| The soil physical and chemical properties and soil soluble organic carbon content in eight different types of grassland in Qinghai Province were investigated.The grassland types included montane meadow,alpine meadow,alpine meadow steppe,temperate steppe,alpine steppe,alpine desert,temperate desert steppe and temperate desert.The effects of land use patterns(grazing,returning farmland to forest,secondary seabuckthorn forest and farming)on soil physical and chemical properties and soil soluble organic carbon content in alpine meadow soils were studied as well.At the same time,the structure characteristics of soil soluble organic matter were studied by using ultraviolet-visible light and fluorescence spectra.It aimed to provide basic data for grassland health evaluation,sustainable development and full play of ecological functions in Qinghai Province,and provide theoretical basis for grassland ecosystem maintenance and rational management and utilization.(1)The soils of montane meadow,alpine meadow and alpine meadow steppe contained more silt and clay than that in other types of grassland soils and contained a certain amount of fine sand.The contents of coarse sand in the soils of temperate steppe,temperate desert,temperate desert steppe and alpine desert were high,while the contents of silt and clay were low.In alpine meadow soils under different utilization patterns,the grazing land soils contained many fine particulate matters such as silt and clay,returning farmland to forest had less and secondary seabuckthorn forest land even less.The farming land soil had the least small particulate matters.The variation of coarse sand contents in alpine meadow soils under different utilization modes were contrary as for silt and clay,and the difference of fine sand contents was small.(2)The soil pH values in montane meadow,alpine meadow and alpine meadow steppe soils ranged from 7 to 8 in 0-10 cm and 10-20 cm soil layers,while in other types of steppe were greater than 8.The soil pH values in montane meadow soils were the lowest,which were significantly lower than that in alpine meadow and alpine meadow steppe(P<0.05).The pH values of temperate desert steppe,alpine desert and temperate desert soils were high,but there were no significant differences among them(P>0.05).Except for alpine steppe,the soil pH values of other types of steppe in 10-20 cm soil layers were slightly higher than that in 0-10 cm soil layers.In alpine meadow soils under different utilization patterns,the soil pH value of grazing land in 0-10 cm soil layer was significantly lower than that of other three land use patterns of grassland(P<0.05).There was no significant differences among the pH values of farming land,secondary seabuckthorn forest land and returning farmland to forest(P>0.05).The soil pH value of farming land in 10-20 cm soil layer was significantly higher than that of returning farmland to forest and grazing land(P<0.05),but there was no significant difference between secondary seabuckthorn forest land and farming land soils(P>0.05).With increasing soil depth,the soil pH values increased in all land use patterns of alpine meadow.(3)The soil organic carbon contents of montane meadow and alpine meadow were significantly higher than that of other types of steppe(P<0.05).There was no significant difference in soil organic carbon content between temperate desert steppe and alpine desert steppe(P>0.05).The content of soil organic carbon in temperate desert soil was the lowest.Except for alpine steppe,soil organic carbon contents of other types of steppe decreased with increasing soil depths.The ranking of soil organic carbon contents in alpine meadow under different land use patterns in 0-10 cm and 10-20 cm soil layers was grazing land > returning farmland to forest > secondary seabuckthorn forest > farming land,and the difference was significant(P<0.05).The contents of soil organic carbon in alpine meadow soils under different utilization patterns decreased gradually with increasing soil depth except for that in farming land soils.(4)The soil total nitrogen contents in alpine meadow,alpine meadow steppe and montane meadow were high.Except for alpine steppe,the soil total nitrogen contents of other types of steppe decreased with increasing soil depth.The contents of total phosphorus and potassium in soils were inconsistent with increasing soil depth.The contents of soil total nitrogen and total phosphorus in grazing land were high,and they tended to decrease with increasing soil depth.The soil total nutrients of returning farmland to forest soils were high,and the changes of soil total nutrients in secondary seabuckthorn forest soils and farming land soils were inconsistent.(5)There was a significant(P<0.05)or extremely significant(P<0.01)positive correlation between soil organic carbon,total nitrogen and the amounts of clay respectively in different types of steppe soils and in different land use types of alpine meadow soils,and they were also significantly(P<0.05)or extremely significantly(P<0.01)negatively correlated with soil pH values and the amounts of coarse sand.There were no significant correlations between total phosphorus and total potassium contents and other soil physical and chemical properties(P>0.05).(6)The effect of steppe types on soil organic carbon,total nitrogen,coarse sand,silt and clay contents were greater than that on soil total phosphorus,total potassium,fine sand contents and pH values.Soil total potassium contents of montane meadow,alpine meadow,alpine meadow steppe and temperate steppe were below the middle level.Soil total phosphorus and potassium contents of alpine steppe were slightly absent.The content of each nutrient in the temperate desert steppe was below middle level.The contents of each nutrient in alpine desert and temperate desert soils were low,and more than one nutrient were in an extremely lacking state.Soil organic carbon,total nitrogen,coarse sand and total potassium contents of steppe soils were significantly affected by utilization modes,while the soil total phosphorus,fine sand,silt and clay contents and pH values were slightly affected.(7)The contents of soil soluble organic carbon in alpine meadow,montane meadow,alpine meadow steppe and temperate steppe were higher than that in alpine desert,temperate desert and alpine steppe.Soil soluble organic carbon contents of different steppe types varied with increasing soil depth.Under different utilization modes in alpine meadow,the content of soluble organic carbon in surface soil layer of grazing land was high,while that in deeper soil layer of returning farmland to forest and secondary seabuckthorn forest were high.The contents of soluble organic carbon in different soil layers of farming land were low.The contents of soil soluble organic carbon tended to decrease with increasing soil depth.(8)In different types of grassland,the contents of soil soluble organic carbon were significantly negatively correlated with the contents of soil coarse sand(P<0.05)and soil pH values(P<0.01),which were significantly positively correlated with the contents of soil clay(P<0.01),soil organic carbon and total nitrogen(P<0.01).Under different utilization patterns of alpine meadow,the contents of soluble organic carbon in 0-10 cm layers were significantly negatively correlated with the contents of coarse sand(P<0.01)and soil pH values(P<0.05),which were significantly positively correlated with the contents of soil silt(P<0.05),soil clay(P<0.05),soil organic carbon and total nitrogen(P < 0.05).There were no significant correlations between soil soluble organic carbon and other physical and chemical properties in 10-20 cm soil layers(P>0.05).(9)Through spectral analysis,it can be concluded that the soil soluble organic matter of eight types of grassland in Qinghai Province contained aromatic substances,and the soil soluble organic matter structures of alpine meadow,montane meadow,alpine meadow steppe and temperate steppe were complex,which had a high degree of aromaticity and condensation and had many polycyclic aromatic hydrocarbons with complex structures.However,the aromaticity and condensation degree of soluble organic matter in desert grasslands that with low vegetation coverage and low organic matter contents were low,the structures were simple,and the organic matters were easy to be decomposed and be lost.The complexity of soil soluble organic matter structure in most steppe types showed a trend of 0-10 cm soil layers higher than that in 10-20 cm soil layers.Soil soluble organic carbon contents,aromatization degree and humification degree in 0-10 cm soil layers of grazing land were high.The contents of soluble organic carbon in 10-20 cm soil layers of returning farmland to forest and secondary seabuckthorn forest were high,but the degrees of aromatization and humification were low.The degrees of aromatization and humification of soil soluble organic matter decreased with increasing soil depth. |
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