Xueyong Zhao

EFFECTS OF SALINITY ON GROWTH AND COMPATIBLE SOLUTES OF CALLUS INDUCED FROM POPULUS EUPHRATICA

SummaryThe present study aimed to evaluate the response to salinity of Populus euphratica, which is more salt-resistant than other poplar cultivars, at the cellular level. To this purpose, callus was induced from shoot segments of P. euphratica on Murashige and Skoog (MS) medium supplemented with 0.5 mg l−1 (2.2 μM) 6-benzyladenine (BA) and 0.5 mg l−1 (2.7 μM 1-naphthaleneacetic acid (NAA). Callus was transferred to MS medium supplemented with 0.25 mg l−1 (1.1 μM) BA and 0.5 mg l−1 NAA. The relative growth rate of callus reached a maximum in the presence of 50 mmol l−1 NaCl and growth was inhibited with increasing NaCl concentrations. Examination of the changes of osmotic substances under salt stress showed that accumulation of proline, glycine betaine, and total soluble sugars increased with increasing salt concentrations. The results indicate that the response of the callus of P. euphratica to salt stress is similar to that of the whole plant.

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.

Floristic Composition of Vegetation and the Soil Seed Bank in Different Types of Dunes of Kerqin Steppe

The floristic composition of the vegetation and the soil seed bank in different types of dunes of Kerqin steppe were compared. The aim of this study was to examine the influence of desertification on the floristic composition of the vegetation and soil seed bank. An indirect germination method was used to study the seed bank. Thirty species of the vegetation and 25 of the seed bank were identified, 23 species being common to both. Differences between four types of dunes (mobile sand dune, MSD; semifixed sand dune, SSD; fixed sand dune, FSD; interdunal lowland, ILD) in floristic composition of the vegetation and the soil seed bank were examined using nonparametric methods. In the four types of dune, species composition varied significantly except between site MSD and site SSD as well as between site FSD and site ILD in aboveground vegetation. As for soil seed bank, the number of species and seeds differed significantly only between site FSD and site ILD. Species diversity revealed that species numbers increased in the sequence of MSD, SSD, FSD and ILD. The dominant species of aboveground vegetation in site MSD and site SSD were Agriophyllum squarrosum and Setaria viridis, respectively. While in both site FSD and site ILD, the dominant was Eragrostis pilosa. Of total germinated seed, the most abundant species in site MSD were Agriophyllum squarrosum, but the dominant species was Eragrostis pilosa in the other three sites. In each site, more than half of the species recorded in aboveground vegetation were found in the seed bank, even more in the seed bank of site FSD and site ILD. In site FSD and site ILD, high correspondence was observed between the species composition of the aboveground vegetation and of the associated soil seed bank. Spearmans rank correlation coefficients were 0.295, 0.046, 0.704, and 0.612 for site MSD, site SSD, site FSD and site ILD, respectively. These results indicated that desertification development decreased correlation between the seed bank and vegetation in Kerqin Steppe.

Biophysical properties as determinants for soil organic carbon and total nitrogen in grassland salinization.

Grassland salinization causes considerable changes to soil and vegetation, which can lead to changes in soil organic carbon (C) and total nitrogen (N). These changes have complex causal relationships. A significant correlation between soil organic C and total N and any soil or vegetation property does not necessarily imply a significant direct effect of the property on soil organic C and total N. In this study, a field survey was conducted to investigate the changes in soil organic C and total N in grassland along a salinity gradient in Hexi corridor, China, and the direct and indirect effects of soil and vegetation properties on both stocks were quantified using a path analysis approach. Significant decrease in soil organic C and total N contents were observed with increasing salinity. Both had significant positive correlations with the Normalized Difference Vegetation Index (NDVI), soil water, and fine particles (silt+clay) content (p<0.01) and significant negative correlations with soil EC, and sand content (p<0.01). NDVI, fine particles content and soil water content had positive direct effects on soil organic C and total N stocks. Soil EC affected soil organic C and total N stocks mainly through its indirect negative effect on NDVI, soil texture, and water content. NDVI, soil texture, and moisture also indirectly affected soil organic C and total N stocks via changes in each other. These indirect effects augmented each other, although in some cases indirect effects worked in opposing directions.

A comparison of phenotypic plasticity between two species occupying different positions in a successional sequence

We compared the phenotypic plasticity of two greenhouse-grown species (Corispermum macrocarpum and Salsola collina) occupying different positions in a successional sequence in Horqin Sandy Land, by treating with different population density and the availability of soil nutrients and water. The same species can exhibit different patterns of plasticity in response to different environmental factors. In the soil nutrient treatments, the plasticity pattern of S. collina could be described as “master-of-some”. However, in the soil-water and population-density treatments, it showed no significant difference from C. macrocarpum in the reaction norm for plasticity. It was similar to a “jack-of-all-trades” plasticity pattern. Contrary to the previous conclusion that late successional species had higher reproductive allocation than early successional species, in this successional sequence, the late species had lower reproductive allocation in all treatments. Reproductive allocation of both species increased with the increase in water availability and also increased with a decrease in nutrient levels. However, density had no effect on reproductive allocation. Although the root:shoot ratio increased with decreasing water availability, there were no differences in the plasticity pattern for this trait in both species. Root:shoot ratio was, however, not significantly affected by nutrient availability and density. In a word, the plasticity patterns of invaders are adapted to the analyses of succession.

