Spatial heterogeneity of soil organic matter and its response to disturbance in karst peak cluster depressions
By using geostatistic methods, this paper studied the spatial variation and distribution of soil organic matter as well as its ecological processes and related mechanisms in four typical disturbed areas (cropland, man-made forest, secondary forest, and primary forest) of karst peak cluster depressions in northwest Guangxi of China. Eighty soil samples (0-20 cm) were collected from an aligned grid of 10 m x 10 m for the analysis of soil organic matter. The soil organic matter content increased significantly (P < 0.05) with the decrease of disturbance and the vegetation succession from crop to man-made forest to secondary forest to primary forest. Soil organic matter content had good spatial autocorrelation in all of the four typical disturbed areas, but its spatial heterogeneity differed. Gaussian model fitted best to the semivariance functions of soil organic matter content in the study areas except secondary forest area where exponential model fitted well. In cropland area, the spatial autocorrelation of soil organic matter was at medium level, with the C0/(C0 + C) being 26.5%; while in the other three areas, the spatial autocorrelation was at high level, with the C0/(C0 + C) being 9.0%-22.6%. The range and scale of the spatial autocorrelation of soil organic matter in cropland and man-made forest areas were larger than those in the other two areas, possibly due to the strong human disturbance and the homogeneity of low energy. The range of the spatial autocorrelation of soil organic matter in primary forest area was large due to the high vegetation coverage, while that in secondary forest area was the lowest due to the diverse vegetation communities and their uneven distribution. The low fractal value (D) of semivariance functions of soil organic matter in man-made forest and primary forest areas suggested that a strong spatial dependence existed, while the high D in cropland and secondary forest areas suggested a great random variance of spatial distribution of soil organic matter occurred. The spatial pattern of soil organic matter presented a unimodal distribution in cropland and man-made forest areas, a concave distribution in secondary forest area, and a gibbous distribution in primary forest area. To reduce human disturbance would be helpful to the soil quality improvement, rapid vegetation restoration, and ecological reconstruction of karst degenerative ecosystems.
Available from: Shujuan Liu
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ABSTRACT: Spatiotemporal heterogeneity of soil nutrients was put forward to indicate the distribution and changes of soil
nutrients. Comparing with non-karst regions, former studies had shown that soil nutrients were inversed in karst peak-cluster depression, namely nutrient-inversion. The reasons for nutrients inversion in karst peak-cluster depression were obscure. Therefore, this study aimed to explore the impacts of vegetation succession and topographical attributes on surface soil nutrients and validate whether it is universal for nutrients inversion.
An experiment was designed to compare the distribution of soil nutrients under three mainly vegetations to theoriginal forest. In order to explore the temporal and spatial variability of soil nutrients and its controlling factors, the soil samples of topsoil (0—15cm) were collected in two typical Karst peak-cluster with four vegetation succession stages (grassland, shrubland, plantation forest, original forest) in northwest Guangxi, southwest China. A total of 202 samples were collected from twelve sampling lines (each vegetation type had three sample lines for comparison), including 48 grassland samples, 46 shrubland samples, 44 plantation forest samples and 64 original forest samples. Analysis of variance (ANOVA) was utilized to analyze the differences of topsoil nutrient contents for different vegetation types and topographical attributes,
respectively. The results showed that there were significant differences of topsoil nutrients (soil organic carbon, total nitrogen, carbon/ nitrogen ratio, total phosphorus, total potassium, available nitrogen, available phosphorus, and available potassium) and soil pH values among different vegetation succession stages. Except for the total potassium, other nutrient contents increased with vegetation succession. The slope position had different effects on soil nutrient contents. Many influencing factors affected the variability of nutrients at the same time, such as human disturbance and its intensity, soil physical properties, vegetation community, ground litter and so on. Carbon and nitrogen of original forest, as well as phosphorus and potassium of other vegetation succession stages, was accorded with the “nutrient depression effect", which meant that nutrient reduced with the increase of elevation. In addition, carbon and nitrogen of grassland, shrubland and artificial forest increased with elevation. It was due to the disturbance of human activities.
