| Soil water is the basis of the ecosystem of plant survival, and it also is a most active part in the hydrological cycle which affects plant growth, ecological environment and the rational allocation and efficient use of water resources. The region of depression between karst hills in the southwest of China has the characteristics of strong landscape heterogeneity, high temperature and rainy but with uneven distribution, obvious hydrological two-dimensional structure, barren and discontinuous soil, and severe runoff of soil and water. In addition, sharp contradictories between human being and land, and human irrational activities lead to heavy damage on the eco-hydrology pattern and to the occurrence of varying degrees of drought and rocky desertification. While soil water has the decisive effect on plant growth, vegetation restoration, water and heat balance, and system stability, which plays an significant role in the evolution and reconstruction of ecosystems. Based on the controlled experiment and long-term monitoring of six most common land-use types, i.e., burning, cutting, cutting plus with root removal, enclosure, maize plantation, and pasture of Guimu No. 1 plantation, the study was conducted to analyze the temporal effects and spatial pattern of surface soil moisture under these land-use types, and to reveal the influencing mechanism of soil moisture in the region of depression between karst hills, southwest of China. Main results are as follows:(1) Rainfall in the region has the characteristics of poor stability with large variation amplitude, rich precipitation but with uneven distribution in a year and great variation in interannual precipitation, average annual precipitation being 1409 mm and the rainfall from April to September accounting for 77.8%, severe seasonal drought and flood and abundant precipitation in rainy season with low water efficiency and occasional drought in winter.(2) The seasonal variation trend of surface soil water in the slope in the region of depressions between karst hills showed patterns of "one-ascending, two-descending, and one-maintaining" and "one-slowness, two-quickness, and one-gentleness", which basically accords with the seasonal variation of precipitation. The seasonal variation trend of surface soil water differed under varying land-use types. The seasonal variation of both cutting plus with root removal and maize plantation presented a trend of descending first, and then descending, and last ascending, which of burning, cutting, and enclosure showed a wave mode of ascending first, and then descending, and last ascending, while which in the pasture of Guimu No.1 plantation appeared a whole descending trend. The monthly variation of surface soil water under all the varying land-use types could be predicted by wave curve with four-descending and three-ascending. The ten-day variation of surface soil water differed under varying land-use types, but all present regular wave curves with the lowest value in the middle of October.(3) The monthly variation of surface soil water under different land-use types in three precipitation years (i.e., dry year, rainy year, and normal year) had their characteristics, which was affected by recent precipitation. The monthly variation of surface soil water in the dry year showed N type trend, that in the rainy year did W type trend, while that in the normal year did an inverted N shape. The tendency of surface soil water under different land-use types showed high similarities. Both enclosure and the pasture of Guimu No.1 plantation under the three precipitation years had higher surface soil water content, while maize plantation had the lowest soil water content, and cutting plus with root removal followed. The variation of surface soil water content under different land-use types presented in the order of rainy year> normal year> dry year, while the variation coefficient of surface soil water content did in the order of dry year> rainy year> normal year.(4) The best fitting models for semivariance of surface soil water under the different land-use types in dry and rainy seasons in the selected precipitation years were Gaussian model, exponential model, and spherical model, respectively. All the ratioes of nugget to sill [C0/(C0+C)] were less than 50%, which indicated surface soil water had evident spatial structure and fine spatial continuance with moderate or strong spatial autocorrelation. Surface soil water under the same land-use type in the dry and rainy seasons in the same precipitation year appeared similar spatial autocorrelation.(5) According to water retention of soil, the six land-use types in the region of depression between karst hills could be divided into two categories. One is water conservation type, which included burning, cutting, and enclosure, the other one is water consumption type including cutting plus with root removal, maize plantation and the pasture of Guimu No.1 plantation. Based on the soil physical properties and microbial properties, the soil restoration effects of enclosure was best, that of burning was following, while that of maize plantation and cutting plus with root removal was the worst, which went against the quick restoration of soil ecological function. On the whole, the effects of enclosure and burning on soil water and fertilization were the best.(6) The relationships between soil water content and soil nutrients, and soil microbial properties differed in varying land-use types in the region of depressions between karst hills. For the slope position, surface soil water under the land-use types of burning, cutting, enclosure, and maize plantation had the feature of lower slope> upper slope> middle slope, while that of the pasture of Guimu No.1 plantation and cutting plus with root removal had the feature of lower slope> middle slope> upper slope. Through the analysis of vegetation community characteristics under the six land-use types, it was found that the comprehensive effects of tree density and coverage had the same rule as surface soil water, which suggested that vegetation community structure had important effects on surface soil water in the region of depression between karst hills. |
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