Xiaoan Zuo

Spatial pattern and heterogeneity of soil organic carbon and nitrogen in sand dunes related to vegetation change and geomorphic position in Horqin Sandy Land, Northern China

To assesses the effect of geomorphology, topography, and vegetation changes on spatial pattern of soil organic carbon (C) and total nitrogen (N) in sand dunes, we used the quantitative methods to examine the spatial heterogeneity of vegetation cover, soil organic C, and total N in an 11-year naturally restored mobile dune (RMD11) and a 20-year naturally restored mobile dune (RMD20) that had been fenced to exclude grazing in Horqin Sandy Land, northern China. Our results showed that the vegetation cover, plant density, species number and diversity, soil organic C, and total N increased from RMD11 to RMD20 and increased from the 50 × 50-m plot (crest) to the 100 × 100-m plot (slope) in each dune. Geostatistical analysis showed that the spatial structural variance accounted for the largest proportion of the total sample variance in vegetation cover, soil organic C, and total N in each dune plot. Calculated spatial autocorrelation ranges of vegetation cover, soil organic C, and total N increased from RMD11 to RMD20, indicating that longer time since vegetation restoration results in a more homogeneous distribution of vegetation cover, soil organic C, and total N in sand dunes. In addition, the spatial continuity of vegetation cover, soil organic C, and total N decreased from the 50 × 50-m plot (crest) to the 100 × 100-m plot (slope) in each dune. These results suggest that the spatial distribution of soil organic C and total N in sand dunes is associated closely with geomorphic position related to the dune crest and slope, relative elevation of sampling site, and vegetation cover. Understanding the principles of this relationship between them may guide strategies for the conservation and management of semiarid dune ecosystems.

Influence of dune stabilization on relationship between plant diversity and productivity in Horqin Sand Land, Northern China

Plant diversity–productivity relationship is a debated issue in ecology. However, different relationships between plant diversity and productivity have been observed in response to changes of environmental factors. In this study, vegetation cover, species richness, above-ground net primary productivity (ANPP), soil organic carbon (C) and total nitrogen (N) were measured at three stages of dune stabilization (mobile dune, semi-fixed dune and fixed dune) in Horqin Sand Land, Northern China. Relationships between species richness and ANPP were also examined at different stages of dune stabilization. Results showed that vegetation cover, species richness, ANPP, soil organic C and total N increased with dune stabilization. Half U-shaped, positive and no relationship between species richness and ANPP were found in mobile dune, semi-fixed dune and fixed dune, respectively. Species richness was significantly positively correlated to soil organic C and total N in mobile dune and semi-fixed dune, and ANPP was positively correlated to soil organic C and total N in fixed dune. A positive relationship between species richness and ANPP was also found along the habitat gradient of dune stabilization. These results suggest that dune stabilization following restoration succession of degraded vegetation greatly influences relationships between species richness and ANPP in sand dune ecosystems. Soil organic C and total N determining changes of vegetation cover, species richness and ANPP drive the positive relationship between species richness and ANPP along the dune stabilization gradient. Thus, the degree of dune stabilization and vegetation composition, influenced by important soil properties, are important factors to determine relationships between plant diversity and productivity in sand dune ecosystems.

Allometric Effects of Agriophyllum squarrosum in Response to Soil Nutrients, Water, and Population Density in the Horqin Sandy Land of China

We monitored the allometric effects for greenhouse-grown Agriophyllum squarrosum plants in response to variations in population density and the availability of soil nutrients and water. Biomass allocations were size-dependent. The plasticity of roots, stems, leaves, and reproductive effort was “true” in response to changes in nutrient content. At a low level of soil minerals, plants allocated more resources to the development of roots and reproductive organs than to leaves, but data for stem allocations were consistent for tradeoffs between the effects of nutrients and plant size. The plasticities of leaf allocation and reproductive effort were “true” whereas those of root and stem allocations were “apparent” in response to fluctuations in soil water, being a function of plant size. Decreasing soil water content was associated with higher leaf allocation and lower reproductive effort. Except for this “apparent” plasticity of leaf allocation, none was detected with population density on biomass allocation. Architectural traits were determinants of the latter. For roots, the determining trait was the ratio of plant height to total biomass; for stems and reproduction, plant height; and for leaves, the ratio of branch numbers to plant height.

Effects of Shrub Litter Addition on Dune Soil Microbial Community in Horqin Sandy Land, Northern China

To evaluate the effects of shrub litter addition on microbial abundance in sandy dunes of Horqin Sandy Land, shrub litters of Artemisia halodendron, Caragana microphylla, and Salix gordejevii were collected and added to mobile dune, semi-mobile dune, semi-fixed dune and fixed dune at 0 cm (on soil surface) and 10 cm deep in the sand. After one year, the soil bacterial, actinomycetic, and fungal abundance were measured at the litter added sites. Results showed that the abundance of bacteria, actinomycete, and fungi in soils with litter addition was significantly higher than that in soils without additional litter. A. halodendron litter stimulated an increase in soil bacteria and fungi higher than those produced by C. microphylla and S. gordejevii litters; C. microphylla litter stimulated fungi less than the other two litters, but stimulated actinomycetes more than the other two litters. Therefore, A. halodendron litter was considered the best to enrich soil microbial abundance. The overall effect of litter addition on microbial abundance was higher at 10 cm deep layer than that observed at the surface layer. The results suggest that shrub litters can be used as straw checkerboards or nutrient sources to accelerate dune stabilization in sandy land and to promote soil restoration in sandy dunes. Additionally, the results indicate increased soil microbial abundance with shrub litter distributed not only on the soil surface but also underground.

