Precipitation Variability Affects Aboveground Biomass Directly and Indirectly via Plant Functional Traits in the Desert Steppe of Inner Mongolia, Northern China.

Clarifying the response of community and dominance species to climate change is crucial for disentangling the mechanism of the ecosystem evolution and predicting the prospective dynamics of communities under the global climate scenario. We examined how precipitation changes affect community structure and aboveground biomass (AGB) according to manipulated precipitation experiments in the desert steppe of Inner Mongolia, China. Bayesian model and structural equation models (SEM) were used to test variation and causal relationship among precipitation, plant diversity, functional attributes, and AGB. The results showed that the responses of species richness, evenness, and plant community weighted means traits to precipitation changes in amount and year were significant. The SEM demonstrated that precipitation change in amount and year has a direct effect on richness, evenness, and community-weighted mean (CWM) for height, leaf area (LA), specific leaf area (SLA), leaf dry matter content (LDMC), leaf nitrogen content (LNC), and leaf carbon content (LCC) and AGB; there into CWM for height and LDMC had a direct positive effect on AGB; LA had a direct negative effect on AGB. Three dominant species showed diverse adaptation and resource utilization strategies in response to precipitation changes. A. polyrhizum showed an increase in height under the precipitation treatments that promoted AGB, whereas the AGB of P. harmala and S. glareosa was boosted through alterations in height and LA. Our results highlight the asynchronism of variation in community composition and structure, leaf functional traits in precipitation-AGB relationship. We proposed that altered AGB resulted from the direct and indirect effects of plant functional traits (plant height, LA, LDMC) rather than species diversity, plant functional traits are likely candidate traits, given that they are mechanistically linked to precipitation changes and affected aboveground biomass in a desert steppe.

Effects of elevation and slope aspect on the distribution of the soil organic carbon associated with Al and Fe mineral phases in alpine shrub-meadow soil.

Mountain ecosystems store a large amount of soil organic carbon (SOC) sensitive to global climate change. The SOC associated with Al and Fe minerals is important for SOC retention because of the ubiquitous nature and highly reactive surface properties of these minerals. Topography is also known to impact the distribution and transformation of SOC by creating different microclimates. However, the effect of topography on the distribution of organo-mineral associations has seldom been reported. This study uses a selective dissolution method to quantify the soil carbon (C) fractions associated with Al and Fe minerals in alpine shrub-meadow soil. Na-pyrophosphate (PP), HCl-hydroxylamine (HH) and dithionite-HCl (DH) were used to quantify organo-metal complexes, SOC associated with short-range order (SRO) phases and crystalline phases, respectively. Results suggest that the Al and Fe mineral-associated C accounted for a small proportion of SOC (less than 30%) in each extraction. A higher concentration coupled with a lower percentage of SOC was found in the A horizon compared to the B horizon. A significant correlation was observed between Fe and C in PP and HH extractions, whereas Al was significantly correlated with C in DH extractions. Elevation and slope aspect strongly influenced soil biotic and abiotic parameters, as well as organo-mineral associations. The C fractions extracted by PP and HH were significantly higher in the NE slope aspect than the SW slope aspect. These fractions were positively correlated with soil water content and negatively correlated with soil pH. The C fractions extracted using DH decreased with increasing elevation and were positively correlated with DH extractable Al. Our results highlight the role of topography on the distribution of organo-mineral associations, which should be considered during the assessment of SOC stability in alpine soils.

Pressure-Dependent Light Emission of Charged and Neutral Excitons in Monolayer MoSe2.

Tailoring the excitonic properties in two-dimensional monolayer transition metal dichalcogenides (TMDs) through strain engineering is an effective means to explore their potential applications in optoelectronics and nanoelectronics. Here we report pressure-tuned photon emission of trions and excitons in monolayer MoSe2 via a diamond anvil cell (DAC) through photoluminescence measurements and theoretical calculations. Under quasi-hydrostatic compressive strain, our results show neutral (X0) and charged (X-) exciton emission of monolayer MoSe2 can be effectively tuned by alcohol mixture vs inert argon pressure transmitting media (PTM). During this process, X- emission undergoes a continuous blue shift until reaching saturation, while X0 emission turns up splitting. The pressure-dependent charging effect observed in alcohol mixture PTM results in the increase of the X- exciton component and facilitates the pressure-tuned emission of X- excitons. This substantial tunability of X- and X0 excitons in MoSe2 can be extended to other 2D TMDs, which holds potential for developing strained and optical sensing devices.

Correlatively Dependent Lattice and Electronic Structural Evolutions in Compressed Monolayer Tungsten Disulfide.

