| Grassland ecosystem carbon cycle plays an extremely important role in the global carbon cycle in terrestrial ecosystems, the soil organic carbon pool also plays an important role in grassland ecosystem carbon cycle, the stability of soil organic carbon cycle and turnaround is also an important indication of grassland ecosystem functions changed. Therefore, the study of the stability of soil organic carbon dynamic process of grassland desertification process can make scientific measures to restore degraded grassland. With the method of substituting temporal serial with spatial serial by selecting a representative of desert grassland with six sandy grasslands with different sandy habitat types (hard sierozem, sandy sierozem, fixed sand dune, semi-fixed sand dune, semi-mobile dune, mobile sand dune) in grassland as test objects, and taking different development stages of shrub patches and grassland patches of degraded sandy grassland in four different sandy habitats (interdunal lowland, fixed sand, semi-fixed and mobile sand dune) by shrub, Caragana intermedia, in Yanchi County of Ningxia, Northwest China. The soil samples at the depth of 40 cm soil layer were collected at representative sites in six sandy grasslands with different sandy habitat types during the peak of growth period in April 2015. Soil bulk density, soil organic carbon, soil total P, soil total N, soil enzyme activity, labile organic carbon, particulate organic carbon and soil particle composition were determined, and related distribution pattern along soil profiles were calculated. In addition, the grassland plant community structures and features were calculated.0-10 cm soil samples were collected from different development stages of shrub patches and grassland patches of degraded sandy grassland in four different sandy habitats by shrub, Caragana intermedia, with the soil organic carbon, labile organic carbon, and particulate organic carbon contents and soil particle composition were analyzed, and related distribution pattern along soil profiles were calculated. We measured the potential of organic carbon mineralization of the different development stages of shrub patches and grassland patches of degraded sandy grassland in four different sandy habitat types, and six sandy grasslands with different sandy habitat types in desert steppe region using a laboratory soil incubation experiment. After 31 and 34 days incubation experiment, the mineralization of soil organic carbon distribution pattern along soil profiles and between shrub patches and grassland patches were calculated. The mineralization of soil organic carbon and related main soil factors are discussed for six sandy grasslands. This study researched the progress of grassland degraded succession in desert steppe region, it described the response mechanism of soil organic carbon stability and spatial variation to soil environmental factors in different stages of sandy grasslands and shrub patches. The results were as followed:1. The mainly percentage of particle composition was fine sand at size of 50-250μm. Along with the increased soil layer, the soil bulk density and soil particle composition changed irregularly, but the soil organic carbon, total nitrogen and total phosphorus gradually decreased along the soil profiles. Although, the percentage of clay particles, fractal dimension, soil organic carbon, soil total nitrogen, total phosphorus and surface soil enzyme activity decreased. The fixed sand was a sensitive stage and also a reversal of soil texture and soil fertility levels, The fixed sand was the stage of coarse-grained and soil texture suffered the risk of soil erosion gradually increased, also the gradual loss of soil physical stability. Soil texture is coarse this stage of the threshold, the risk of soil erosion suffered gradually increased, soil physical stability is gradually lost.2. The species richness was 9, 12,9,9,9 and 5 respectively in the communities, of which the desertification was from hard sierozem to mobile sand dune. For life form composition of all communities along the desertification succession, the dominant species were the perennial herbaceous plants from hard sierozem to sandy sierozem, and the dominant species were the annual herbaceous plants from fixed sand dune to semi-fixed sand dune, then, the dominant species were semi-shrubs from semi-mobile dune to mobile sand dune. Species number of Gramineae in the community was the highest dominant species in the semi-mobile sandy stage. Species number of Chenopodiaceae is stable after sandy sierozem. Species number of Compositae and Leguminosae disappeared or decreased continuously along the fixed and semi-fixed sand sandy succession gradient and their importance in community also decreased gradually. The species diversity was the highest in sandy sierozem, and decreased then in fixed sand, and increased again in mobile sand dune. Along with succession, the ecological dominance and species diversity which performed the opposite trend. However, the community evenness showed decreases firstly and then increased gradually again. There was a high degree of similarity between two adjacent communities along with succession. There was a significant negative correlation among annual herbaceous plants dominance, silt and clay soil and soil particle fractal dimension (P<0.05), while it showed a significant positive correlation (P<0.05) between Leguminosae