Scale dependence of plant species richness and vegetation-environment relationship along a gradient of dune stabilization in Horqin Sandy Land, Northern China

Ecological patterns and processes in dune ecosystems have been a research focus in recent years, however the information on how dune stabilization influences the spatial scale dependence of plant diversity is still lacking. In this study, we measured the plant species richness, soil properties and altitude across four spatial scales (1, 10, 100 and 1,000 m2) at three different dune stabilization stages (mobile dune, semi-fixed dune and fixed dune) in Horqin Sandy Land, Northern China. We also examined the relationships between plant species richness, community composition and environmental factors along the gradient of dune stabilization. Our results showed that plant species richness increased with the increase of spatial scales in each dune stabilization stage, as well as with the increase of dune stabilization degrees. Canonical correspondence analysis (CCA) showed that plant distributions in the processes of dune stabilization were determined by the combined environmental gradient in relation to soil organic carbon (SOC), total nitrogen (TN), carbon/nitrogen (C/N), pH, electrical conductivity (EC), soil water content (SWC), fine sand (FS), very fine sand (VFS), silt and clay (SC), and altitude. Plant species richness was significantly and positively correlated to SOC and TN in mobile dune, and significantly and positively correlated to SOC, TN, C/N, VFS and SC in semi-fixed dune. However, no significant correlation between plant species richness and environmental factors was observed in fixed dune. In addition, plant species richness in different dune stabilization stages was also determined by the combined gradient of soil properties and altitude. These results suggest that plant species richness has obvious scale dependence along the gradient of dune stabilization. Soil resources depending on dune habitats and environmental gradients caused by dune stabilization are important factors to determine the scale dependence of species diversity in sand dune ecosystems.

Land cover changes and the effects of cultivation on soil properties in Shelihu wetland, Horqin Sandy Land, Northern China

Land cover change plays an essential role in the alternation of soils properties. By field investigation and applying satellite images, land cover information in the Shelihu wetland was carried out in an area of 2,819 hm2 in 1985, 1995, 2000, 2005, 2010 and 2011, respectively, in Horqin Sandy Land. A total of 57 soil sampling sites across Shelihu were chosen in wet meadow (CL0), cropland (CL) and sandy land (SL) according to the spatial characteristics of water body change. Soil texture, organic carbon (SOC), total nitrogen (TN) and total phosphorus (TP) contents, electrical conductivity (EC) and pH were measured at the soil depths of 0–10, 10–20 and 20–40 cm to examine the influence of agricultural conversion and continuous cultivation on soil properties. The results showed that the study area was covered by water body in 1985, which gradually declined afterwards and then reclaimed rapidly at a mean annual rate of 132.1 hm2/a from wet meadow to cropland since 1995. In 2011, water body was drained and the area was occupied by 10.8% of CL0, 76.9% of CL and 12.3% of SL. Large amounts of SOC, TN and TP were accumulated in the above depths in CL0. Soil in CL0 also had higher EC and silt and clay fractions, lower pH than in SL and CL. Soil in SL was seriously degraded with lower contents of SOC, TN and TP than in CL and CL0. SOC, TN content and EC in CL decreased with the increase of cultivation age, while pH showed a reverse trend with significance at plough horizon. The agricultural conversion in Shelihu was driven by the comprehensive factors of precipitation reduction, economic development and intense competitions for irrigation water. Continuous cultivation in this process is not sustainable because of SOC degradation and nutrient content reduction. The key point is that conventional tillage and removal of residuals induced further land degradation. Wetland reclamation for immediate economic interests led to greater costs in the long-term environmental restoration in Horqin Sandy Land.

Effect of spatial scale and topography on spatial heterogeneity of soil seed banks under grazing dis- turbance in a sandy grassland of Horqin Sand Land, Northern China

