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The abstract is available here: https://uscholar.univie.ac.at/o:1639163

The impact of the long-term uneven precipitation distribution model on the diversity and community composition of soil C-fixing microorganisms in arid and semiarid grasslands remains unclear. In 2015, we randomly set up five experimental plots with precipitation gradients on the natural restoration grassland of the Loess Plateau (natural precipitation, NP; ± 40% natural precipitation: decreased precipitation (DP), DP40; increased precipitation (IP), IP40; ± 80% natural precipitation: DP80; IP80). In the third and fifth years after the experimental layout (spanned two years), we explored the cbbL-genes, which are functional genes in the Calvin cycle, harboring microbial diversity and community composition under different precipitation treatments. The results showed that the increase in mean annual precipitation significantly changed the cbbL-harboring microbial alpha diversity, especially when controlling for 40% natural precipitation. The response of the dominant microbial communities to interannual increased precipitation variation shifted from Gammaproteobacteria (Bradyrhizobium) to Betaproteobacteria (Variovorax). The structural equation model showed that precipitation directly affected the cbbL-harboring microbial diversity and community composition and indirectly by affecting soil NO

Microbial necromass plays an essential role in soil organic carbon (SOC) accumulation. Nevertheless, how microbial necromass carbon (C) concentrations and their contributions to SOC sequestration change from litter to mineral soil and what factors influence its accumulation remain poorly understood. To address this knowledge gap, we performed a field experiment to investigate the compositional distribution characteristics of microbial necromass C and its contributions to SOC sequestration in an oak forest (Quercus wutaishanica) litter-mineral soil profile of the Chinese Loess Plateau. The present study estimated the microbial necromass C concentrations based on the microbial cell wall's biomarker amino sugars. Our results demonstrated that microbial necromass C increased from the Oi1 to Oa layers but decreased from the Ah1 to AB horizons. The highest accumulation of microbial necromass C was found at the litter-mineral soil interface (i.e., total microbial necromass in the Oa layer was 39.5 Mg ha−1, and Ah1 was 22.8 Mg ha−1). The contribution of total microbial necromass C to SOC increased from Oi1 to Ah2. Specifically, the total microbial necromass C accounted for 40.7%, 47.7%, and 37.0% of the average bulk SOC in the Ah1, Ah2, and AB horizons of the oak forest mineral soil, respectively. The ratio of fungal to bacterial necromass C decreased from litter to mineral soil, indicating that the relatively higher bacterial necromass C increasingly accumulated in the deeper litter layers and upper mineral soil horizons. Fungal and bacterial necromass C increased with increasing labile organic C, nitrogen (N), and labile inorganic phosphorus (P), suggesting that higher easily accessible soluble nutrients lead to higher microbial biomass levels, which in turn lead to higher microbial necromass accumulation. Overall, our findings suggested that microbial demand for C or N influences the quantity of soluble nutrients and further lead to changes in microbial necromass C decomposition/accumulation.

Although forests account for only 27% of the total land area, they store approximately 80% of the aboveground carbon (C) and 40% of soil C globally. However, there is currently little information regarding the input and distribution of photoassimilates of trees in plant–soil systems. To quantify the belowground C input and allocation to plant–soil pools, we pulse labeled 5-year-old pioneer (Populus davidiana) and climax (Quercus wutaishanica) species with 13CO2 under field conditions. The 13C allocation dynamics were traced in the leaves, branches, roots, and soil microorganisms, rhizosphere and bulk soil under Populus davidiana and Quercus wutaishanica over 21 days. 13C recovery (% of assimilated 13C) in the leaves of Populus davidiana and Quercus wutaishanica decreased from nearly 90% at 6 h after labeling to 40% and 45% at 21 days, respectively. Continuous assimilate allocation from above- to belowground increased 13C recovery in roots from 0.4% at 6 h after labeling to 9.5% in Populus davidiana and from 1.5% to 15% in Quercus wutaishanica at 21 days after labeling. The recently assimilated C was detected in the soil immediately after labeling. The 13C amounts in the bulk and rhizosphere soil of the climax species Quercus wutaishanica was two-fold greater than that under the pioneer species Populus davidiana. The total belowground net C input (including that in roots) by Populus davidiana and Quercus wutaishanica was 109 and 283 g C m−2 yr−1 (top 20 cm of soil), respectively, including rhizodeposition of 4.2 and 28 g C m−2 yr−1. Consequently, the belowground C allocation and soil C sequestration increase from pioneer to climax tree species.

