[Spatial heterogeneity of soil organic carbon and total nitrogen of sandy grassland in the restoration of degraded vegetation in Horqin Sandy Land, northern China].

Based on the field investigation and lab analysis, we used geostatistics method to examine the spatial heterogeneity of soil organic carbon and total nitrogen in a 0-year fenced mobile dune (MD11), an 11-year fenced mobile dune (MD11) and a 20-year fenced mobile dune (MD20) in Horqin Sandy Land. The results showed that the average values of vegetation cover, species number and diversity, soil organic carbon and total nitrogen (0-20 cm) contents increased with the increase in fenced age of mobile dunes. Geostatistical analysis showed that the proportions of relative structural variance C/(C0 + C) of soil organic carbon and total nitrogen in each dune were over 65% , which suggested that they had obvious spatial autocorrelation. Calculated ranges of spatial autocorrelation for soil organic carbon and total nitrogen were 58.39 m and 91.00 m (MDO), 28.59 m and 23.61 m (MD11) and 63.31 m and 61.05 m (MD20), respectively. The analysis from semivariance calculated parameters and spatial distributed maps showed that the spatial heterogeneity of soil organic carbon and total nitrogen firstly increased from MDO to MD11 then decreased from MD11 to MD20. Correlation analysis indicated that there were significantly positive correlations among vegetation cover, species richness and diversity index, soil organic carbon and total nitrogen (p < 0.01) . The analyzed results indicate that the spatial distributions of soil organic carbon and total nitrogen are most strongly related the topography features and vegetation changes in sand dune ecosystems prone to wind erosion. The accumulation and spatial haterogeniety of soil organic carbon and total nitrogen in the restoration of dune degraded vegetation are greatly affected by vegetation restoration succession.

[Challenge to the desertification reversion in Horqin Sandy Land].

Analyses of the changes in desertified land area, water resource availability, land use, and plant productivity in Horqin Sandy Land in recent 50 years showed that from 1950 to the late 1980s, the land desertification in Horqin Sandy Land had a rapid expansion, but reversed since then. The annual runoff of Xiliaohe River decreased consistently, and in 1999, the middle reach at Tongliao section was dried up. In recent 20 years, the water table of Xihu Lake was decreased by about 10 m, and dried up in 2001. The above-ground biomass of grasslands decreased from 520 g x m(-2) in 1937 to 197 g x m(-2) in 2005. The main cause of these results was the change of land use pattern, i. e., the overuse of water resources for re-vegetation or cropland irrigation. Water resources reduction was the major challenge to the desertification reversion in Horqin Sandy Land.

[Responses of Agriophyllum squarrosum phenotypic plasticity to the changes of soil nutrient and moisture contents and population density].

This paper studied the phenotypic plasticity of Agriophyllum squarrosum under effects of soil nutrient and moisture contents and population density. The results showed that with the increase of soil nutrient content, the root/shoot ratio of A. squarrosum was decreased from 0.135 to 0.073. However, soil moisture content and population density had less effect on the root/shoot ratio. The plasticity of reproductive allocation of A. squarrosum as responding to the changes of soil nutrient and moisture contents was a "real plasticity", and the allocation was negatively correlated with soil nutrient content but positively correlated with soil moisture content. When soil nutrient content was high or moisture content was low, the reproductive allocation of A. squarrosum changed larger with plant size. Population density had no effects on the reproductive allocation, while plant size conditioned the allocation. Among the three test affecting factors, soil nutrient content had the greatest effects on the morphological characters and biomass of A. squarrosum.