂O flux（P<0.01）.4.The main controlling factors of soil N₂O flux were different in different grassland types.The gene abundance of ammonia oxidizing bacteria（AOB）and soil nitrate nitrogen content were the main factors affecting soil N₂O flux in the alfalfa monoculture grassland and Elymus sibiricus monoculture grassland.The gene abundance of ammonia-oxidizing bacteria（AOB）and soil ammonium nitrogen content were the main controlling factors of N₂O flux in the mixed grassland.In summary,different types of rain-fed artificial grasslands in the northern foot of Yinshan Mountain are the sources of greenhouse gas N₂O.The mixed sowing of Elymus sibiricus and alfalfa change the physical and chemical properties of soil,thereby regulating the gene abundance of ammonia oxidizing bacteria and reducing the emission of N₂O.In the rain-fed artificial grassland at the northern foot of Yinshan Mountain,the dominant factors affecting N₂O emission were ammonia oxidizing bacteria（AOB）,ammonium nitrogen and nitrate nitrogen.Reasonable establishment of mixed planting artificial grassland is of great significance to increase production and reduce emission.