The Effect Of Soil Physico-chemical Properties And Enzyme Activities On Soil Organic Carbon Of Songyuan Saline-alkali Farmlands In Jilin Province

Soils,acting as an important component in terrestrial ecosystem,contain the largest organic carbon sink.According to the previous researches,the global soil organic carbon(SOC)pool is more than twice that of the terrestrial vegetation carbon pool and nearly three times that of atmospheric carbon pool,thus any small variation of SOC could affect the global carbon cycle and climate change.Previous studies about SOC and its influencing factors mostly focused on forests,wetlands,grasslands and so on.However,the subject about saline-alkali agriculture ecological systems has not been extensively studied.We selected saline-alkali agroecosystems as our research subject and the saline-alkali paddy fields and upland fields both has been included in our research,at the same time,.A variety of data analysis methods were applied to reveal the SOC dynamics and the environmental influencing factors on it.The formation,transformation and decomposition of SOC are regulated by almost all enzymes,so they are comparatively vital in soil carbonc cycling,moreover,soil physico-chemical properties also have important influence on SOC dynamics.Therefore,soil physico-chemical properties and soil enzyme activities has been studied in this research to explore the impacts of these factors on SOC dynamics,and then analyzes all influencing factors together to discuss the main factors affecting the SOC.This research area is located in the Songyuan of western Jilin Province which belongs to one of the three major saline-alkali regions in the world and the Northeast China Transect of IGBP Terrestrial Transects.Three saline-alkali upland farmlands(U1,U2 and U3)as well as three saline-alkali paddy farmlands(P1,P2 and P3)were chosen for the research.Topsoil(0-30cm)and subsoil(30-60cm)samples were taken from study sites during the growing periods of maize and rice in 2016,respectively in May,July and September,corresponding to the seedling stage(?),tillering to heading stage(?)and fructicative stage(?)for rice,and the seedling stage(?),jointing to heading stage(?)and ripening stage(?)for maize,respectively.Soil samples were taken back to the lab for the measurement of SOC content,and soil p H,IV exchangeable sodium percentage(ESP),bulk density and electrical conductivity,and soil amylase,invertase and catalase and polyphenol oxidase activities.According to the analysis of variance,we found that the SOC content of topsoil was significantly higher than the content of subsoil no matter at which sampling sites and crop growing periods.During the different growing periods of crops,a drastic decrease of SOC content was observed in the July and showed an increase trend to the September.In addition,the mean value of SOC in paddy fields was the highest at P1 site,medium at P2 site and the lowest at P3 site;and in upland fields,the value of SOC was the highest at U1 site,medium at U2 site and the lowest at U3 site.Correlation analysis showed that soil p H,ESP,and bulk density were negatively correlated with SOC in paddy and upland fields,which illustrated that the p H,ESP and bulk density had negative effects on SOC and higher p H,ESP and bulk density might inhibit the formation of SOC.Amaong the determined soil enzymes,amylase,invertase and catalase showed significant positive correlation with SOC in paddy fields.For upland fields,soil amylase and invertase had significant positive correlation with SOC,and polyphenol oxidase had significant negative correlation with SOC.From the results,it could concluded that soil hydrolase had a significant positive influence on the SOC.As for oxidoreductase,the catalase had an important positive effect on the SOC in the paddy fields,and the polyphenol oxidase had an important negative effect on the SOC in the upland fields.Finally,we combined soil physico-chemical properties and soil enzyme activities together to obtain the main influencing factors for SOC of songyuan saline-alkali agroecosystems in Jilin province through the principal component analysis.The results showed that in the paddy fields,the main negative impact factors on SOC was soil p H.For the upland fields,soil p H and ESP had more significant negative effect on SOC.By comparing the main soil quality indicators between upland fields and paddy fields,we could notice that the constraint of ESP on paddy SOC was not as much as on upland SOC.This might be attributed to the farming practices of rice planting that perennial irrigation condition could reduce the degree of soil alkalization.This study provided technical supports for improving the research in SOC dynamics and the effect factors of it in songyuan saline-alkali farmlands and had certain significance to soil resource conservation,environmental management and sustainable agricultural development in research area.