| Farmland is a carbon pool with the largest reserve and highest activity among the three terrestrial ecosystems in the world,and it has dual properties of carbon source and carbon sink.Soil carbon pool is the largest carbon pool in terrestrial ecosystem,with 3.3 times of atmospheric carbon pool and 4.5 times of biological carbon pool.Soil organic carbon（SOC）is an important part of soil carbon pool.Its content and distribution characteristics influence the regional soil quality and fertility,and affect plant growth and soil microbial activity.SOC is also essential in providing sources of nutrients required and maintenance of high yield for crop growth.SOC plays a significant role in the global carbon cycle research field.The west of Jilin Province belongs to the transect of the northeast of the ecological fragile zone of global climate change,where is one of the world’s three saline-alkaline soil distribution areas,and is also the largest soda saline soil areas in China.The part influenced by salinization is close to 36%of the whole study area,that restricts the sustainable development of agricultural production and ecological system.In recent years,large-scale land development projects have been carried out in the study area,and the utilization rate of land resources has been improved by using the“reduce alkalinity by water”measure,which has a good prospect for agricultural use and a huge potential of carbon sequestration.Four paddy fields with different saline-alkali levels（P1,P2,P3 and P4）and one experimental field in Da’an city were used as the research sites.At the same time,field in situ,indoor incubation and test simulation experiments were conducted based on a large number of continuous field investigations and basic data analysis,multiple mathematical statistical methods such as regression analysis,multiple comparison（LSD）,correlation analysis and variance analysis were used to analyze the experimental data.The kinetic model was used to fit the results of carbon mineralization.Based on analyzing the characteristics of saline-alkali paddy fields in western Jilin province,a systematic study on the characteristics of SOC input and output was carried out.The soil constraints of internal factors including soil physical,chemical and biological properties,and constraints of the external factors including land cultivation age,soil depth（0～20 cm,topsoil and 20～40 cm,subsoil layer）rules are focused studied.The soil carbon sink/source of paddy field under saline-alkali stress characteristics and potential are analyzed,to reveal the SOC cycle change rule and its influencing factors and clarify the dynamic evolution process of carbon cycle in saline-alkali land,so as to provide powerful data support and theoretical basis for the SOC pool change rule in western Jilin and our whole nation.The main results are as follows:（1）The soil pH value ranged from 8.70 to 9.26 and soil alkalinity（ESP）ranged from6.49%to 14.43%in Qianguo Irrigated area in western Jilin Province.P1 and P2 sites were light alkalinity,while that of soils in P3 and P4 sites were moderate alkalinity soils.The alkalinity intensity of the four sites was P4>P3>P2>P1.The electrical conductivity was in the range of 0.225～0.429 ms cm-1,and the salt content was in the range of 0.026%～0.179%.Among them,P1 was slightly salinized,P2 and P3 were non-salinized,and P4 was moderately salinized soil.The degree of salinization was P4>P3>P2>P1.（2）The average values of SOC,total carbon（TC）and soil inorganic carbon（SIC）were16.876±0.436 g kg-1,24.861±0.786 g kg-1 and 8.934±0.041 g kg-1 in the study area.The SOC concentrations of all sites differed significantly,showing a significant surface enrichment characteristic that the result is P1>P2>P3>P4.It had a significant positive correlation with the development years and the degree of salinization.In SIC,there was a certain enrichment phenomenon in the subsoil,and TC concentration did not show a consistency in the longitudinal distribution.（3）The process of SOC mineralization in Qianguo Irrigation district in western Jilin Province conforms to first-order kinetic law,Ct=C0(1-e-kt)（correlation index R2=[0.984-0.991]）,and the change of SOC cumulative mineralization amount（CO2-C）with incubation time is consistent with the trend of exponential model.In the process of incubation,the CO2-C was similar,and the total amount of CO2-C was consistent with its SOC concentration.With the increase of salinization degree,the potential value of SOC mineralization（C0）decreased,and C0>Ct（Ct is the accumulated amount of SOC mineralization after t time）,and the value of topsoil layer was higher than that of subsoil layer.（4）The initial rate of SOC mineralization（V0）was P2>P1>P3>P4.The average rate of SOC mineralization（VA）was 0.149 g kg-1 d-1.For P1,P2,P3 and P4 sites,the values were 0.147,0.193,0.151 and 0.107 g kg-1 d-1,respectively.The instantaneous rate of SOC mineralization showed a trend of first high and then low in the whole incubation period.Therefore,the SOC mineralization cycle can be divided into four periods,namely rapid reaction period,rapid fluctuation period,dynamic stationary period and slow cooling period.On the 70th day of the incubation,the rate of SOC mineralization decreased to 7.54%-10.79%of the initial value.The longitudinal distribution had strong correlation with SOC content.（5）Pearson correlation analysis showed that there was a significant negative correlation between ESP and SOC mineralization rate and cumulative SOC mineralization amount.The higher the soil salinization degree was,the lower the SOC mineralization rate was.Organic carbon turnover rate constant k was highly positively correlated with ESP,moderately positively correlated with EC and C0/SOC（An important index reflecting soil carbon sequestration capacity）,and negatively correlated with SOC and pH.（6）The soil enzyme activity is an important index that participates in the soil carbon cycle field.Two hydrolases,amylase（AMY）and invertase（INV）,and two oxidoreductases,catalase（CAT）and polyphenol oxidase（PPO）,which were closely related to soil carbon cycle were selected in this study.AMY,INV and CAT in different topsoil layers were higher than those in the subsoil layer,while PPO was the opposite.INV activity was greatly affected by salinity.There was no significant difference in CAT activity in P2,P3 and P4 in the topsoil.Ct,C0 and k were significantly correlated with the activities of four enzymes.（7）Effects of straw application on amino sugars（a significant soil organic matter pool）in upland fields had been received much attention,however,those in paddy fields remain unknown.A three-year experiment including three treatments was conducted:one without straw application（CK）,one with straw application at rates of 2500 kg hm-1 y-1（S1treatment）,and one with 5000 kg hm-1 y-1（S2 treatment）.S2 stimulated a higher increase in glucosamine（GluN）,muramic acid（MurA）,galactosamine（GalN）,fungal-derived GluN（GluNF）,and total amino sugars than SA1 in the rhizosphere and bulk soil in the first year.The concentration of MurA in rhizosphere soil was 10.95%-16.35%higher than that in bulk soil.At the same time,the effects of straw addition on the accumulation of soil microbial residues and SOC cycling and transformation could be inferred.The analysis results of amino sugars showed that the three-year continuous straw returning promoted the accumulation of MurA,GluN,GalN,GluNF and total amino sugars in rhizosphere and bulk soils. |
