Effects Of Nitrogen Addition On Soil Organic Carbon Components And Root Exudates Of Bothriochloa Ischaemum

Nitrogen is the main limiting factor affecting terrestrial ecosystem carbon sequestration function.The extensive use of fossil fuels and chemical fertilizers leads to the significant increase of nitrogen input to the ecosystem,and affects the plant-soil carbon trend.Plant root exudates act as a bridge between the plant and the soil.At present,nitrogen fertilizer addition is one of the main ways of grassland management.Therefore,it is of important theoretical significance to clarify the change of carbon cycle in terrestrial ecosystem by nitrogen application.In this paper,the typicial vegetation in the loess hilly-gully region bothriochloa ischaemum as the research object.We analyzed the effect of different supplemental levels(0,2.5,5,7.5,10,15 g N m^-2yr^-1)on soil organic carbon and root exudate of bothriochloa ischaemum,and the mechanism of nitrogen supplemental on soil in the hilly region of Loess Plateau to reveal the relationship between soil organic carbon and root exudates and soil carbon,nitrogen and phosphorus components,and to provide data support and management of degraded grassland.The main research results are as follows:(1)Nitrogen addition increased the content of organic carbon(SOC),nitrate nitrogen(NH₄⁺-N)and ammonium nitrogen(NO₃^--N)in the rhizosphere.Compared with no nitrogen application,15 g N m^-2 yr^-1(N15)treatment increased by 30.5%,60.8%and 28.8%,respectively.The content of rhizosphere was significantly higher than that of non-rhizosphere,which showed obvious rhizosphere effect.Nitrogen addition significantly increased the aboveground biomass,underground biomass,N content of leaf and P content of root of bothriochloa ischaemum.The aboveground and underground biomass of N15 treatment were 1.44 times and 2.04 times of those without N application.Compared with those without N application,N content of leaf and P content of root increased by 33.9%and 45%,respectively.However,compared with no nitrogen application,stem carbon content of N15 treatment was reduced by 8.1%.(2)Nitrogen addition increased the content of easily oxidized organic carbon,soluble organic carbon and microbial biomass carbon content,and the content in the rhizosphere was significantly higher than that in the non-rhizosphere,which showed obvious rhizosphere effect.The microbial biomass carbon content reached the maximum in 10 g N m^-2 yr^-1(N10)treatment,and the rhizosphere content was 1.89times that of the treatment without nitrogen application.The contents of granular organic carbon,light fraction organic carbon and heavy organic carbon were increased by nitrogen addition,and the rhizosphere was higher than the non-rhizosphere,but the differences were not significant.Rhizosphere carbon pool management index values of different nitrogen addition treatments were all greater than 100,indicating that the rhizosphere fertility was increased in all treatments.The sensitivity of particulate organic carbon and soluble organic carbon was higher.(3)Nitrogen supplementation significantly increased the relative total content of the exudates of bothriochloa ischaemum.With the increase of nitrogen fertilizer application,alcohols,amino acids,sugar acids and short chain organic acids were significantly increased,but nitrogen supplementation had no significant effect on the contents of amines and long chain organic carbon.Nitrogen supplementation significantly increased the content of short-chain carbon,but had no significant effect on long-chain carbon,and the content of long-chain organic acids in the secretion was higher than that of short-chain organic acids.Nitrogen supplementation increased the contents of organic acids and amino acids secreted by bothriochloa ischaemum root system.The contents of oxalic acid,alpinic acid,pyrrole-2-carboxylic acid,mandelic acid,3-hydroxybutyric acid and imidazole acrylic acid were increased by nitrogen addition,while the contents of 2-ketyl iso-valeric acid were decreased.The contents of?-ketone isomeric acid first decreased and then increased with the increase of nitrogen supplemental level.The contents of 2,3-dimethylsuccinic acid,2-ketobutyric acid,isoleucine,aspartic acid,oxalanine,alanine and citrulline increased first and then decreased with the increase of nitrogen supplemental level,while the contents of cycloleucine,serine and valine increased with the increase of nitrogen supplemental level.(4)Soil easily oxidized organic carbon content was positively correlated with TP content and SOC content,indicating that soil easily oxidized organic carbon had a strong dependence on the input,fixation and decomposition of rhizosphere soil carbon sources.The microbial biomass carbon content of rhizosphere soil was positively correlated with TN and NO₃^--N,and TN and NO₃^--N could predict the microbial biomass carbon content of soil.Soil organic carbon was positively correlated with soil TN,while the content of soil organic carbon in light group was negatively correlated with total nitrogen.Except for 3-hydroxybutyric acid,the other organic acids were positively correlated with SOC and negatively correlated with pH to varying degrees.Oxalic acid and tartaric acid were significantly positively correlated with TP.For amino acid components in secretion,pH was positively correlated with L-isothreonine,threonine,serine and cycloleucine,while negatively correlated with isoleucine,alanine,cannianine,aspartic acid,citrate and valine.The effects of environmental changes on the secretion of plant root exudates are complex.