Effects Of Experimental Warming And Precipitation Manipulation On The Key Processes Of Soil Carbon Cycle In Abandoned Farmland Of The Loess Hilly Region

Natural restoration of abandoned farmlands is important for reconstructing degraded ecosystems in the Loess Plateau.It not only sequesters carbon and stabilizes the soil carbon pool,but also plays an important role in the global carbon cycle and in mitigating climate change.Studying the influences of climate change on the soil carbon pool,the key processes of the soil carbon cycle in abandoned farmlands,and their mechanisms is important for understanding the soil carbon cycle process in vulnerable ecological environments.It can also provide an important scientific reference for improving the carbon sink effect and carbon pool stability of vegetation restoration areas.This study used an abandoned farmland(restored for 12 years)in the loess hilly region as the research object and conducted a field warming experiment using open top chambers and precipitation-change experiments using homemade equipment that can intercept and increase rainfall.By combining field monitoring and indoor analysis,we investigated the differences in soil carbon pool stability between simulated warming and altered precipitation scenarios(25% and 50% precipitation reduction and 25% and 50% precipitation enhancement)and their interactions.We further analyzed the effects of different climate change scenarios on ecosystem carbon fluxes,soil organic carbon fractions,and soil carbon emissions of the abandoned farmland and their relationships with soil carbon pool stability.We then revealed the mechanisms by which climate change affects soil carbon pool stability.The main findings are as follows:(1)Warming,precipitation enhancement,and their interactions significantly increased the contents and allocation proportions of the soil organic carbon fractions,improved the carbon activity index and carbon pool management index,and reduced soil carbon pool stability.The contents of the soil organic carbon fractions increased significantly after warming and generally showed positive and negative associations with precipitation enhancement and reduction,respectively.Moreover,warming increased the allocation proportion of soil labile organic carbon,which was significantly higher under the precipitation enhancement treatments than under the precipitation reduction treatments.The opposite was true for soil recalcitrant organic carbon.The interactive effects of warming and precipitation enhancement on the characteristics of the soil organic carbon fractions were much greater than the main effect of a single climate change factor.However,precipitation reduction with warming could further reduce the content and allocation proportion of soil labile organic carbon.The activity(carbon pool activity and carbon pool activity index)and quality(carbon pool index and carbon pool management index)of the soil carbon pool under the warming treatment was 7.66%–32.71% higher than that under the control.In contrast,25% and 50% precipitation enhancement improved the activity and quality of the soil carbon pool by 1.64%–22.24% and 6.66%–45.69% ,respectively,compared with that of the corresponding precipitation reduction treatments.These differences further increased to25.53%–59.10% and 51.94%–139.18% ,respectively,under the interactive treatment of warming and precipitation variation.Therefore,warming,precipitation enhancement,and their interactions decreased soil carbon pool stability compared with precipitation reduction and its interactions with warming.(2)Precipitation reduction and its interactions with warming reduced the net CO₂ absorption of abandoned farmland,which was mainly affected by the changes in the proportions of Compositae and Leguminosae.Net ecosystem CO₂ exchange(NEE)was significantly affected by climate change and showed clear interannual variability.Compared with that of the control,warming increased and decreased the NEE in the summers of 2018 and 2019,respectively.The NEE was significantly lower under the precipitation enhancement treatments than under the precipitation reduction treatments.In particular,the NEE was negative under all treatments in the summer of 2018,thereby indicating that the abandoned farmland acted as a carbon sink.However,one year later,the overall carbon sequestration capacity of the abandoned farmland declined.Notably,precipitation reduction and its interactions with warming induced the abandoned farmland to shift from a carbon sink to a carbon source.Under the precipitation enhancement conditions in 2019,warming further enhanced the carbon sequestration capacity of the abandoned farmland.However,warming with precipitation reduction could decrease the net CO₂ emissions of the ecosystem.The plant diversity,plant family composition,and plant carbon,nitrogen,and phosphorus contents explained 95.71% of the variation in ecosystem carbon fluxes under the climate change scenarios.Among them,the NEE was positively correlated with the percentage of Compositae,plant diversity and evenness,but negatively correlated with the percentage of Leguminosae and the leaf carbon and nitrogen contents.The percentage changes in Compositae and Leguminosae under different climate change scenarios were the dominant factors affecting the carbon source/sink function of abandoned farmland and could change the NEE and leaf carbon and nitrogen contents,thereby indirectly influencing the soil organic carbon fractions.(3)Warming and precipitation enhancement accelerated the CO₂ emissions from the abandoned farmland soil into the atmosphere.The soil microbial metabolic characteristics were the major drivers of the changes in soil CO₂ emissions.The dynamic characteristics of soil CO₂ emissions displayed clear single-peak curves in both 2018 and 2019,with the maximum and minimum emission rates observed in summer and winter,respectively.Warming,precipitation enhancement,and their interactions significantly increased the soil CO₂ emissions.However,the opposite was true for precipitation reduction and its interactions with warming.Soil CO₂ emissions were positively correlated with precipitation,but warming interacted with precipitation change had an inhibitory effect on soil CO₂ emissions.Warming,precipitation enhancement,and their interactions significantly increased the soil dissolved organic nitrogen and available phosphorus contents,potential activities of?-1,4-xylosidase and microbial N-acquiring and P-acquiring enzymes,microbial carbon use efficiency(CUE),and microbial biomass turnover rate(BTR).In contrast,precipitation reduction and its interaction with warming significantly increased the potential activities of?-1,4-glucosidase,cellobiohydrolase,and oxidases,as well as the microbial nitrogen and phosphorus use efficiencies.Soil CO₂ emissions were positively correlated with the soil organic carbon fractions and were coordinately regulated by the soil enzyme activities,CUE,and BTR.Structural equation modeling revealed that soil moisture rather than soil temperature was the crucial climate factor affecting soil CO₂ emissions.Climate change could directly or indirectly affect the soil physicochemical properties via plant characteristics and then influence the soil enzyme activities,CUE,BTR,and soil organic carbon fractions,which ultimately affect soil CO₂ emissions.(4)Soil carbon pool stability was synergistically regulated by the soil organic carbon fractions,soil CO₂ emissions,and ecosystem carbon fluxes under climate change conditions,whilst the soil organic carbon fractions had the greatest effect amongst them.The carbon pool activity,carbon pool activity index,carbon pool index,and carbon pool management index,which were used to represent soil carbon pool stability in this study,had significant negative relationships with the NEE and allocation proportion of soil recalcitrant organic carbon,but were significantly and positively correlated with ecosystem respiration,gross ecosystem productivity,soil organic carbon fraction contents,allocation proportion of readily oxidized organic carbon(ROOC),and soil CO₂ emissions.The contents and allocation proportions of soil organic carbon fractions explained 74.83% of the variance of soil carbon pool stability,with ROOC having the largest contribution(58.00% ).Moreover,the effects of the key processes of the carbon cycle on soil carbon pool stability were in the order of soil organic carbon fractions>soil CO₂ emissions>ecosystem carbon fluxes.The NEE did not have a direct effect,but did have an indirect effect on soil carbon pool stability by affecting the soil organic carbon fractions and soil CO₂ emissions.Similarly,we found that precipitation rather than warming was the key climate change factor affecting the soil carbon pool stability of the abandoned farmland in the loess hilly region.Structural equation modeling revealed that climate change can influence soil carbon pool stability directly by affecting the soil organic carbon fraction contents.In addition,climate change can also alter the soil organic carbon fractions and soil CO₂ emission by changing the NEE,soil enzyme activities,CUE,and BTR and thus affect soil carbon pool stability indirectly.