| The changing of land use plays an important role in affecting soil carbon storage. After logging, the forest changed into the other land use patterns, which may lead to significant loss of soil organic carbon. Recent studies suggest that after forest changing to the other land uses, a large number of soil organic carbon was decomposed and released, because of the destruction of soil aggregates and physical protection loss of the soil organic carbon. Currently, it was unclear the dynamics process of soil carbon pool and the affect of mechanism under different land use, the main reason was soil organic carbon was composed of different organic carbon fractions. So study the different fractions of soil organic carbon is the key to reveal the impact of soil organic carbon on different land use. We selected secondary forest, plantations (pinus plantation, larix plantation, picea plantation, fraxinus plantation, larix fraxinus mixed) and farmland for the study as they are representative at the northeast temperate regions of China. To provide a scientific soil carbon pools management, the effect of stability on soil aggregate and fractions of organic carbon fractions was studied when land use changed. The results were as follows:1. Different land use patterns (secondary forest, plantations and farmland) has great impact on water stable aggregates. When the particle diameter is>2mm, secondary forest the largest amount contains of the water stable aggregates, the next is in the plantation, the lowest is in the farmland. When the particle size is <0.25mm, the water stable aggregates variation is exactly the opposite. Compared to the other land uses, the secondary forest, mixed forest and fraxinus plantation were more helpful to increase the proportion of large diameter water-stable aggregates.Under different land use patterns, forest anthropogenic interference with small and soil structure highly stability, Soil water stable aggregates mean weight diameter (MWD) and geometric mean diameter (GMD) in order:secondary forest, followed by mixed forest, Ash forest, coniferous forest, farmland is the minimum, Secondary forest, mixed forest and farmland compared to soil water stable aggregates MWD and GMD were increased by58.1%,52.3%and41.0%,35.1%; The farmland of soil water stable aggregates is highest in the fractal dimension, secondary forest and plantation is low, especially mixed and larch plantations are more lower.2. Among different land usages, all forests are significantly different to farmland(P<0.05), different soil aggregates exhibited decreasing with decreasing particle size of soil aggregates, and particle size>2mm aggregates is the highest. The order for the content of organic carbon in soil aggregates on the surface of different land uses was secondary forest> fraxinus plantation>mixed>coniferous forest>farmland. For seven land usage patterns, the contribution rate of organic carbon in soil aggregates shows a consistent sequence:diameter>2mm aggregates is the highest, followed by2～1mm,1-0.5mm,0.5-0.25mm,<0.25mm is the minimum. For different land use patterns, Secondary forest and fraxinus plantations <0.25mm,0.5～0.25mm and1～0.5mm aggregates are highest in the enrichment factor of soil aggregates organic carbon, enrichment factor of coniferous and mixed forests concentrated in <0.25mm,0.5～0.25mm are maximum. Under different land use, soil aggregates organic carbon in1～0.5mm aggregates has the maximum correlation coefficient with soil organic carbon indicating that1～0.5mm aggregates is the main carriers for soil organic carbon.3. After the secondary forest changing into plantations and farmland, The content and the distribution ratio of soil aggregates light fraction organic carbon (LFOC) are significantly decreased. All different land uses shows in2-lmm the content and distribution ratio of soil aggregates LFOC are the highest, in0.5-0.25mm are the lowest.>0.25mm soil aggregates content of LFOC order:fraxinus plantation>secondary forest>mixed>coniferous forest>farmland,>0.25mm aggregates distribution ratio of LFOC order:plantation>secondary forest>farmland. Under land use the distribution ratio and content of soil aggregates POC, the overall average aggregate size increases with the increasing trend. Different land use patterns showed particle diameter>2mm the content of soil aggregates POC were highest, particle size0.5～0.25mm were minimum.>0.25mm soil aggregates POC content of the order:secondary forest, raxinus plantation, mixed forest, coniferous forest, the last was farmland;>0.25mm the distribution ratio of soil aggregates POC order:fraxinus plantation> mixed> secondary forest> coniferous forest> farmland.>0.25mm soil water stable aggregates (WSA) and the content of soil organic carbon, LFOC and POC showed a significant positive correlation (P <0.05). Secondary forests into plantations, farms, especially coniferous forest and farmland cultivated soil aggregates and POC LFOC large loss, mainly>0.25mm WSA.4. The order for the content of soil aggregates’free particulate organic carbon (fPOM) under different land uss was:>2mm>2～1mm>1～02.5mm>0.25～0.053mm>0.053mm. From the surface to the subsurface, in the particle size>2mm aggregates, after the secondary forest changed into farmland, soil aggregates fPOM content decreased the most8.00g.kg-1. Especially in diameter>2mm aggregates, and all secondary forest and fraxinus plantation, mixed the content of soil aggregates fPOM was not significant difference (P>0.05).The soil aggregates occluded particulate organic carbon (oPOM:oPOM<i.6and oPOM1.6-2.0) is higher than the soil aggregates fPOM among different land use. Each particle soil aggregates oPOM content decreased with the particle size decreasing. From secondary forest into farmland, soil aggregates0POM1.6.2.0levels decreased significantly more than50%of the surface, In diameter>2mm aggregates, secondary forest, fraxinus plantation and farmland the content of soil particulate organic carbon occluded POM<1.6were significant difference (P<0.05), its secondary forest, fraxinus plantation, mixed difference were not significant (P>0.05).The content of soil mineral combined carbon (Mineral:Mineral>0.053mmand Mineral<0.053mm) decreased with depth, and with trending for reduced size of the aggregates among different land use. Soil intra-aggregate particulate organic matter fractions:fPOM<oPOM<Mineral under different land uses. It means soil minerals combined carbon is more stable than oPOM in soil, therefore, organic matter attached to the mineral particles appears to be more resistant mineralization. Secondary forest, plantation the content of soil mminerals combined carbon were higher than farmland, among all the particle size of secondary forest and plantation showed significant differents (P>0.05), fraxinus plantation, and its secondary forests, mixed difference was not significant (P>0.05).The contribution rate of particulate organic carbon in soil aggregates decreased with the depth of soil, and with decreasing soil particle size under different land use. Different land use changes the contribution rate of soil aggregates, oPOM is the largest in>0.25mm aggregates, the contribution rate of soil mineral combined carbon is also the highest reflected in the large particle aggregates. Thus, different land usages changes, not only affected the total SOC contents, but also the relative importance of different SOC pools at the aggregate level, and be able to change the structure, stability and function of soil organic carbon fractions. |
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