Characteristics Of Soilaggregates In Continuous Eucalyptus Plantation And Their Response To Concentrated Flow On Slope

Eucalyptus as one of the four fast-growing tree species,has a prominent position in the development process of forestry industry in China.It has been more than 100 years since Eucalyptus was introduced to China and has been popularized in a large area,Eucalyptus plantations can be found everywhere because of the superior water and heat conditions and the rich land resources in south of our country.Eucalyptus can be continuously cultivated Several generations even more than a dozen generations on the same land due to its biological characteristics of strong germination ability.The pure Eucalyptus plantation under long-term management has shown a trend of decline in soil fertility and declining fertility,coupled with frequent man-made operations and unreasonable land use,soil erosion has occurred from time to time.Therefore,it is significant for scientific and rational management of Eucalyptus plantation to study the characteristics of soil aggregate stability and the mechanism of slope erosion.In this paper,1?4 generations of Eucalyptus plantation were planted in Huangmian forest,the basic physical and chemical properties,aggregate characteristics and stability,and denudation characteristics of soil aggregates in three soil layers A,B1 and B2,were analyzed qualitatively and quantitatively by wet-sieving method,LB method and concentrated-flow scouring experiment,the characteristics of soil aggregates in continuous-planting fast-growing pure plantation were studied.The conclusions reached are as follows:1.The physical and chemical properties of soil were measured by corresponding methods,and the relationship between each index and the change of successive planting was explored by analyzing soil bulk density,porosity,saturated water content,mechanical composition,p H value and organic matter.(1)The results showed that the soil bulk density,porosity and saturated water content decreased with the increase of successive planting times and soil depth,the soil bulk density and total porosity of different soil layers changed obviously in the first and second generations,but in the third and fourth generations on the contrary.The soil saturated water content of A layer and B1layer of the first generation forest was significantly different from the other three stands,the level of soil saturated water in layer B2 of the fourth generation forest was the lowest(30.99%).(2)The soil p H value increased and then decreased with the increase of Eucalyptus generations,the first generation was the most acidic,the third generation was the weakest,soil organic matter content decreased with the increase of successive generations;The free Fe-Al-Mn oxides decreased with the increase of successive planting times,and the variation range of each oxide in different soil layers was different,the content of Fe oxide decreased most obviously,but the basic content of Mn oxide was lower.(3)The mechanical composition of soil fluctuated with the increase of successive generations,showing a trend of first increasing and then decreasing.The lowest sand content appeared in the third generation forest,the highest in the second generation forest,the highest in the fourth generation forest,the lowest in the first generation forest and the lowest in the proportion of the total clay content.2.On the basis of analyzing the particle size distribution of soil aggregates in different generations of Eucalyptus plantation by traditional wet-sieving method,combining with LB method,the distribution characteristics of soil aggregates were evaluated from different angles.The results of wet-sieving showed that the content of macroaggregate in different soil layers of 1?4generations was A>B1>B2,and the content of macroaggregate decreased with the increase of successive generations.The average mass diameter(MWD)of soil aggregates and the geometric mean diameter(GMD)of soil aggregates were slightly different in different soil layers,which showed as first generation forest>second generation forest>third generation forest>fourth generation forest.The content of soil macroaggregate decreased and the stability of soil macroaggregate also decreased after the successive planting of Eucalyptus plantation.The mass percentage of soil aggregates under different successive generations of Eucalyptus was measured by LB method,and the order of particle size distribution was MWD_FW(fast wetting)<MWD_WS(pre-wetting oscillation)<MWD_SW(slow wetting).The content of soil aggregates with grain size>2 mm decreased with the increase of successive generations.Under the three treatments,the content of grain diameter>2 mm in B2 layer of the fourth generation forest was the lowest.The MWD and GMD of 1?4 generations of plantation decreased with the increase of successive generations,which indicated that the stability of aggregates decreased with the increase of successive generations.In addition,the relative dissipation index(RSI),the relative mechanical fragmentation index(RMI)and the aggregate stability characteristic parameter(AS)increased with the successive generations of Eucalyptus.3.The correlation analysis between the stability parameters of soil aggregates and soil physical and chemical properties showed that the soil porosity and organic matter treated by wet sieving method and LB method were positively correlated with the stability of soil aggregates.Under LB treatment,non-capillary porosity and saturated water content and aggregate stability had significant or extremely significant positive correlation,while soil bulk density,capillary porosity and aggregate stability had significant negative correlation.4.With the increase of scouring distance,soil depth and flow gradient,the destruction degree(stripping coefficient?)of aggregates increased in a parabola shape,when Wr/Wi and>0.25 mm aggregate mean weight diameter(MWD)decreased,the stability of soil aggregates decreased,the migration distance increased,the particle size decreased and the morphology became round gradually,and the difference was significant when the migration distance was>18 m.The MWD value of soil aggregate after scouring had a significant negative correlation with RMI index,and the more sensitive the soil aggregate was to its fragmentation,the more significant the correlation was with free Fe-Al oxide and amorphous Fe-Al oxide,there was a significant correlation with free Fe-oxide.