Transition-metal dichalcogenides (TMDs) are promising materials for optoelectronic devices. Their lattice and electronic structural evolutions under high strain conditions and their relations remain open questions. We exert pressure on WS2 monolayers on different substrates, namely, Si/SiO2 substrate and diamond anvil surface up to ∼25 GPa. Structural distortions in various degree are disclosed based on the emergence of Raman-inactive B mode. Splits of out-of-plane B and A1' modes are only observed on Si/SiO2 substrate due to extra strain imported from volume decrease in Si and corrugation of SiO2 surface, and its photoluminescence (PL) quenches quickly because of decreased K-K transition by conspicuous distortion of Brillouin zone. While diamond anvil surface provides better hydrostatic environment, combined analysis of PL and absorption proves that pressure effectively tunes PL emission energy and enhances Coulomb interactions. Knowledge of these distinct pressure tunable characteristics of monolayer WS2 improves further understanding of structural and optical properties of TMDs.

Pressure confinement effect in MoS2 monolayers.

With ever increasing interest in layered materials, molybdenum disulfide has been widely investigated due to its unique optoelectronic properties. Pressure is an effective technique to tune the lattice and electronic structure of materials such that high pressure studies can disclose new structural and optical phenomena. In this study, taking MoS2 as an example, we investigate the pressure confinement effect on monolayer MoS2 by in situ high pressure Raman and photoluminescence (PL) measurements. Our results reveal a structural deformation of monolayer MoS2 starting from 0.84 GPa, which is evidenced by the splitting of E(1)2g and A1g modes. A further compression leads to a transition from the 1H-MoS2 phase to a novel structure evidenced by the appearance of two new peaks located at 200 and 240 cm(-1). This is a distinct feature of monolayer MoS2 compared with bulk MoS2. The new structure is supposed to have a distorted unit with the S atoms slided within a single layer like that of metastable 1T'-MoS2. However, unlike the non-photoluminescent 1T'-MoS2 structure, our monolayer shows a remarkable PL peak and a pressure-induced blue shift up to 13.1 GPa. This pressure-dependent behavior might enable the development of novel devices with multiple phenomena involving the strong coupling of the mechanical, electrical and optical properties of layered nanomaterials.

[Effects of grazing disturbance on soil active organic carbon in mountain forest-arid valley ecotone in the upper reaches of Minjiang River].

Effects of grazing disturbance on the soil carbon contents and active components in the four vegetations, i.e., artificial Robinia pseudoacacia plantation, artificial poplar plantation, Berberis aggregate shrubland and grassland, were studied in the mountain forest-arid valley ecotone in the upper Minjiang River. Soil organic carbon and active component contents in 0-10 cm soil layer were greater than in 10-20 cm soil layer at each level of grazing disturbance. With increasing the grazing intensity, the total organic carbon (TOC), light fraction organic carbon (LFOC), particulate organic carbon (POC) and easily oxidized carbon (LOC) contents in 0-10 cm soil layer decreased gradually in the artificial R. pseudoacacia plantation. The LFOC content decreased, the POC content increased, and the TOC and LOC contents decreased initially and then increased with increasing the grazing intensity in the artificial poplar plantation. The POC content decreased, and the TOC, LFOC and LOC contents decreased initially and then increased with increasing the grazing intensity in the B. aggregate shrubland. The POC and TOC contents decreased, and the LFOC and LOC contents decreased initially and then increased with increasing the grazing intensity in the grassland. The decreasing ranges of LOC, LFOC and POC contents were 0.1-7.9 times more than that of TOC content. There were significant positive relationships between TOC and LOC, LFOC and POC, suggesting that the active organic carbon components could reflect the change of soil total carbon content.

[Relationships between grazing-path and Berberis aggregate population characteristics in upper reaches of Minjiang River, Southwest China].

Taking the Berberis aggregate shrubs in the ecotone of dry valley and montane forests in upper reaches of Minjiang River as study objects, and by the methods of tracking grazing and field survey, this paper studied the characteristics of habitat utilization by livestock and the features of grazing-path. The major factors affecting the features of grazing-path were screened by redundancy analysis (RDA), and the relationships of the grazing-path features with the coverage, size class, and distribution pattern of the shrubs were analyzed. It was shown that the distribution pattern of the grazing-path could intuitively reflect the characteristics of the habitat utilization by livestock, being in accordance with the results of tracking grazing. The Morisita index at 5 m scale could objectively reflect the distribution type of the grazing-path. Sample plots 1, 2 and 6 presented a contagious distribution of grazing-path, while the other plots showed regular distribution. In slope scale, the coverage and height of the shrubs were the notable affecting factors, which had negative correlations with the grazing-path features. There was a significant negative correlation between the coverage of B. aggregate population and the area of grazing-path. The population structure of B. aggregate had a close correlation with the distribution of grazing-path. The ratio of the long axis to short axis of the shrubs was averagely 1.29, and the shape of the shrubs approached to round. It was considered that the grazing-path landscape and the livestock on the grazing-paths had the function of reconstructing the shape of the shrubs. The directionality of the population pattern of B. aggregate was generally in line with the distribution type of grazing-path, but actually, they were opposite in distribution. The patches of the shrubs were in aggregated or uniform distribution in the areas deviated from the grazing-path.

