Long-Term Tillage Effects On Soil Microbial Diversity,CO₂ Emission And The Underlying Mechanisms In Wheat-Pea Rotation Field In The Semi-Arid Loess Plateau,China

Soil management practices have the potential to modify the diversity and function of microbes and carbon dioxide emission?CO₂?in agricultural fields.Soil microorganisms in agricultural field influence soil quality and fertility by being involved in the cycling of the major essential elements?C,N,P,and S?through decomposition and mineralization.Soil microbes also influence the above-ground ecosystems by contributing to plant nutrition and health and emission of CO₂ through soil respiration and microbial decomposition of soil organic matter.The aim of the study was to investigate long-term tillage effect on soil microbial diversity,CO₂ emission in wheat-pea rotation field and the underlying mechanism with soil physical and chemical properties.The study was conducted at the Rainfed Agricultural Experimental Station of Gansu Agricultural University,located in Anding district,Dingxi,Gansu Province,P.R.China.The long-term tillage experiment was initiated in 2001,this thesis reports on experimental data collected at pre-sowing,flowering and pre-harvest stages of the field pea during the 2016,2017 and 2018 cropping seasons.The treatments in the study included conventional tillage with stubble removed?T?,no-tillage with stubble removed?NT?,conventional tillage with stubble incorporated?TS?and no-tillage with stubble retained?NTS?.The main findings of the study are summarized as follows:1.No-tillage with stubble retained?NTS?and no-tillage?NT?increased microbial diversity.NTS increased number of bacteria 16S rRNA OTUs while mostly,NT increased archaea16S rRNA OTUs and fungi ITS OTUs in 0-10 cm bulk soil.The 10-30 cm bulk soil had higher number of bacteria 16S rRNA?1.2-8.3%?and fungi ITS?3.5-12.5%?OTUs compared to 0-10 cm bulk soil across treatments in pre-sowing,flowering and pre-harvest stages.The rhizosphere soil consistently had lesser bacterial?14.0-21.1%?and fungal?26.0-44.9%?OTUs diversity when compare to 0-10 cm bulk soil.Tillage and stubble effect mostly significantly influenced archaea and bacteria 16S rRNA diversity in the 0-10and 10-30 cm bulk soil but not fungi.Treatment effect on the microbial communities in bulk soil and rhizosphere soil determined by PERMANOVA did not show significant influence.The dominant archaeal,bacterial and fungal phyla,classes and genera in 0-10and 10-30 cm bulk soil or rhizosphere soil did not differ significantly among the treatments.The rare fungal genus Kurtzmanomyces?phylum Basidiomycota;class Agaricostilbomycetes?occurred in the different tillage treatments at flowering stage and pre-harvest rhizosphere soil but was absent in their bulk soil whereas it was present in pre-sowing bulk soil.2.Total carbon emission?TCE?in 2016,2017 and 2018 differed significantly among treatments.Treatment NTS had the greatest TCE when compared to T with ranking of NTS>NT>TS>T in 2017 but T had greatest TCE compared to NTS and NT with ranking of T>TS>NT>NTS in 2016 and T>TS>NTS>NT in 2018.In 2016 and 2018,NTS was a carbon sink and increased carbon emission efficiency?CEE?by 43.4%(0.76 kg ha^-1)and47.8%(0.67 kg ha^-1)respectively,compared to T.The source of variation in mean Rs and TCE was mainly from year effect and interaction effect between year and treatment while the source of variation in CEE was as a result of year and treatment effects.3.No-tillage with stubble retained?NTS?improved soil physical and chemical properties.NTS mostly increased soil moisture content and decreased temperature across the 0-30 cm soil depth when compared with T.NTS mostly had significantly higher soil chemical indices in the 0-10 cm bulk soil.NTS had significantly higher soil chemical?pH,total organic carbon;TOC,total nitrogen;TN,Olsen P,NH₄⁺-N,NO₃^?-N,soil microbial biomass carbon;SMBC and soil microbial biomass nitrogen;SMBN?content in the 0-10cm bulk soil.Tillage and stubble effects showed varying significant influence on the soil chemical properties.4.Soil microbial community showed both positive and negative relationship with soil properties.Soil physical and chemical properties,respiration?Rs?,and TCE overlaid with the NMDS?Non-metric multidimensional scaling?ordinations did not show a clear effect of the composition of the microbial community by treatments.However,Mantel tests showed that soil microbial community composition was occasionally significantly positively correlated with soil physical and chemical properties,mean Rs and TCE.Bacteria showed the highest number of significant Mantel tests correlations with the soil properties measured in pre-sowing and pre-harvest stages of the study.Principal component analysis?correlation-based?determined association and it showed that the first component explained 30-67%of the total variation in microbial community composition and indicator soil properties measured in the study.In conclusion,in the rainfed Loess Plateau no-tillage with stubble retention improved the soil microbial diversity and physical and chemical properties.Tillage practices affected CO₂ emission differently in dry and wet years,but no-tillage with stubble retained was a carbon sink with the significantly highest carbon emission efficiency in both dry and wet years.Bulk soil had greater microbial diversity than rhizosphere soil,no-tillage with stubble retained increased bacteria diversity and no-tillage increased archaeal and fungal diversity in the soil.Identification of Kurtzmanomyces in agricultural soils was uncommon during our literature search and this may be the first report of Kurtzmanomyces in agricultural soil and under different tillage systems in the northwestern loess plateau of China.No-tillage and stubble retention practices influenced bacterial and fungal diversity and CO₂ emission through improved soil physical and chemical properties which have the potential to affect the habitat and activity of soil microbes.