Long-term Conservation Tillage Effects On Carbon Emissions Of Loessal Soil And Its Underline Mechanisms In Semi-arid Areas

In order to reduce global warming and develop low carbon agriculture,it is significant to study and formulate scientific and rational cultivation measures to reduce carbon emission from farmland soil.This study involve a 16 consecutive years long-term different tillage experiment located in the semi-arid Loess Plateau;a rainfed agricultural region,LI-8100 A automated soil CO₂ flux system（LI-COR Inc,Lincoln,NE,USA）was used to determine the soil respiration of spring wheat and field pea double-sequence rotation at different growth stages.The treatments consisted of no tillage with straw（NTS）,no tillage straw cover（NT）,conventional tillage with straw mulch（TS）and conventional tillage without straw（T）.We study the effects of soil respiration,carbon emissions and soil carbon balance and explore the main mechanism which includes soil temperature,soil moisture,soil organic carbon content,and soil total porosity.The main results are as follows:1.The soil respiration rate changed under different tillage practices.It showed a single peak curve and was high during the night,and at 12:00-14:00 and 4:00-6:00,respectively to achieve the maximum value and the minimum value during the day.The maximum value during the whole growth season of the spring wheat was reached at filling stage and at podding stage in field pea plots.2.Conservation tillage reduced soil carbon emissions in both spring wheat and field pea plots.No tillage with straw mulch（NTS）had the greatest effect.Compared with conventional tillage（T）,NTS decreased soil carbon emissions,respectively,by 21.90% and 33.54% in spring wheat and field pea soils.Under the conservation tillage,the dry farmland showed a "sink" of atmospheric CO₂,and the effect of conservation tillage was better than that of conventional tillage,whereas NTS was the best.3.Compared with conventional tillage,conservation tillage reduced soil temperature in 0-15 cm soil layer during the crop growing season.,No tillage with straw mulch（NTS）,respectively,reduced soil temperature by 0.33℃and 0.43℃of 0-15 cm on average in spring wheat and field pea.In the 0-15 cm,soil temperature and soil respiration rate had significant（P<0.05）or had highly significant（P<0.01）positive correlation;and highly significant positive correlation with soil carbon emissions（P<0.01）.Conservation tillage reduced the soil respiration rate by lowering the soil temperature,and then reducing carbon emissions.The correlation between 0-5cm soil depth temperature and soil respiration rate was higher than 5-10 cm and 10-15 cm soil depth temperature.The variation range of soil respiration temperature sensitivity coefficient Q10 was 1.45².26,and the deep layer of the sensitivity of soil respiration to soil temperature was higher than that of shallow layer.4.Irrespective of spring wheat or field pea plots,compared with conventional tillage（T）,conservation tillage improved soil moisture of 0-30cm;NTS had the greatest effect,,it respectively,improved soil moisture by 5.94% and 7.94% in spring wheat and field pea plots.There was no significant correlation between soil moisture and soil respiration rate and carbon emissions（P<0.05）in 0-30 cm.The effects of soil moisture on soil respiration rate decreased with the increase of soil depth.5.Compared with conventional tillage,conservation tillage significantly improved soil organic carbon content in 0-30 cm soil layer,and no tillage with straw mulch（NTS）was the best.Compared with the conventional tillage（T）,NTS increased organic carbon content of 0-30 cm soil layer by 34.70% and 27.23% in spring wheat and field pea,respectively.There was extremely significant negative correlation（P<0.01）between soil carbon emissions and soil organic carbon content of 0-10 cm.There was no significant correlation between soil organic carbon and the soil moisture at 10-30 cm.Conservation tillage increased soil organic carbon content and reduced carbon emissions.6.In the spring wheat and field pea,conservation tillage increased the total porosity at 0-30 cm soil layer and decreased the soil bulk density of 0-30 cm layer,and the effect of no tillage with straw mulching（NTS）was the best.Compared with conventional tillage（T）,the total soil porosity of 0-30 cm soil layer,respectively,increased by 8.92% and 6.28% in spring wheat and field pea field under the NTS treatment,while the average soil bulk density was decreased by10.43% and 7.63%.The carbon emission was highly significant（P<0.01）or had significant（P<0.05）negative correlation with 0-10 cm soil depth total porosity.In dry land,conservation tillage increased soil total porosity in the 0-30 cm soil layer,thus reducing the soil carbon emissions.7.The grain yield of spring wheat and field pea were individually increased by 29.25% and 33.09% than conventional tillage under no tillage and straw mulching（NTS）.,The carbon emission efficiency were individually increased by 57.69% and 76.74%..Conservation tillage significantly reduced the soil carbon emissions in dry farmland,whiles conservation tillage significantly increased the carbon emission efficiency,and the NTS effect was the greatest.Overall,conservation tillage practices such as no tillage and straw mulching（NTS）improved soil physical properties and increase grain yield.In addition NTS also reduced soil carbon emissions and improve the carbon emission efficiency by reducing soil temperature and increasing total porosity and soil organic carbon content in the semi-arid Loess Plateau,NTS was found to be the most reasonable agricultural measure to increase crop grain yield and carbon sink and reduce soil carbon emission.