In addition, Our study indicates that vegetation succession plays a positive role in soil nutrient accumulation and thegovernment should keep on carrying out to protect vegetation from human disturbance. Phosphorus (available phosphorus 3. 55—11. 41 mg/ kg) and potassium (available potassium 64. 05—105. 75 mg/ kg) are always limiting nutrient factors in karst peak-cluster depression regions. Additionally, nutrient-inversion is not a common phenomenon and if there was no interference of human, the distribution of soil nutrients in Karst peak-cluster depression will accord with other regions,following the “nutrient depression effect".
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ABSTRACT: Based on the investigation and analysis of seven soil mineral substance variables, nine vegetation factors, four topographical factors, and ten soil physicochemical factors in the 200 m x 40 m dynamic monitoring plots in farmland, forest plantation, secondary forest, and primary forest in the depressions between hills in karst region, and by using traditional statistical analysis, principal component analysis (PCA), and canonical correlation analysis (CCA), this paper studied the compositional characteristics and roles of soil mineral substances as well as the coupling relationships between the mineral substances and the vegetation, topography, and other soil properties. In the depressions, soil mineral substances were mainly composed of SiO2, Al2O3, K2O, and Fe2O3, whose contents were obviously lower than the mean background values of the soils in the world and in the zonal red soils at the same latitudes. The soil CaO and MgO contents were at medium level, while the soil MnO content was very low. The composition of soil mineral substances and their variation degrees varied with the ecosystems, and the soil development degree also varied. There was a positive correlation between vegetation origin and soil origin, suggesting the potential risk of rock desertification. Due to the high landscape heterogeneity of the four ecosystems, PCA didn't show good effect in lowering dimension. In all of the four ecosystems, soil mineral substances were the main affecting factors, and had very close relationships with vegetation, topography, and other soil properties. Especially for SiO2, CaO, and MnO, they mainly affected the vegetation species diversity and the soil organic matter, total nitrogen, and total potassium. This study indicated that soil mineral substances were the one of the factors limiting the soil fertility and vegetation growth in the depressions between hills in karst region. To effectively use the soil mineral resources and rationally apply mineral nutrients would have significances in the restoration and reconstruction of karst degraded ecosystems.
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ABSTRACT: Soil aggregates and their organic carbon distributions were studied under six ecosystems, i. e., farmland (short for ST), dry land (HD), grassland (CD), shrubbery (GC), plantation (RGL) and secondary forest (CSL), in a karst canyon region of China by a combination of field investigation and laboratory analysis. The result showed that, soil aggregates were dominated by particles with sizes>8 mm in the ecosystems except HD under dry sieving, and basically presented a trend of decreasing firstly, then increasing and finally decreasing along with particle sizes decreasing; while soil aggregates were dominated by particles with sizes > 5 mm in the ecosystems except HD under wet sieving and decreased with decreasing of particle sizes. The mean mass diameter (MMD) was in the order of ST>CD>RGL>CSL>GC>HD and the geometric mean diameter (GMD) was ST>CD>RGL>CSL>HD>GC by dry sieving, and MMD was RGL>CSL>GC>CD>ST>HD and GMD was CSL>RGL>GC>CD>ST>HD by wet sieving. Therefore, MMD and especially GMD of wet sieving were more accurate than that of dry sieving to evaluate soil aggregates quality in the karst cannon region. The fractal dimension (D) of mechanical stability in soil aggregates followed the order of CD>HD>ST>RGL>CSL>GC and the water stability was in the order of GC>CSL>RGL>HD> CD>ST. The higher the SOC content was, the larger values of D, MMD, GMD became, and the more sense the soil structure made. Soil organic carbon content was highest in the aggregate particles with sizes ranging from 0.25 to 0.053 mm, and the content in some particles with sizes > 5 mm was lowest. However, the contribution rate of particles with sizes > 5 mm was largest to soil organic carbon, which gradually decreased with the decrease of particle size.
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