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Natural vegetation succession is a process of interactions between plants and various environmental factors. The dynamics of soil microbes during successional stages have been intensively explored in the past two decades. The effect of rhizosphere interactions of interspecies on microbial properties, however, has received less attention. During the early stage of succession on a Loess Plateau soil from China, we investigated microbial properties in the rhizospheres of three dominant grass species: Artemisia capillaris , Artemisia sacrorum and Stipa bungeana . Experiments with monocultured and polycultured potted plants were designed to determine the effects of rhizosphere interactions on plant growth and soil microbial properties. The results showed that root biomass, shoot biomass, plant height, microbial biomass carbon (C), microbial biomass nitrogen (N), invertase and urease activities, Gram-negative and Gram-positive bacteria were significantly higher for A. capillaris than for A. sacrorum and S. bungeana in both monocultures and polycultures. Lower root biomass, shoot biomass, plant height, microbial biomass C, microbial biomass N, invertase and alkaline phosphatase activities, and levels of microbial phospholipid-derived fatty acids in polycultures relative to the monocultures indicated that rhizosphere interactions led to lower plant growth and soil microbial activities. Our study suggests that rhizosphere interactions had a significant effect on the microbial properties of plant rhizospheres. The advantage of A. capillaris over A. sacrorum and S. bungeana in growth characteristics and rhizosphere microbial conditions is likely responsible for the dominance of A. capillaris in the early stage of succession in abandoned cropland on the Loess Plateau. [ABSTRACT FROM AUTHOR]

The effects of reforestation on carbon (C) sequestration in China's Loess Plateau ecosystem have attracted much research attention in recent years. Black locust trees ( Robinia pseudoacacia L.) are valued for their important use in reforestation and water and soil conservation efforts. This forest type is widespread across the Loess Plateau, and must be an essential component of any planning for C sequestration efforts in this fragile ecological region. The long-term effects of stand age on C accumulation and allocation after reforestation remains uncertain. We examined an age-sequence of black locust forest (5, 9, 20, 30, 38, and 56 yr since planting) on the Loess Plateau to evaluate C accumulation and allocation in plants (trees, shrubs, herbages, and leaf litter) and soil (0-100 cm). Allometric equations were developed for estimating the biomass of tree components (leaf, branch, stem without bark, bark and root) with a destructive sampling method. Our results demonstrated that black locust forest ecosystem accumulated C constantly, from 31.42 Mg C/ ha (1 Mg = 10 g) at 5 yr to 79.44 Mg C/ha at 38 yr. At the 'old forest' stage (38 to 56 yr), the amount of C in plant biomass significantly decreased (from 45.32 to 34.52 Mg C/ha) due to the high mortality of trees. However, old forest was able to accumulate C continuously in soil (from 33.66 to 41.00 Mg C/ha). The C in shrub biomass increased with stand age, while the C stock in the herbage layer and leaf litter was age-independent. Reforestation resulted in C re-allocation in the forest soil. The topsoil (0-20 cm) C stock increased constantly with stand age. However, C storage in sub-top soil, in the 20-30, 30-50, 50-100, and 20-100 cm layers, was age-independent. These results suggest that succession, as a temporal factor, plays a key role in C accumulation and re-allocation in black locust forests and also in regional C dynamics in vegetation. [ABSTRACT FROM AUTHOR]

Abstract: Soil organic carbon (SOC) is an important component of agricultural soil. Understanding the storage of carbon (C) and nitrogen (N) helps us understand how ecosystems would respond to natural and anthropogenic disturbances under different management strategies. Although the quantity of SOC stocks in the Loess Plateau has been previously investigated, the distribution and storage of C in its deep soil layers has been poorly studied to date. The present study was conducted in the Zhifanggou Watershed on the Loess Plateau to evaluate the effect of the types of land use on the distribution and storage of SOC and total nitrogen (TN) at different soil depths. Ten types of land use were investigated: woodland (Robinia pseudoacacia), shrubland (Caragana korshinskii; Hippophae rhamnoides), natural grassland, artificial grassland (Medicago sativa), orchard, check-dam cropland, terraced cropland (Setaria italica; Zea mays), and sloped cropland. The results show that land use significantly affects the SOC and TN content. The reconversion of sloped croplands into forestlands and grasslands improve their SOC and TN content. Natural grassland, terraced cropland with Z. mays, and the artificial grassland had the highest SOC content within 0–60 cm, 60–100 cm, and 150–500 cm layers, respectively. The SOC and TN content in the top layer were higher than those in the deep layer. Natural grassland had the highest SOC and TN storage within 0–40 cm layer, followed by shrubland I (C. korshinskii), check-dam cropland, woodland, and shrubland II (H. rhamnoides), orchard and artificial grassland, whereas the sloped cropland, and the terraced cropland with S. italica had the lowest values. The SOC and TN storage between the top (0–20 cm) and deep layers (0–100 cm, 0–300 cm, and 0–500 cm) can be modeled using linear function (y = ax + b). The results suggest that natural grasslands may be the optimal choice for SOC and TN sequestration in the Zhifanggou Watershed. Shrublands, woodland, artificial grasslands, orchards, terrace cropland with Z. mays and check-dam croplands are also recommended for their great contribution to SOC and TN storage in the area. [Copyright &y& Elsevier]

Slope aspect, as an important topographic factor on the Loess Plateau in China, may influence the productivity and relative characteristics of Artemisia sacrorum in this area. Three slope aspects (sunny, half-sunny, and shady slopes) were chosen to study the effects of slope aspect on the biomass, carbon:nitrogen:phosphorus (C:N:P) stoichiometry, and non-structural carbohydrate (NSC) concentrations in A. sacrorum in 2014. The maximum A. sacrorum biomass was detected in the half-sunny slope, and the ratio of belowground to aboveground biomass significantly changed in the different slope aspects. Slope aspect significantly affected the N:P ratio in the aboveground and the C:P and N:P ratios in the belowground. The maximum C:P and N:P ratios in the rhizosphere and non-rhizosphere soils were reached in the sunny slope, whereas no significant change was observed in C:N ratio. The maximum NSC concentrations occurred in different A. sacrorum tissues, that is, aboveground in the sunny slope and belowground in the half-sunny slope. The ratio of soluble sugar to starch concentrations in the aboveground significantly differed among the three slope aspects, but that in the belowground showed no significant difference. Slope aspect was the main environmental factor affecting the plant nutrient and stoichiometry on the Loess Plateau in China. [ABSTRACT FROM AUTHOR]