Biomass Energy, Carbon and Nitrogen Stores in Different Habitats along a Desertification Gradient in the Semiarid Horqin Sandy Land

ABSTRACT The Horqin Sandy Land is one of the most seriously desertified regions of China. We sampled total aboveground and belowground biomass, biomass energy, and the C and N stores in five different habitats along a desertification gradient in the Horqin Sandy Land: interdunal lowland, fixed sand dune, semifixed sand dune, semimobile sand dune, and mobile sand dune habitats. These habitats correspond to the potential, light, moderate, severe, and most-severe levels of desertification. We also sampled soil organic C and N stores at all sites. Along the spectrum from potential desertification to most-severe desertification, total aboveground and belowground biomass decreased by 89 and 91%, respectively, versus decreases of 90 and 91% for the aboveground and belowground biomass energy stores, 91 and 87% for the total aboveground biomass C and N stores, 91 and 96% for the belowground biomass C and N stores, and 90 and 83% for the soil organic C and N stores.

Artificial root exudates and soil organic carbon mineralization in a degraded sandy grassland in northern China

Plant root exudates contain various organic and inorganic components that include glucose, citric and oxalic acid. These components affect rhizosphere microbial and microfaunal activities, but the mechanisms are not fully known. Studies concerned from degraded grassland ecosystems with low soil carbon (C) contents are rare, in spite of the global distribution of grasslands in need of restoration. All these have a high potential for carbon sequestration, with a reduced carbon content due to overutilization. An exudate component that rapidly decomposes will increase soil respiration and CO2 emission, while a component that reduces decomposition of native soil carbon can reduce CO2 emission and actually help sequestering carbon in soil. Therefore, to investigate root exudate effects on rhizosphere activity, citric acid, glucose and oxalic acid (0.6 g C/kg dry soil) were added to soils from three biotopes (grassland, fixed dune and mobile dune) located in Naiman, Horqin Sandy Land, Inner Mongolia, China) and subjected to a 24-day incubation experiment together with a control. The soils were also analyzed for general soil properties. The results show that total respiration without exudate addition was highest in grassland soil, intermediate in fixed dune and lowest in mobile dune soil. However, the proportion of native soil carbon mineralized was highest in mobile dune soil, reflecting the low C/N ratio found there. The exudate effects on CO2-C emissions and other variables differed somewhat between biotopes, but total respiration (including that from the added substrates) was significantly increased in all combinations compared with the control, except for oxalic acid addition to mobile dune soil, which reduced CO2-C emissions from native soil carbon. A small but statistically significant increase in pH by the exudate additions in grassland and fixed dune soil was observed, but there was a major decrease from acid additions to mobile dune soil. In contrast, electrical conductivity decreased in grassland and fixed dune soil and increased in mobile dune. Thus, discrete components of root exudates affected soil environmental conditions differently, and responses to root exudates in soils with low carbon contents can differ from those in normal soils. The results indicate a potential for, e.g., acid root exudates to decrease decomposition rate of soil organic matter in low carbon soils, which is of interest for both soil restoration and carbon sequestration.

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 grazing exclusion on carbon sequestration and the associated vegetation and soil characteristics at a semi-arid desertified sandy site in Inner Mongolia, northern China

Li, Y., Zhao, X., Chen, Y., Luo, Y. and Wang, S. 2012. Effects of grazing exclusion on carbon sequestration and the associated vegetation and soil characteristics at a semi-arid desertified sandy site in Inner Mongolia, northern China. Can. J. Soil Sci. 92: 807-819. Chinas Horqin Sandy Land is a region vulnerable to disturbance that has been subjected to serious desertification, mainly because of overgrazing. We investigated whether the establishment of grazing exclosures in areas with active sand dunes would benefit vegetation recovery and improve soil quality. The results showed that along the age sequence of grazing exclosure for 8, 13, and 26 yr, plant cover, species number, and above-ground biomass increased, the soils water-holding capacity, fine particle content, organic C, total N, total P, available N and K, and electrical conductivity also increased, and the soil coarse sand content, pH, and bulk density in the top 20 cm of the soil decreased. However, the greatest improvements compared with the continuous grazing sites occurred in the 13- and 26-yr exclosures. Based on the area of heavily and severely desertified land in the study region and the results of the present study, the amount of C sequestered in the top 20 cm of the soil could reach 7.8 Mt after 26 yr of grazing exclusion. Our results confirm that grazing exclusion is a positive way to restore desertified ecosystems and has a high potential for sequestering soil C and improving soil quality in the semi-arid Horqin Sandy Land.