Response And Mechanism Of Soil Respiration And Soil Carbon Fraction To Precipitation Changes In Alfalfa-Winter Wheat Rotation System On The Longdong Loess Plateau

Alfalfa（Medicago sativa L.）and winter wheat（Triticum aestivum L.）rotation is a typical rotation pattern in the Loess Plateau region,an area susceptible to global climate changes and the associated risks of extreme precipitation events.This rotation balances grain production,forage production,and soil health,but the essential carbon cycling processes within this system remain debated.Consequently,the patterns and mechanisms through which precipitation changes affect critical carbon cycle processes warrant deeper investigation.To address this need,I carried out a detailed field study from 2018 to 2022 at the Qingyang Grassland Agro-Ecosystems Research Station.Three crop rotation systems were established:1-year alfalfa rotation with 3 years of winter wheat（AWWW）,2 years alfalfa rotation with 2 years of winter wheat（AAWW）,and 3 years alfalfa rotation with1 year of winter wheat（AAAW）.The continuous winter wheat-recreation（WWWW）was used as a control.I also set up three precipitation treatments using rainfall shelters,comprising a 30%reduction in precipitation（P-30）,natural precipitation（CK）,and a30%increase in precipitation（P+30）.The objective was to elucidate the dynamic characteristics of crop productivity and soil carbon fraction in crop rotation systems under these precipitation variations.Furthermore,I sought to clarify the primary controlling factors of soil respiration and their regulatory mechanisms.Key results from this study are summarized as follows:（1）Crop productivity and soil water content:The soil volumetric water content,aboveground biomass,leaf area index,net photosynthetic rate,and winter wheat grain yield significantly increased with precipitation（P<0.05）.The soil volumetric water content within 0-3 m for winter wheat following 2 or 3 years of alfalfa was not significantly different from continuous winter wheat（P>0.05）.The grain yield under CK and P+30 scenarios was 21.8%and 22.8%higher than under P-30（P<0.05）,respectively.System biomass rose with alfalfa planting years.The AAAW system outperformed others with total biomass higher by 11.8%,15.0%,and 10.0%（P<0.05）,respectively.（2）Soil carbon and enzyme activity:Precipitation changes had no substantial effect on soil total carbon,organic and inorganic carbon,or the carbon pool management index in the rotation system（P>0.05）.Soil total carbon,organic,and inactive organic carbon content significantly increased in alfalfa’s fourth year compared to the first year（P<0.05）,though the carbon pool management index decreased with alfalfa cultivation years.The total carbon content of WWWW,AWWW,and AAWW was significantly higher than AAAW by 3.8%,5.8%,and 3.6%,respectively,from 2021to 2022（P<0.05）.Soilβ-glucosidase,cellobioglucosidase,β-xylosidase activities,and microbial biomass carbon content correlated positively with precipitation and increased with alfalfa planting years.These enzymes were higher under AAAW by 32.6%,32.6%,and 20.1%,and microbial biomass carbon content was 99.8%higher than WWWW（P<0.05）.（3）Soil respiration and its components:In continuous winter wheat,the average soil respiration rate under different precipitation scenarios ranged from 1.31 to 1.59μmol m^-2 s^-1.The annual cumulative CO₂ emissions were between 503.29 to 597.75 g C m^-2,with 63.9%to 65.8%occurring in the growing season.In contrast,alfalfa’s average soil respiration rate was significantly higher,56.1%,52.9%,and 46.8%more in the P-30,CK,and P+30 scenarios（P<0.05）,respectively.Precipitation changes did not significantly affect winter wheat’s soil respiration rates in the rotation system but did alter the cumulative CO₂ emissions proportion（P>0.05）,with increased precipitation elevating this proportion and reduced precipitation decreasing carbon emission efficiency（P<0.05）.Cumulative CO₂ emissions and carbon emission efficiency grew with alfalfa planting years,with emissions in the AAAW system 45.2%,14.3%,and 8.5%higher than other systems from 2021 to 2022（P<0.05）.Soil temperature,water content,and aboveground biomass were vital for regulating soil respiration.Temperature sensitivity improved with rising precipitation but fell with more alfalfa cropping years.Contrarily,soil respiration’s response threshold to volumetric water content had the opposite behavior.Seasonal droughts significantly influenced alfalfa soil respiration rates,with spring and summer droughts having differing impacts on soil heterotrophic and autotrophic respiration（P<0.05）.This pattern was attributed to the opposite response of the leaf area index and photosynthetic rate to spring and summer drought and the contrary feedback of microorganisms to spring drought versus the rainfall pulse effect following summer drought.（4）Diurnal lag patterns and regulatory pathways:A pronounced diurnal lag pattern exists between soil respiration and soil temperature within the crop rotation system,with lag times(LT_Ts-Rs)spanning from 1 to 10 hours.This lag time in both alfalfa and winter wheat is governed by two interconnected pathways.The first pathway is the photosynthetic material transport pathway,representing the diurnal dynamic lag between soil respiration and photosynthetically active radiation(LT_Rs-PAR).The second is the heat transfer pathway,embodying the diurnal dynamic lag between soil temperature and photosynthetically active radiation(LT_Ts-PAR).Together,LT_Ts-Rs is the aggregate of LT_Rs-PAR and LT_Ts-PAR.Meanwhile,the heat transfer pathway can be further dissected into two components:one involving air temperature and photosynthetically active radiation(LT_Ta-PAR),and the other soil temperature and air temperature(LT_Ts-Ta),both exhibiting a diurnal dynamic hysteresis effect.Notably,both LT_Rs-PAR and LT_Ts-Tawere found to be significantly and negatively correlated with soil water content（P<0.05）,highlighting the influence of soil water content on LT_Ts-Rs through the regulation of these underlying processes.In summary,future increases in precipitation are likely to enhance the productivity of alfalfa-winter wheat rotation systems.The soil moisture content in the 0-3 m profile of winter wheat,when preceded by 2-3 years of alfalfa cultivation,showed no significant difference compared to a continuous winter wheat cropping system.The grain yield of winter wheat and cumulative soil CO₂ emissions rose with the extension of alfalfa planting years;however,the increase in biomass overtaken the rise in CO₂emissions,thereby favoring carbon sequestration within the system.An increase in the frequency of summer droughts is expected to accelerate soil carbon emissions in the future.The 3-year alfalfa and 1-year winter wheat rotation emerged as a preferable option,aligning with food security and carbon management objectives.Additionally,this study quantified the regulatory pathways affecting the lag time between soil respiration and soil temperature,revealing how soil volumetric water content influences these dynamics.This research is essential for clarifying the relationship between various factors affecting lag time and offers a solid foundation for the accurate modeling of soil carbon emissions in terrestrial ecosystems.