dominance and silt, and soil fractal dimension, and soil total nitrogen, and total phosphorus, and then there was a significant positive correlation between species diversity and the content of silt (P<0.01), there was a significant positive correlation between species diversity and soil total nitrogen, and total phosphorus(P<0.05), there was a significant positive correlation between at size of 50-250μm sand and soil total nitrogen, and soil fractal dimension, and total phosphorus(P<0.01). Based on the non-equilibrium ecology theory, which could well document the severely degraded sandy grassland community succession, and indicated the progress from Leymus secalinus and Stipa capillata Linn community, Artemisia scoparia and Lespedeza davurica community, Pennisetum centrasiaticum and Sophora alopecuroides community, Setaria viridis community, Pennisetum centrasiaticum community to Cynanchum komarovii community.3. The soil labile organic carbon (LOC) content and soil particulate organic carbon (POC) content decreased from hard sierozem, sandy sierozem, fixed sand dune, semi-fixed sand dune, semi-mobile dune to mobile sand dune. The soil particulate organic carbon content at the depth of 0～1Ocm showed significant differences (P<0.05) among different sandy habitats; Along with the increased soil depth, LOC, POC, LOC/SOC and POC/SOC evolved from uniform distribution to gradient distribution. The soil organic carbon stability decreased with the degree of soil desertification and soil depth. The soil organic carbon fractions in four sandy habitats had significantly differences between the medium development stages of shrub patches and grassland patches (P<0.01), which showed that the soil organic carbon, particulate organic carbon, labile organic carbon and POC/SOC in shrub patches were significantly higher (P<0.01) than that of grassland patches in four sandy habitats, and however, there was not significantly differences of LOC/SOC between the medium development stages of shrub patches and grassland patches (P>0.01). The soil organic carbon in the medium development stages of shrub patches and grassland patches appeared a peak in fixed sand, similarly, the labile organic carbon, particulate organic carbon and LOC/SOC appeared peaks of the various developmental stages in fixed sand, and then reduced gradually from fixed sand to mobile sand dune.4. The accumulative amount of SOC mineralization and daily average amount of SOC mineralization in six sandy grasslands experienced rapid mineralization, slow mineralization and stable mineralization. With the degree of soil desertification, The accumulative amount of SOC mineralization and daily average amount of SOC mineralization in hard sierozem, sandy sierozem and fixed sand dune were bigger than the other sandy habitats, while the mineralization rate of SOC were smaller than the other sandy habitats, and the mineralization rate of SOC in fixed sand dune was the lowest one in the all sandy habitats. The daily average amount of SOC mineralization and accumulative amount of SOC mineralization decreased along with the increased soil depth, and the surface soil mineralization rate in hard sierozem, sandy sierozem and fixed sand dune were lower than subsoil, while the surface soil mineralization rate in the semi-fixed sand, semi-mobile and mobile sand is greater than the subsoil. It showed a significant negative correlation (P<0.01) between organic carbon mineralization rate and urease activity, and catalase activity, and phosphatase activity, and showed a significant negative correlation (P<0.05) with invertase activity. While the accumulative amount of SOC mineralization and urease, catalase, invertase and phosphatase showed a significant positive correlation (P<0.01), the highest correlation with urease. It showed a significant negative correlation (P<0.01) between organic carbon mineralization rate and LOC, POC, LOC/SOC and POC/SOC(P<0.01), while the POC and LOC/SOC with mineralization rates of organic carbon and organic carbon mineralization showed the most relevant. It was a significant positive correlation (P<0.01) between organic carbon mineralization and soil bulk density. the higher of total nitrogen, total phosphorus and organic carbon content, the more of the mineralization of soil organic carbon accumulation amount; the higher of the silt and clay, fractal dimension, the greater of the cumulative amount of organic carbon mineralization.5. The accumulative amount of SOC mineralization and daily average amount of SOC mineralization in shrub patches were higher than the corresponding grass patches of four sandy habitats. There was a turning point in the fixed sand in shrub patches and grass patches of four sandy habitats. After 34 days incubation experiment, the soil organic carbon mineralization rate and cumulative mineralization have experienced rapid mineralization, slow mineralization and stable mineralization. The soil organic carbon mineralization rates were 4.976%,6.709%,13.725% and 10.174% from interdunal lowland, fixed sand, semi-fixed to mobile sand dune, while in grassland patches were 4.187%, 7.876%,8.043% and 7.865%. The organic carbon mineralization rate between patches of fixed sand as follows:grassland patches> shrub patches, and for interdunal lowland, semi-fixed sand and mobile sand dune are as follows:shrub patches> grassland patches. |
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