| Soil organic carbon and nitrogen are important components of soil nutrients,both of which reflect the soil quality status,which is an important index to evaluate soil fertility and sustainable land use.Soil organic carbon is mainly stored in the aggregates,which is one of the most important factors affecting the soil agglomeration structure.Its distribution in the aggregates directly affects the formation and stability of the soil water-stable aggregate structure,and the formation of aggregates is considered the most important mechanism of soil carbon sequestration.Nitrogen is the necessary nutrient for plant growth and is one of the most vulnerable elements in the plant ecosystem photosynthesis and primary production process,it plays an important role in terrestrial ecosystem function.In recent years,due to the large area of rubber plantation in Xishuangbanna,leading to the serious destruction of the original forest,followed by the deterioration of the ecological environment,soil fertility decline,soil erosion and other issues become increasingly serious.In this study,we selected the different plantation ages and different altitude rubber forests in Xishuangbanna,and analyzed the characteristics of soil physical and chemical properties,the stability of water-stable aggregates,the characteristics of organic carbon and nitrogen in water-stable aggregates and their correlation,so as to study the mechanism of organic carbon and nitrogen in aggregates of rubber forest soil.At the same time,the trend of soil carbon and nitrogen change under the change of vegetation was analyzed,compared with the original forest,to provide the basis reference for reasonable planting and soil fertility restoration of the rubber forest in Xishuangbanna,the main findings are as follows:?1?The soil moisture content of rubber forest in Xishuangbanna was 21.01-31.51%,soil bulk density was 1.45-1.82 g.cm-3,total soil porosity was 31.22-43.50%,and the proportion of sand components accounted for 14.01%,proportion of silty components accounted for 69.91%,clay composition accounted for 16.08% in rubber and virgin forest soil,the soil type was mainly sandy loam,followed by sandy loam and loam;the soil organic carbon content was 5.16-20.13 g.kg-1,soil total nitrogen content 0.14-2.06 g.kg-1,soil total phosphorus content 0.24-0.32 g.kg-1,soil total potassium content 8.01-19.75 g.kg-1,the content of total organic carbon,total nitrogen and total phosphorus in the 1950 s still showed a downward trend,but nearly a decade has recovered,and the total potassium content has been increasing since the 1990 s,which has benefited from more scientific and reasonable human management.?2?The average content of organic carbon and nitrogen in soil was as follows: virgin forest > 15a> 5a> 25 a,it decreased with soil depth,and it changed on 8.32-11.95 g · kg-1,0.51-1.28 g · kg-1 of soil organic carbon and nitrogen content under different altitude rubber plantation,the lowest value appeared at 850 m,and the effect of altitude on the total nitrogen content of rubber forest was higher.The influence of age and altitude on soil organic carbon and nitrogen content in rubber forest was mainly in the surface?0-15 cm?soil.?3?Soil water-stable aggregates content of particle size >2mm was as follows: virgin forest > 15a> 25a> 5a,soil water-stable aggregate content of particle size <0.25 mm was as follows: 5a> 25a> 15a> virgin forest,the stability of soil structure was as follows: virgin forest > 15a> 25a> 5a,and the soil stability was higher than that of the lower soil.The influence of age on the soil structure stability of rubber forest was higher than that of the lower soil.The soil structure of rubber forest was the worst at 850 m,the soil integrated structure of rubber forest was the most stable at 950 m,and the surface soil structure of rubber plantation was the most stable at 1050 m,but its vertical stability was the worst.?4?The soil organic carbon and nitrogen contents of soil water-stable aggregates were as follows: virgin forest> 15a> 5a> 25 a,and the soil organic carbon and nitrogen content was lowest 850 m,the average values on the particle size of >2mm?2-0.25mm?<0.25 mm were 8.07 g · kg-1,9.27 g · kg-1,6.99 g · kg-1 and 0.5g · kg-1,0.56 g · kg-1,0.40 g · kg-1.The content of organic carbon and nitrogen in water-stable aggregates of each particle size decreased with the soil depth,and it was as follows: 2-0.25 mm,> 2mm,<0.25 mm in the same soil layer,large aggregate?> 0.25mm?organic carbon and nitrogen content accounted for the main part,accounting for more than 60%,was the main contribution of soil organic carbon and nitrogen carrier.?5?There was a significant positive correlation between total organic carbon and nitrogen content and organic carbon and nitrogen content of water-stable aggregates?P <0.01?.The distribution of water-stable aggregates was closely related to the content of organic carbon and nitrogen.The soil C / N in different forest types was as followed: virgin forest> 5a> 25a> 15 a,the soil C / N values of the forest soil in different forest types were in the range of 8.32-17.59,of which only level 850 m was higher than 15,its mean is 17.09.The relationship between total organic carbon content and soil total nitrogen content was very significant?P <0.01?,the content of organic carbon had little effect on soil C / N,but the nitrogen content directly affected soil C / N,the C / N of water-stability aggregates directly affect the increase or decrease of soil total N;The total C / N of the soil was significantly correlated with the C / N of the water-stable aggregates?P <0.01?,the soil C / N interaction between the water-stable aggregates was significant?P <0.01?,Indicating that the soil aggregates between carbon and nitrogen interrelated and mutually constrained,and always maintained at a balanced level.?6?In general,the stability of the soil structure has been deteriorated,the organic carbon and total nitrogen content has been decreased,and the soil quality has been decreased after the conversion of the original forest to the rubber forest,but the reasonable management has been carried out,the ecological environment tends to be stable during the growth of rubber forest,and its soil erosion resistance can be strengthened,soil and water conservation function can be a certain extent recovery. |
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