| The Loess Plateau is characterized by fragmented and intricate terrain,and the soil and vegetation properties are highly spatially heterogeneous.The spatial variability of soil properties largely affects the vegetation properties,while the latter feedback the ecosystem function.The small watershed is not only the basic hydraulic unit for water-diverting,but also the basic unit of soil erosion control on the Loess Plateau.Hence,a small watershed was selected in the wind-water erosion crisscross region with the most fragile ecological environment on the Loess Plateau to systematically investigate the spatial changes of soil properties and vegetation characteristics.Using classical statistics and geo-statistical methods,the spatial variability and distribution patterns of soil properties and vegetation characteristics?above-ground biomass,community coverage,species diversity?,and the effects of soil environmental factors on plant community characteristics were revealed.Furthermore,the effects of slope aspects and plant community types on soil organic carbon and total nitrogen were also analyzed.The results will be helpful in restoration and rational construction of vegetation,as well as evaluation of ecosystem structure and function in the wind-water erosion crisscross region.The main results were as the follows:?1?There was a certain correlation among the physical and chemical properties of soils in the surface layer of 0-20 cm in the small watershed,and these soil properties showed obvious spatial distribution patterns.Soil water content was significantly positively correlated with clay and silt content?P<0.01?,significantly negatively correlated with bulk density and sand content?P<0.01?.There was a significantly positive correlation between soil organic carbon and total nitrogen,mineral nitrogen,total phosphorus and clay content?P<0.01?,and a significantly negative correlation with bulk density?P<0.01?.In the process of soil formation,the spatial variabilities of soil saturated hydraulic conductivity,sand content,total phosphorus and available phosphorus were mainly affected by the structural factors,while the spatial variabilities of soil water content,bulk density,clay content,silt content,soil organic carbon,total nitrogen and mineral nitrogen were caused by both the structural factors and random factors.The spatial distribution of saturated hydraulic conductivity and total phosphorus was relatively uniform,but the distribution patterns of the other soil properties were patchy.Soil water content,clay,silt content,soil organic carbon,total nitrogen and mineral nitrogen were all higher near the outlet of the watershed.?2?The plants in the small watershed were mainly composed of Leguminous,Gramineous and Compositae.The above-ground biomass?AGB?,coverage?C?,Patrick richness index?R?,Shannon diversity index?H?,Simpson's dominance index?D?and Pielou's evenness index?J?of the community all showed certain spatial variability.The spatial variability of AGB was determined together by the structural factors and the random factors,while the variabilities of C,R,H,D and J were mainly determined by the structural factors.The spatial distribution pattern of AGB was patchy,both AGB and C were gradually increasing from northeast to southwest along the channel direction of the small watershed;while the spatial distribution patterns of R,H,D and J were relatively fragmented,and the first three indexes were obviously higher at the top of the semi-shady slope,but D was the opposite.AGB and C were chiefly positively correlated with soil water content,organic carbon and mineral nitrogen,while R,H,D and J were mainly affected by soil saturated hydraulic conductivity,bulk density,sand and silt content.?3?Slope aspects and plant community types significantly affected soil organic carbon?SOC?,total nitrogen?TN?contents and density,and ratio of carbon-nitrogen?C/N?in the small watershed.The semi-shady slope had higher SOC and TN contents than that in the semi-sunny slope and the gully-head,but C/N in semi-sunny slope was higher than that in semi-shady slope and the gully-head.There was no significant difference in SOC and TN contents among Leguminous plant communities,however,they were all significantly higher than those communities in Compositae and Gramineae communities.The contribution rate of SOC density and TN density in the surface layer of 0-20 cm to the 60 cm profile was both 51%.The average SOC density of the 1 m soil depth in the small watershed was estimated to be 2.13 g·cm-3,which was far lower than that of other areas of the Loess Plateau and the national grassland. |
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