Soil seed banks play an important role in the distribution and composition of plant communities in semiarid grassland ecosystems. However, information on how spatial scale influences the spatial heterogeneity of soil seed banks in a grassland under grazing disturbance is still lacking. Based on field sampling and greenhouse germination, we measured the species composition and seed density of soil seed banks at different spatial scales (30 m×30 m, 30 m×60 m and 30 m×90 m) along a topographical gradient in a sandy grassland in Horqin Sand Land, Northern China. By applying geostatistical methods, we examined how spatial scale and topography affected the spatial distribution of soil seed banks in the study area. Our results showed that the total number of species in soil seed banks, as well as the number of dominant annuals, increased with the increase of spatial scales. Seed density in soil seed banks decreased with the increase of spatial scales due to an increase in the slopes and relative heights of the sampling points. Geostatistical analysis showed that the relative structural variance (C/(C0+C)) of seed density and species richness were over 65% for all spatial scales, indicating that these variables had an ob- vious spatial autocorrelation and the spatial structured variance accounted for the largest proportion of the total sample variance. Spatial autocorrelation of seed density in soil seed banks increased with the increase of measured scales, while that of species richness showed a reverse trend. These results suggest that the total number of spe- cies in soil seed banks is spatial scale dependent and lower topography may accommodate more seeds. Spatial distribution of seed density in soil seed banks is also scale dependent due to topographic variation. Grassland management, therefore, needs to consider local grazing disturbance regime, spatial scale and topography.

Long-term grazing effects on vegetation characteristics and soil properties in a semiarid grassland, northern China

Understanding the responses of vegetation characteristics and soil properties to grazing disturbance is useful for grassland ecosystem restoration and management in semiarid areas. Here, we examined the effects of long-term grazing on vegetation characteristics, soil properties, and their relationships across four grassland types (meadow, Stipa steppe, scattered tree grassland, and sandy grassland) in the Horqin grassland, northern China. Our results showed that grazing greatly decreased vegetation cover, aboveground plant biomass, and root biomass in all four grassland types. Plant cover and aboveground biomass of perennials were decreased by grazing in all four grasslands, whereas grazing increased the cover and biomass of shrubs in Stipa steppe and of annuals in scattered tree grassland. Grazing decreased soil carbon and nitrogen content in Stipa steppe and scattered tree grassland, whereas soil bulk density showed the opposite trend. Long-term grazing significantly decreased soil pH and electrical conductivity (EC) in annual-dominated sandy grassland. Soil moisture in fenced and grazed grasslands decreased in the following order of meadow, Stipa steppe, scattered tree grassland, and sandy grassland. Correlation analyses showed that aboveground plant biomass was significantly positively associated with the soil carbon and nitrogen content in grazed and fenced grasslands. Species richness was significantly positively correlated with soil bulk density, moisture, EC, and pH in fenced grasslands, but no relationship was detected in grazed grasslands. These results suggest that the soil carbon and nitrogen content significantly maintains ecosystem function in both fenced and grazed grasslands. However, grazing may eliminate the association of species richness with soil properties in semiarid grasslands.

Effects of habitat disturbance on the pollination system of Ammopiptanthus mongolicus (Maxim) Cheng f. at the landscape-level in an arid region of Northwest China

Ammopiptanthus mongolicus is an ecologically important species in the arid region of Northwest China. Habitat disturbance can significantly affect plant mating success and ultimately species viability. Pollen limitation of plant reproduction occurs in many plant species, particularly those under habitat disturbance. However, previous investigations have demonstrated differences in pollen limitation between conserved and disturbed sites. We compared the phenology, pollen limitation, pollinators and breeding system of both sites to determine whether habitat disturbance has generated changes in these plant components. We found that the species differed in four aspects. First, blooming duration and flowering peak were longer in the disturbed site than in the conserved site. Second, A. mongolicus can be pollen-limited and pollen limitation was more intense in the conserved site than in the disturbed site. Third, Anthophora uljanini was found to be a frequent pollinator in the conserved site, while Apis mellifera was the most effective and frequent flower visitor. More pollinator visits were recorded in the disturbed site, which could explain the differences in reproductive success. Finally, seed set was higher in the disturbed site than in the conserved site. We found that outcrossing was dominant in both sites and that agamospermy and self-pollination played complementary roles to ensure reproduction. Differences in flower production influenced by artificial selection and pollinator type explain the different seed set in both sites, whereas habitat disturbance cause changes differences in the pollination process and limits pollen flow. The balance between artificial management and mating success is crucial to analysis of the pollination process and manipulation of A. mongolicus population size.

Comparative reproductive biology of Apocynum venetum L. in wild and managed populations in the arid region of NW China

Apocynum venetum L. (dogbane) is one of the ecologically important species in the arid region of Northwest China. To select plants with higher flowering rate and fruit production, we investigated the following characteristics of A. venetum in wild and managed populations: flowering dynamics, pollen viability, pollen limitation, floral visitors and breeding system. We found that the species showed four reproductive characteristics. First, the flower production period and flowering peak were different between the wild and managed populations, longer in the managed. Second, A. venetum was pollen-limited, and pollen limitation was more intense in the wild population than in the managed. Third, in the wild, Apis mellifera L. was found to be frequent pollinator, Ophion luteus L. being the most frequent and effective visitor in the managed. Finally, the pollen ovule rate was 36.2. Self-pollination was dominant and played an important role to assure production in the breeding system. Differences in flower production influenced by artificial selection and pollinator type explain different fruit production in managed and wild populations, further, proper management could promote re-vegetation or restoration of degraded A. venetum in this region.