The Microbial Driving Mechanism Of Alfalfa Growth Years Affecting Soil Carbon Emission In The Rainfed Area Of The Loess Plateau

The severe soil erosion in the rainfed area of the Loess Plateau not only destroys the ecological environment but also reduces soil quality and soil carbon sequestration.Due to its nutritional value and ecological functions,alfalfa(Medicago sativa)has played an important role in ecological restoration and soil quality improvement in this area.However,the continuous cultivation of alfalfa for many years will cause serious degradation of the grassland and a decrease in forage yield.The effect of soil carbon sequestration will also change due to the extension of the alfalfa planting years.At present,there are few studies on the microbial mechanism of CO₂ emission from alfalfa artificial grassland in the Loess Plateau.In this study,alfalfa artificial grassland with different cultivation ages(2,9,16 and18 years)in the rainfed area of the Loess Plateau was used as the research object,and farmland(maize field)was used as the control.The LI-8100 open-circuit soil carbon flux measurement system was used to measure the soil respiration rate,and the Illumina Mi Seq sequencing technology was used to study the community structure,abundance,diversity of soil bacteria and CO₂-fixing functional gene cbb L.Additionally,we explored the relationship between biotic and abiotic factors and soil carbon emissions by combining soil physicochemical factors and the calculation of soil CO₂–C emissions during the growth period,and revealed the influence of alfalfa cultivating ages on soil carbon emissions and its microbiological mechanism in the rainfed area of the Loess Plateau.To provide a theoretical basis for the sustainable utilization of artificial alfalfa grassland in the rainfed area of the Loess Plateau.The main conclusions of this study are as follows:(1)The soil respiration rate of alfalfa in the rainfed agricultural region of the Loess Plateau showed a bimodal trend,reaching two peaks on June 15 and July 30,respectively.The average soil respiration rate of the whole growth period was 16 years(3.67±1.22?mol·m^-2·s^-1)>9 years(3.39±1.14?mol·m^-2·s^-1)>cropland(3.20±1.22?mol·m^-2·s^-1),18years(3.19±1.00?mol·m^-2·s^-1)>2 years(2.84±1.03?mol·m^-2·s^-1).Soil carbon emission and emission efficiency increased first and then decreased with the extension of alfalfa growth years.Soil carbon emission reached the highest value in 16 years,and carbon emission efficiency reached the highest value in 9 years.(2)The dominant phylum of soil bacterial community were Actinobacteria(19.49%-28.39%),Proteobacteria(22.09%-24.69%),Acidobacteria(12.54%-15.46%),Chloroflexi(9.41%-10.49%)and Planctomycetes(5.62%-8.35%)in the rainfed area of Loess Plateau.The relative abundance of Actinobacteria in alfalfa soil was higher than that in farmland and increased with the extension of alfalfa planting years.The relative abundance of Proteobacteria in alfalfa soil was lower than that in farmland,and the change was stable with the extension of alfalfa planting years.The relative abundance of Acidobacteria and Chloroflexi increased first and then decreased with the extension of alfalfa planting years,and the relative abundance of Planctomycetes decreased with the increase of alfalfa planting years.Arthrobacter(2.22%-3.15%)and Microvirga(2.00%-2.44%)were the dominant bacterial genera that have been classified,and the relative abundance of both decreased first and then increased with the extension of alfalfa cultivating years.(3)The dominant phylum of soil CO₂-fixing bacteria community in the rainfed area of Loess Plateau were Proteobacteria(82.93%-89.25%)and Actinobacteria(10.14%-15.09%).The relative abundance of Proteobacteria increased first and then decreased with the extension of alfalfa planting years,and reached the highest value(89.25%)in 9 years.The relative abundance of Actinobacteria decreased first and then increased with the extension of alfalfa planting years,among which 16 years was the highest and 9 years was the lowest.Compared with farmland,planting alfalfa could increase the relative abundance of Proteobacteria and Actinobacteria,and reduce the relative abundance of Planctomycetes.The dominant genera of CO₂-fixing bacteria were Starkeya(5.96%-13.23%)and Bradyrhizobium(3.06%-11.47%),among which the relative abundance of Bradyrhizobium was significantly different between treatments.(4)Redundancy analysis showed that soil available phosphorus and microbial biomass carbon were the main factors affecting the community structure of soil bacteria.Soil available phosphorus and nitrate nitrogen were the main factors affecting the community structure of soil CO₂-fixing bacteria.(5)The soil respiration rate of alfalfa of different years in the rainfed area of the Loess Plateau was affected by both soil temperature and moisture,but the temperature had a stronger effect on it.Soil microbial biomass carbon was the main factor affecting soil carbon emission,and Gemmatimonadota and Armatimondetes were the main biological groups affecting soil carbon emission.