| The purpose of this study was to elucidate the effect of plantation type and age on forest land and grassland those were converted from cropland in relation to the variation of aggregate content, stability and aggregate-associated organic carbon content. Soil samples were collected from five types of land in the southern part of the Loess Plateau including:artificial pure locust forest land converted from slope cropland with plantation ages of 6 years (FL06) and 15 years (FL15),29years (FL29),40years (FL40),56years (FL56), artificial alfalfa grassland converted from slope cropland with plantation ages of six years (GL06) and 15 years (GL15) and the neighboring long-term conventional slop cropland (CK).The results revealed that:(1) After croplands were converted to forest land and grassland, in 0-20cm and 20-40cm soil layer,dry aggregate content of every land decreased with aggregate size decreasing.in 0-20cm,>2mm aggregate fractions significantly surpassed 31.82%-42.49% compare to slope cropland as a result of afforestation; with plantation age prolonging, aggregate content descend at first and then increased,and at last descend after frost plantation; aggregate content increased with plantation age prolonging after grass plantation. In 20-40cm soil layer,for>2mm dry aggregate fractions, with plantation age prolonging, aggregate content increased at first and then descend,and at last increased after frost plantation,and increased with plantation age prolonging after grass plantation.For water stable aggregate,in 0-20cm soil layer,soil aggregates contents were dominated by>2mm and<0.25mm.>2mm aggregate fractions under grass lands increased significantly by 16.87%and 104.45%than that of forest lands after 6 and 15 years of afforestation.>0.25mm aggregate fractions under plantation of different ages and types was in the order of FL40>FL29>FL56>GL15>FL06>GL06>FL15>CK.In 20-40cm soil layer, aggregates contents increased with decreasing aggregate size.(2) Under the condition of dry sieve, both the mean weight diameter (MWD) and geometric mean diameter (GMD) value of forest and grass lands were higher than that of cropland in 0-20cm soil layer.MWD and GMD value of grass lands increased with plantation age prolonging. MWD value of grass lands decreased by 4.73%and 0.42%than that of forest lands after 6 and 15 years of afforestation. In 20-40cm soil layer, MWD and GMD value of forest and grass lands increased by 31.15%-17.63%and 30.66%-57.18%than that of cropland.For water stable aggregate,both the MWD and GMD value of forest and grass lands were higher than that of cropland in 0-20cm soil layer. In the two soil layers, MWD and GMD value increased at first and then descend with plantation age prolonging.(3) Total organic carbon contents in 0-20cm layer were higher than in the 20-40cm layer in all the sample plots. There was an distinct trend that,after forest plantation,total organic carbon contents both in 0-20cm and 20-40cm soil layer firstly decreased then increased,and at last decreased along with plantation age going on.Total organic carbon contents were the highest under FL40 both in the two soil layers,with the maximum value of 18.19g.Kg-1 and 6.12g.Kg-1,respectively.After grass plantation,total organic carbon contents in 0-20cm increased with plantation age going on,and in 20-40cm soil layer had the opposite tendency.Under the condition of dry sieve,in 0-20cm soil layer,the averaged organic carbon contents under all the sample plots in every particle size increased with the aggregate size decreasing,and in>2mm,1-2mm,0.5-1mm,<0.25mm aggregate fractions, FL40 had the maximum organic carbon contents. In 20-40cm soil layer, the averaged organic carbon contents in every particle size decreased first then increased along with the aggregate size decreasing.Along with the water stable aggregate size decreasing, the averaged organic carbon contents under all the sample plots in every particle size increased at first and then descend in 0-20cm soil layer, on the whole, organic carbon contents in 1-2mm aggregates fractions was the highest, while<0.25mm aggregates fractions had the lowest organic carbon contents. Organic carbon contents both in>2mm and 1-2mm water stable aggregates fractions after foest plantation increased at first and then descend along the afforestation age extending, and both were the highest under FL40.Under the condition of dry sieve, there was a trend that contribution rate of aggregate-associated organic carbon to its responding total organic carbon in aggregates fractions firstly decreased then increased,with aggregate size decreasing. The contribution rate in>2mm aggregates fractions was in the order of FL06>FL40>FL56>FL29>GL15> GL06>CK>FL56. In 20-40cm soil layer, the averaged contribution rate decreased with aggregate size decreasing.Under the condition of wet sieve, in 0-20cm soil layer, the contribution rate in>2mm and <0.25mm aggregates fractions were dominated,and there was a trend that the contribution rate in aggregates fractions, with aggregate size decreasing, firstly decreased then increased.In 20-40cm soil layer, the contribution rate in<0.25mm aggregates fractions was highestAnd in this layer, the contribution rate increased with aggregate size decreasing. |
PDF Full Text Request