Composition And Contribution To Soil Carbon Pool Of Typical Plants Root Exudates In The Mountainous Area Of Southern Ningxia

The promotion of the policy of returning farmland to forest and grassland and the vigorous implementation of water and soil conservation measures have greatly increased the area of vegetation restoration in the loess hilly area,and the content of soil organic carbon has been increasing.However,the contribution mechanism of plant root exudates to the increase of soil organic carbon is still unclear.Therefore,in this study,the typical plants in the mountainous area of southern Ningxia:Stipa bungeana and Stipa grandis were used as research materials,the root exudates and soil were cultivated and collected to determine the composition and content of the root exudates;the rhizobox method withδ¹³C isotope was used for the experiment.Monitored the changes ofδ¹³C in root-soil-soil microorganisms,conducted research on the composition of root exudates and their effects on soil carbon pool transformation.Understand the effects of root exudates on soil organic carbon pool during the growth of different plants.Discriminated the mechanism of plant rhizosphere on soil organic carbon input,and provided a theoretical basis for revealing that plant growth can increase soil carbon sequestration capacity and improve soil fertility quality in the process of vegetation restoration in the Loess Plateau.The main conclusions are as follows:（1）The relative content of esters in the root exudates of the two plants ware higher,and the content of the two esters of 1-propylene-1,2,3-tricarboxylic acid-tributyl ester and acetyl tributyl citrate were both high,it shows that they may be the main substances secreted by these two plants to adapt to the arid living environment of the Loess Plateau.The common substances in the root exudates of the two plants had no difference in the content.（2）After five months of planting,the soil organic carbon content in the rhizosphere of Stipa bungeana was 4.92%higher than that of the non-rhizosphere,which was 2.97%in Stipa grandi.The microbial biomass carbon content in the rhizosphere of Stipa bungeana was 30.30%higher than that of the non-rhizosphere,which was 35.71%in Stipa grandis.Humus carbon content in the rhizosphere of Stipa bungeana was 29.47%higher than in the non-rhizosphere,which was 5.11%in Stipa grandis:It shows that the root exudates of plants can increase the content of soil organic carbon and activity of microorganisms to increase soil organic carbon.（3）After the root exudates increased,the microbes in the rhizosphere soil were more active than the non-rhizosphere soil,and after the root exudates inputed into the soil,the rhizosphere will produce a stimulating effect to reduce the organic carbon of the rhizosphere soil and the non-rhizosphere soil.Root exudates had a certain influence on the content of humus in rhizosphere soil.（4）The difference in soil organic carbon and its components was not obvious,but the overall soil humus carbon content of Stipa grandis is more and larger.The soil humus carbon of Stipa grandi was constantly decreasing with the growth of plants;the degree of change and response time of rhizosphere soil were basically faster than that of non-rhizosphere soil,which means that the increase of root exudates will produce rhizosphere stimulating effect and make the soil organic carbon decreased,but after the short-term increased in root exudates ends,the soil organic carbon will return to the state before the fluctuation.（5）As the incubation time progressed after 13C pulse labeling,the organic carbon componentδ¹³C of Miscanthus rhizosphere soil was larger than that of non-rhizosphere soil.The 13C-SOC labeling of rhizosphere soil increased by 40.79%after 65 days compared with0 day.,The non-rhizosphere soil increased by 37.06%;the rhizosphere soil ¹³C-DOC marked a decrease of 71.79%compared to the 0 day mark at 65 days,and the non-rhizosphere soil decreased by 59.80%;the rhizosphere soil ¹³C-MBC marked 65 days after the marking.Day 0 day decreased by 55.56%,and non-rhizosphere soil decreased by41.41%;indicating that root exudates have a rapid effect on soil DOC,MBC and other active organic carbon components,and can enter the soil through the transformation of microorganisms and become soil organic carbon.（6）After root exudates enter the soil,they will gradually migrate to the non-rhizosphere soil.Due to the action of microorganisms,¹³C-SOC will be solidified near the root system.When plant exudates enter the rhizosphere soil,root microorganisms will quickly gather in the rhizosphere soil and decompose the root exudates for use.For humus carbon,when the humus carbon ¹³C in the rhizosphere soil decreases,the content in the non-rhizosphere soil increases.The change trend of soil DOC and MBC of Stipa grandis is more consistent with that of Miscanthus.The photosynthetic products of plants enter the soil through root exudates and are used by microorganisms in a short time.Some of the root exudates will be transferred to the non-rhizosphere soil.In the middle,the soil organic carbon is formed later,and most of it exists in the rhizosphere soil and rhizosphere microorganisms.