Soil Aggregate And Organic Carbon Stability At Different Altitudes In Helan Mountains

Mountain ecosystems are carbon sources and carbon sinks,and the stability of soil organic carbon has an important impact on global climate change.The stability of soil organic carbon is closely related to the physical protection of soil aggregates and the chemical properties of soil organic carbon(SOC).The mountain forest ecosystem in arid and semi-arid areas is sensitive to climate change.Study on the chemical structure of SOC and aggregate stability at different altitudes in the region is of great significance to study the stability and sequestration of soil organic carbon.The Helan Mountains,which are representative of the arid mountain forest system in the aeolian sand area,are located at the boundary between the temperate steppe and desert areas in China.The vegetation types of Helan Mountain are complex and diverse and the ecosystem is highly vulnerable.In view of this,we selected the 0-20 and 2040 cm soil layers at different altitude gradients(1380-2438 m)in Helan Mountains as the research object.Wet-sieving methond and Solid-state 13C NMR spectra(CPMAS13C-NMR)were used to separate aggregates and to determine the chemical structure of organic carbon respectively.The change characteristics of aggregate associated carbon and the chemical structure characteristics of SOC were used to clarify the influence of physical and chemical protection of SOC on the storage of organic carbon,we expected to provide a scientific basis for studying the stability mechanism of soil organic carbon under climate change.The results showed that:(1)Soil physicochemical properties varied significantly with increasing altitude.In the 0-20 cm soil layer,SOC,total nitrogen(TN),pH,sand content and N/P,C/P increased significantly with increasing altitude,while soil bulk density(BD)and clay content decreased with altitude,the total phosphorus fluctuated with the increase in altitude.In addition to the fluctuation of pH in the 20-40 cm soil,the change trend of other indicators with the increase in altitude was consistent with that of the 0-20 cm soil layer.(2)The distribution and stability of soil aggregates were significantly affected by altitude and the interaction of altitude and soil layers.With the elevation gradient,the main soil aggregates of Helan Mountains changed from microaggregates(0.25-0.053 mm)to macroaggregates(>0.25 mm).The mean weight diameter(MWD)and geometric mean diameter(GMD)in the middle and high altitudes(21392248 m)were significantly higher than the low altitudes(1380-1650 m),and reach the highest in the middle altitude.The organic carbon of each particle size aggregate increased significantly with the elevation.The organic carbon storage in macroaggregates and microaggregates is significantly higher than that in clay particles(<0.053 mm).The macroaggregates and microaggregates are the main carrier of SOC.(3)The SOC chemical composition were significantly diference along elevation gradient(P<0.05).The chemical composition of SOC at different altitudes in Helan Mountains were ranked in O-alkyl C>Alkyl C>Aromatic C>Carbonyl C.O-alkyl C was the main chemical component of soil organic carbon at different altitudes.The chemical composition of organic carbon varies significantly at different altitudes.As the altitude increases,the aromatic C and carbonyl C increased significantly to the middle altitude and then gradually decreased,while the alky C and O-alkyl C decreased to the lowest at mid altitude and then increased.Aromaticity,alkyl C/O-alkyl C(A/O-A),aliphatic C/aromatic C(Alip/Arom)and hydrophobic C/hydrophilic C(HB/HI)showed a unimodal trend with increasing altitude.The aromaticity,A/OA and HB/HI first increased and then decreased with the increase in altitude,Alip/Arom first decreased and then increased with the increase in altitude,and all reached their peaks at mid-altitude.(4)Soil aggregate stability and aggregate associated organic carbon were sensitive to environmental changes.The stability of soil aggregates and the aggregate associated carbon were significantly affected by the environment factors.The content of macroaggregates,MWD,GMD,and the carbon of each particle size aggregate were significantly positively correlated with altitude,MAP,SOC,TN and C/N ratio(P<0.05),and significantly negatively correlated with pH,BD,and annual mean temperature(MAT)(P<0.05).The chemical compositions of SOC were less affected by the environment.Alkyl C and aromatic C were not significantly affected by environmental factors.Carbonyl C was significantly positively correlated with SOC,TN,TP and sand content(P<0.05),and was negatively related to BD and clay content.(P<0.05).Soil aggregates were closely related to the chemical compositions of SOC,OAlky C was positively related to macroaggregates and microaggregates,while carbonyl C and A/O-A were positively related to the silt and clay fractions,and negatively related to macroaggregates,aromatic C and aromaticity is positively correlated with large aggregates.Soil organic carbon storage was affected by aggregate associated carbon,aggregates and SOC chemical compositions,of which aggregate associated carbon had a significant impact,with an explanation rate of 61%,which is the main path to affect the organic carbon storage of Helan Mountains.In summary,the mid and high elevation of Helan Mountains has high soil aggregate stability and soil organic carbon stability in 1380-2438 m of Helan Mountains.The aggregate associated organic carbon content is a key factor affecting the soil organic carbon storage,and the macroaggregates and microaggtrgates are the main carrier of organic carbon storage.The organic carbon content of the aggregates is a key factor affecting the soil organic carbon storage.The large aggregates and micro aggregates are organic carbon.The main carrier of storage.Low-elevation soils are sensitive to environmental changes.In forest cultivation,it is recommended to reduce human disturbance and increase the protection of soil aggregates to improve the storage of soil organic carbon and the sustainability of the ecosystem.