[Fractal features of soil aggregate structure in slope farmland with different de-farming patterns in South Sichuan Province of China].

By using fractal model, this paper studied the fractal dimension of soil aggregate structure (D) in the slope farmland (CK), its 5-year de-farmed Neosinocalamus affinis plantation (NAP), Bambusa pervariabilis x Dendrocalamopsis oldhami plantation (BDP), Alnus crenastogyne + Neosinocalamus affinis plantation (ANP), and abandoned farmland (AFL) in south Sichuan Province of China, and analyzed the relationships between the D and soil physical and chemical properties. In the de-farmed plantations and abandoned farmland, the contents of > 0.25 mm soil aggregates and water-stable aggregates were increased significantly, compared with those in the slope farmland. The D was 1.377-2.826, being in the order of NAP < BDP < ANP < AFL < CK, and decreased with the increasing contents of > 0.25 mm soil aggregates and water-stable aggregates. Comparing with CK, de-farming increased the soil natural water content, capillary porosity, and contents of soil organic matter, total N, alkali-hydrolysable N, total P, and total K, and decreased soil bulk density, non-capillary porosity, and aeration porosity. There were close relationships between the fractal dimension of soil aggregate structure and the soil physical and chemical properties. All the results suggested that the de-farming of slope farmland was beneficial to the increase of the contents of > 0.25 mm soil aggregates and water-stable aggregates, and the enhancement of soil structure stability. The D could be used as an ideal index to evaluate soil fertility, and planting Neosinocalamus affinis on the de-farming slope farmland was a good measure for the improvement of soil fertility in the research area.

[Effects of long-term fertilization on soil particulate organic carbon and nitrogen in a wheat-maize cropping system].

0-20 cm soil samples were collected from an 18-year wheat-maize rotation field on the North China Plain to study the effects of long-term fertilization on the contents and storages of soil particulate organic carbon (POC), particulate organic nitrogen (PON), incorporated organic carbon (IOC), and incorporated organic nitrogen (ION). The long-term fertilization experiment was designed to include 7 treatments, i.e., chemical NPK (NPK), organic manure (OM), 1/2 organic manure plus 1/2 chemical NPK (1/20 MN), chemical NP (NP), chemical PK (PK), chemical NK (NK), and control (CK). After 18 years experiment, all the fertilization treatments showed higher contents of soil POC, PON, IOC and ION, higher proportions of soil POC to soil total organic carbon (TOC) and of soil PON to soil total nitrogen (TON), and higher C/N ratio of soil particulate organic matter. The storages of soil POC and PON under fertilization treatments were increased by 11.7%-196.8% and 13.0%-152.2%, respectively, compared with the control. The contribution of soil POC to the increased storage of soil TOC ranged from 31.5% to 67.3%, and that of soil PON to the increased storage of soil TON ranged from 14.3% to 100.0%. The storages of soil IOC and ION under fertilization treatments increased by 2.0%-75.0% and 0.0%-69.8%, respectively, compared with the control. Among the fertilization treatments, treatment OM had the highest storages of soil POC, PON, IOC and ION, followed by treatment 1/20 MN, and the treatments of applying chemical fertilizers alone. Balanced application of chemical fertilizers (treatment NPK) showed higher storages of soil POC, PON, IOC and ION than imbalanced application (treatments NP, PK, and NK). It was suggested that applying organic manure or its combination with chemical NPK and the balanced application of chemical NPK could be the keys for the increase of soil POC, PON, IOC and ION contents and storages.

[Research advances in forest soil respiration].

Among the methods of measuring forest soil respiration, infrared CO2 analysis is the optimal one so far. Comparing with empirical model, the process-based model in simulating the production and transportation of soil CO2 has the advantage of considering the biological and physical processes of soil respiration. Generally, soil respiration is positively correlated with soil temperature and moisture, but there are still many uncertainties about the relationships between soil respiration and forest management activities such as firing, cutting, and fertilization. The relationships of soil respiration with vegetation type and soil microbial biomass, as well as the spatial heterogeneity of soil respiration, are the hotspots in recent researches. Some issues and future development in forest soil respiration research were discussed in this paper.