Despite the recognized importance of the contribution of microbial necromass to soil organic carbon (SOC) sequestration, at a global scale, there has been no quantification for cropland, grassland, and forest ecosystems. To address this knowledge gap, the contents of fungal and bacterial necromass were estimated based on glucosamine and muramic acid contents in cropland (986 samples), grassland (278 samples), and forest (452 samples) soils. On an average, microbial necromass C contributed 51%, 47%, and 35% to the SOC in cropland, grassland, and forest soils, respectively, in the first 20 cm of topsoil. The contribution of microbial necromass to SOC increased with soil depth in grasslands (from 47% to 54%) and forests (from 34% to 44%), while it decreased in croplands (from 51% to 24%). The microbial necromass accumulation coefficient (the ratio between necromass and living microbial biomass C) was higher in soil from croplands (41) and grasslands (33) than in forest (20) soils. These results suggest that the turnover of living microbial biomass is faster in grassland and cropland soils than in forest soils, where the latter contains more partially decomposed plant residues. Fungal necromass C (>65% of total necromass) had consistently higher contributions to SOC than bacterial necromass C (32–36%) in all soils due to i) a larger living fungal biomass than bacterial biomass, and ii) fungal cell compounds being decomposed slowly and, thus able to persist longer in soil. The ratio of fungal:bacterial necromass C increased from 2.4 to 2.9 in the order of croplands

Vegetation type and topographic characteristics are important factors that affect the amount and spatial distribution of the forest floor and the soil carbon cycling. However, the relationships among vegetation type, topographic characteristics, forest floor properties, soil properties and soil labile organic carbon (LOC) fractions remain to be elucidated. Here, we conducted a field observational study to explore the relationships between the soil LOC fractions (microbial biomass carbon (MBC), extractable organic carbon (EOC) and permanganate oxidizable carbon (Pox-C)) and forest floor characteristics under various vegetation types (forest, forest-steppe and steppe) and topographic features (slope position and aspect). Our results showed that the vegetation type and topography are both important factors that significantly influence the forest floor properties (i.e., stock, thickness and coverage). The soil total nitrogen and phosphorus were significantly increased, and the soil pH and bulk density were decreased, with an increase in litter stock, coverage and thickness. Simultaneously, the SOC and soil LOC fractions were increased significantly with the increase in litter stock, coverage and thickness. Over 50% of the variation in the SOC and soil LOC fractions was jointly explained by environmental factors, litter properties and soil properties. Our results demonstrated that the SOC and soil LOC fractions are directly influenced by soil physical and chemical properties (e.g., pH, bulk density and soil total nitrogen) and indirectly affected by external environmental factors. Changes in vegetation and topography lead to increases in the litter stock, coverage, thickness and water content in the backslope position and northern slope, which could create favorable soil nutrient and environmental conditions (e.g., higher soil moisture and lower bulk density) to further enhance SOC and soil LOC fraction sequestration. The forest floor potentially links environmental factors and soil properties and serves as an important factor for soil C cycling.

As the “Grain for Green” project goes on in the Loess Plateau, some drawbacks, such as the low survival rate, the appearances of dry soil layers and little old trees are appearing, especially for introduced tree species. Ecological stoichiometry, which indicates the balance of elements and the flow of energy, plays a vital role in exploring biogeochemical cycling in ecosystems. To investigate the carbon (C), nitrogen (N) and phosphorus (P) balance and flow of energy, the stoichiometry characteristics and stoichiometry homeostasis for introduced and natural forests on the Loess Plateau, four typical forests were selected, including three introduced forests composed of Robinia pseudoacacia (R. pseudoacacia), Pinus tabuliformis (P. tabuliformis) and Platycladus orientalis (P. orientalis) and a natural forest composed of Quercus wutaishansea (Q. wutaishansea) and the C, N and P in the plant-litter-soil system were measured. The results showed that the C, N and P contents and C:N:P in leaves, branches, fruits, litters and soils varied widely and were influenced strongly by forest species. The N content in R. pseudoacacia leaves, branches, fruits and litters and the soil C, N and P contents in Quercus wutaishansea were higher than those in other forest species. The soil C:N:P stoichiometry in Quercus wutaishansea was higher than that in introduced forest species, while the reverse was found for plant tissues and litter C:N:P. According to the leaf N:P, with the exception of R. pseudoacacia, growth of the other three forest species was limited by N. The N, P and N:P homeostasis exist in trees, especially in Quercus wutaishansea, forest species and plant tissues had significant effects on the strength of homeostasis. Overall, the results of C, N and P content and stoichiometric homeostasis revealed that the natural forest was more adaptable to the arid environment in the Loess Plateau compared to the introduced forests.

