Effects Of Reclamation On Soil Organic Carbon And Its Components In Northwest Sichuan Alpine-cold Grassland

Soil carbon pool is the largest carbon pool in terrestrial ecosystem,and soil organic carbon is the most active component.The small amplitude of soil carbon can lead to large fluctuations in atmospheric CO₂ concentration.Grassland ecosystem is an important part of terrestrial ecosystem.It plays an important role in regional climate change and global carbon cycle.The alpine grassland in Northwest Sichuan is located in the eastern margin of the Qinghai Tibet Plateau.It is an important recharge area for the Yangtze River and the Yellow River,and its ecological strategic position is very important.In recent decades,driven by population pressure and economic benefits,local residents have cultivated large areas of natural grassland to grow grain?highland barley?and vegetables,and some alpine grassland in northwestern Sichuan are seriously degraded.In recent years,the study of soil organic carbon in grassland has attracted wide attention at home and abroad.At present,the research which reported on the effects of grassland reclamation on soil organic carbon are mostly concentrated on the temperate grassland,while the reports on the alpine meadow are still scarce.These research are limited to the study of the total amount of organic carbon in soil reclamation,but lack of research on soil organic carbon and its components.Therefore,the 3,10,16,27 and 40 years of cultivated alpine-grassland were served as the research objects,the natural grassland?0 years?was served as the control,the soil organic carbon and its components of the alpine-grassland under different reclamation years were measured with the method of space instead of time and the field investigation and indoor analysis technologies.The results showed that:?1?After artificial reclamation of natural grassland in Northwest Sichuan,the soil bulk density,pH and sand content of 0-20,20-40 and 40-60 cm soil layer increased significantly with the increase of reclamation years,while the soil water content,clay and powder content decreased significantly.The surface layer?0-20 cm?performed the most obvious change trend and as the soil deepens,the decrease trend slowed down.After 40years of cultivation,the soil bulk density,pH and sand content of the 0-20 cm soil depth increased by 55%,21.7%and 21.9%?P<0.05?respectively compared with the natural grassland,while the soil water content,clay and powder content decreased by 51.5%,41.8%and 22.2%?P<0.05?compared with the uncultivated grassland.?2?The artificial reclamation of natural grassland in Northwest Sichuan led to the decrease in the content and reserves of soil organic carbon?SOC?,among which,the decrease trend of 0-20 cm soil layer was most obvious.After 40 years of cultivation,the content and reserves of SOC in the soil layer of 0-20 cm were 74.1%and 60%lower than that of uncultivated natural grassland?P<0.05?,and the decrease range and rate of soil layer of 20-40 cm and 40-60 cm were less than 0-20 cm soil layer.In the cultivation process,the content and reserves of SOC in the early stage of reclamation had the fastest decrease rate.In the first 3 years of cultivation,the annual decline rate of the content and reserves in the 0-20 cm soil layer reach 12.3%and 8.1%,respectively.With the increase of reclamation years,the decline rate of SOC decreased gradually.As the other hand,the annual decline rate of organic carbon content and reserves in 20-40 cm and 40-60 cm soil depths were the highest in 3¹0 years,and gradually decreased with the increase of reclamation years.?3?With the extension of reclamation years,the contents of the humic acid carbon?HAC?,the fulvic acid carbon?FAC?and the humin carbon?HMC?were significantly decreased.After 40 years of reclamation,the HAC,FAC and HMC in 0-20 cm soil layer decreased by 81.9%,60.1%and 78.4%,respectively,compared with those of uncultivated grassland?P<0.05?.With the increase of soil depth,the content of HAC,FAC and HMC decreased gradually.The HAC,FAC and HMC content declined fastly in the first years of cultivation,and then declined slower in the later reclamation stage.The HAC/FAC,PQ values?HAC/total humic acid carbon?,the optical density ratio of HA and FA?E4/E6?and the hue coefficient??logK?showed a decreasing trend with the increase of reclamation years,indicating that the degree of soil humification is decreasing and humus tends to age.?4?The content of soil microbial biomass carbon?MBC?,particulate organic carbon?POC?,dissolved organic carbon?DOC?and oxidized organic carbon?EOC?also decreased obviously with the prolongation of the reclamation years.After 40 years,the soil MBC,POC,DOC and EOC in 0-20 cm soil layer decreased by 80.9%,71%,72.7%,80.3%?P<0.05?.The decline characteristic is also similar to the total organic carbon in soil.The soil MBC/SOC,POC/SOC and EOC/SOC all showed a significant downward trend in the 0-20 cm,20-40 cm and 40-60 cm soil layers,and the soil DOC/SOC in 0-20cm showed an upward trend,but the changes in 20-40 cm and 40-60 cm were not obvious.?5?The soil invertase and amylase activity of natural grassland in Northwest Sichuan decreased significantly with the extension of reclamation years.After 40 years of reclamation,the activities of invertase and amylase in the 0-20 cm soil depth decreased by 57%and 50.4%?P<0.05?respectively,in the 3 to 16 years,the decline rates reached the fastest and the annual decline rates were 3.2%and 3.9%respectively.The activity of soil peroxidase and polyphenol oxidase increased gradually,and increased by 275.6%and 40%?P<0.05?compared with that of natural grassland after 40 years of reclamation,whereas there was no obvious change in different reclamation periods.?6?The results of correlation analysis showed that the content of SOC and soil HAC,FAC,HMC,MBC,EOC,DOC and POC were all significantly correlated with soil water content,clay and powder contents,soil invertase and amylase activities?P<0.01?,and were significantly negatively correlated with soil bulk density,pH,and sand content?P<0.01?.The activities of soil peroxidase and polyphenol oxidase were negatively correlated with soil HAC content?P<0.01?,and had no significant correlation with other indexes.