| Semiarid land, which is characterized by fragile hydrologic and environmental conditions, is the main component of land ecosystems and very sensitive to global climate change. A good understanding of the interrelations between land cover alteration and changes in hydrologic, soil as well as vegetation is crucial for maintaining the fragile hydrologic and environmental conditions of semiarid land.A typical area within the Horqin Sandy Land with sand-meadow landscapes was selected for this study. Using the long-term (2003 to 2009) hydrometeorological, soil physicochemical, and remote sensing images data collected in a 9.71 km~2 well-instrumented area of the Horqin Sandy Land. The spatio-temporal variations in hydrology (e.g., soil moisture and groundwater table), soil physicochemical properties (e.g., soil particles size distribution, soil organic matter and total nitrogen et al.) and vegetation as well as the dynamic interrelations of hydrology, soil and vegetation were examined using regression analysis, and multiple comparisons and analyses of variance (ANOVA). The results were listed as follows:1. The top soil moisture (0 ~ 30 cm) was dependent on the local topography as well as soil texture. The moisture of the sandy areas was almost 10% lower than that of the transitional zones, which in turn was about 18% lower than that of the meadow lands. In addition, soil moisture was also influenced by vegetation density, locations with denser vegetation cover tended to have higher soil moisture. However, human activities could alter such relation between soil moisture and vegetation density, natural meadow land incurred long-term reclamation for agriculture and the soil moisture was decreased by 23%.2. Soil moisture at 10 to 30 cm beneath the ground level of the mobile dunes was higher than that for the semi-fixed dunes, which in turn was higher than that for the fixed dunes. This is because the thickness of dry sand layer of the mobile dunes was greater than that of the semi fixed dunes, which in turn was greater than that of the fixed dunes. The dry sand layer can restrict evaporation by increasing the distance of vapor diffusion.3. The water table was controlled by the general topographic relief and localized geological settings. Water table underneath the sandy areas was almost 0.9 m higher than that underneath the transitional zones, which in turn was about 0.1 m higher than that underneath the meadow lands. Also human activities have noticeable effects on water table distribution, natural meadow land incurred long-term reclamation for agriculture and the water table was lowered by 0.3 m.4. The mean annual water table temporal variations were mainly dependent on the precipitation and potential evapotranspiration (PET). For a given year with abnormally large PET (e.g., PET > 2200 mm in 2007) resulted in a more-than-normal soil storage in the subsequent years. The increased soil storage tended to diminish replenishment of infiltrated water to the aquifer. Compared with mean annual water table, annual mean monthly water table exhibited much smaller temporal variations. The replenishment effect of increased precipitation in summer (June to August) was offset by elevated PET, while precipitation in other three seasons was too small to cause a noticeable increase of water table.5. Significant positive correlations ( R 2 > 0.7) were found at a significance level of?= 0.05 among soil silt-clay content, soil organic matter, total N, total P, available N, available P, and soil moisture characteristics (e.g., field capacity). This indicates that the decrease of silt-clay content will likely cause the decrease of soil organic matter, nutrients, and water holding capacity.6. Compared to those at the control meadow (CM, vegetation density > 50%, with no detectable erosion and sand accumulation), the contents of silt and clay at the general meadow (GM, vegetation density < 50%) and cultivated land (CL, primarily covered by maize) were found to be significantly lower by 14.6 and 53.9%, respectively ( P < 0.05). The contents of silt and clay at the semi-fixed dune (SFD, vegetation density 20 to 40%) was found to be significantly lower by 52.5% ( P < 0.05) compared with that of the fixed dune (FD, vegetation density > 40%). The contents of field capacity at the SFD, FD and CL were found to be significantly lower by 44.8, 39.4, and 33.6% ( P < 0.05) compared with that of the CM.7. Compared to those at the CM, the contents of soil organic matter at the SFD, FD, GM and CL were found to be significantly lower by 97.1, 93.6, 70.9 and 56.2% ( P < 0.05), which were significantly lowered by 94.2, 89.1, 72.3 and 40.9% for total nitrogen, and significantly lowered by 86.1, 77.8, 50.0 and 33.3% for total phosphor ( P < 0.05).8. A region with a smaller depth to water table tended to have a better vegetation condition. Vegetation condition was best in the meadow lands where depth to water table was less than 2 m (with the largest NDVI), while it was worst in the sandy areas where depth to water table was greater than 4 m (with the smallest NDVI). Vegetation cover in the transitional zones, where depth to water table varied from 0.6 to 3 m, exhibited noticeable spatial variations (with the medium NDVI).9. The internal changes of vegetation were mainly dependent on the water table, precipitation and PET. Water table explained 42, 38, and 7% of the variations for meadow lands, transitional zones, and sandy dunes, respectively, while PET explained 25, 7, and 30%, respectively,precipitation explained 3, 17, and 8%, respectively. It indicated that the growing condition of vegetation in semiarid area was mainly dependent on groundwater.10. Strong relations were obtained with top soil moisture (0 to 30 cm) data that were lagged by up 2 to 4 weeks with respect to the vegetation conditions during growing season (May to September). The vegetation conditions of meadow lands and sandy dunes were well correlated with soil moisture lagged by 2 weeks, the correlation coefficients were 0.49 and 0.47, respectively, whereas the interrelation in transitional zones performed more randomly. It indicated that soil moisture may be a useful predictor of growing season (warm season) vegetation conditions. |
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