Revegetation in fragile ecosystems is an efficient means to increase aggregate stability and thus reduce soil erosion. However, the influence of large-scale afforestation on soil aggregate stability and erodibility in the Loess Plateau is not well understood. To assess the sustainability and suitability of widespread, long-term planting of plantations in terms of soil aggregate stability (mean weighted diameter (MWD) and geometric mean diameter (GMD)) and erodibility (K), we performed a large-scale investigation of soil aggregate stability and soil erodibility of Robinia pseudoacacia (RP) and Caragana korshinskii (CK) plantations on the Loess Plateau. The results showed that the soil macroaggregate fraction (>0.25 mm) content under RP and CK plantations had a decreasing trend with increasing latitude. Moreover, soil aggregate stability and soil organic carbon (SOC) and total nitrogen (TN) contents in RP and CK plantations decreased with increasing latitude. RP and CK plantations did not always result in improvement of soil aggregate stability and the accumulation of SOC and TN, which depended on the latitude and precipitation conditions. Specifically, RP planting in the south warm temperate forest subzone (STFZ) and north warm temperate forest subzone (NTFZ) could enhance the macroaggregate content (>5 mm), soil aggregate stability and soil nutrients, while CK planting for the improvement of the soil macroaggregate content, soil aggregate stability and soil nutrients in the temperate forest steppe subzone (TFSZ) was more effective than was RP planting. Correlation analysis showed that the latitude, longitude and mean annual precipitation (MAP) were significantly correlated with soil MWD, GMD, K values, SOC and TN across the RP and CK plantations, and soil MWD, GMD, and K values were significantly correlated with SOC and TN in RP and CK plantations. The changing trends of SOC and TN with latitude and longitude were consistent with that of soil aggregate stability with latitude and longitude under RP and CK plantations, indicating that changes in the SOC and TN concentrations will cause changes in soil aggregate stability and erodibility. Additionally, the effects of RP and CK plantations on the soil aggregate stability, erodibility, SOC and TN content were different and distinguished by latitude and MAP. RP plantation resulted in more SOC and TN accumulation, enhanced the soil aggregate stability and decreased erodibility compared with CK plantation in areas with MAP >500 mm or latitude 36° N, CK plantation had slight advantages over RP plantation. This study indicated that the changes in soil aggregate stability, erodibility, SOC and TN accumulation following RP and CK plantations had different patterns along latitude and precipitation gradients. Therefore, the 36° N or 500 mm precipitation threshold may be the dividing line for planting RP and CK plantations. Overall, our study produces unique insights into the relative significance of environmental factors that influence aggregate stability at large scales and is useful for selecting suitable afforestation species and locations to optimize forest management with sustainable production.

This communication described the fabrication of a hierarchy carbon paper, and its application to the gas diffusion layer (GDL) of proton exchange membrane (PEM) fuel cells. The carbon paper was fabricated by growing carbon nanotubes (CNTs) on carbon fibers via covalently assembling metal nanocatalysts. Surface morphology observation revealed a highly uniform distribution of hydrophobic materials within the carbon paper. The contact angle to water of this carbon paper was not only very large but also particularly even. Polarization measurements verified that the hierarchy carbon paper facilitated the self-humidifying of PEM fuel cells, which could be mainly attributed to its higher hydrophobic property as diagnosed by electrochemical impedance spectroscopy (EIS).

The large particle cerium oxide was prepared using oxalic acid as precipitation agent. The effects of preparation conditions on the particle size of cerium oxide were discussed. The results showed that the particle size of cerium oxide could be controlled effectively by the temperature, acidity of the solution, aging time, etc. The optimized preparation process of large particle cerium oxide was obtained. The cerium oxide with size between 50 μm to 150 μm was prepared by the process. Moreover, the cerium oxide particles were dispersed uniformly.