Divergent dynamics between grassland greenness and gross primary productivity across China

peer reviewedGrassland, the most widespread vegetation type in China, has been greening recently.However, the extent to which the greenness has been translated into productivity and the underlying mechanism of the decoupled grassland greenness and productivity remains unclear. In this study, we detected the trend of normalized difference vegetation index (NDVI) and gross primary productivity (GPP) of grassland in China from 2000 to 2019 and analyzed the driving mechanism of the inconsistency between them. It was found that the relative increase rate of productivity (27.27%, p < 0.05) was much greater than that of greenness (14.54%, p < 0.05) across grasslands in China from 2000 to 2019, especially in temperate regions. The temperature and precipitation were the main factors influencing the grassland growth change, and the impact of temperature and shortwave radiation on productivity was greater than on greenness. However, the increase of grassland greenness was not fully translated into productivity in subtropical and tropical grass as well as shrub. This study revealed the dominance of climatic factors in the translation process from ecosystem structure to function, which highlighted the challenge in enhancing carbon uptake capacity of terrestrial ecosystem facing accelerated climate change

Mechanical Spectroscopy: Some Applications On Structural Changes And Relaxation Dynamics In Soft Matter

The general trend in soft matter is to study systems of increasing complexity covering a wide range in time and frequency. Mechanical spectroscopy is a powerful tool for understanding the structure and relaxation dynamics of these materials over a large temperature range and frequency scale. In this work, we collect a few recent applications using low-frequency mechanical spectroscopy for elucidating the structural changes and relaxation dynamics in soft matter, largely based on the author’s group. We illustrate the potential of mechanical spectroscopy with three kinds of soft materials: colloids, polymers and granular systems. Examples include structural changes in colloids, segmental relaxations in amorphous polymers, and resonant dissipation of grain chains in three-dimensional media. The present work shows that mechanical spectroscopy has been applied as a necessary and complementary tool to study the dynamics of such complex systems

Opposite Effects of SiO2 Nanoparticles on the Local α and Larger-Scale α’ Segmental Relaxation Dynamics of PMMA Nanocomposites

The segmental relaxation dynamics of poly(methyl methacrylate)/silica (PMMA/SiO2) nanocomposites with different compositions ( ϕ SiO 2 ) near and above the glass transition temperature were investigated by mechanical spectroscopy. At ϕ SiO 2 &le; 0.5%, the &alpha; peak temperature hardly changes with ϕ SiO 2 , but that of &alpha;&rsquo; relaxation composed of Rouse and sub-Rouse modes decreases by 15 &deg;C due to the increase of free volume. At ϕ SiO 2 &ge; 0.7%, both &alpha; and &alpha;&rsquo; relaxations shift to high temperatures because of the steric hindrance introduced by nanoparticle agglomeration. On the other hand, with increasing ϕ SiO 2 , the peak height for &alpha; relaxation increases at ϕ SiO 2 &le; 0.5% and then decreases at ϕ SiO 2 &ge; 0.7%, but that for &alpha;&rsquo; relaxation shows an opposite behavior. This is because at low ϕ SiO 2 , the short-chain segments related to &alpha; relaxation can easily bypass the particles, but the longer-chain segments related to &alpha;&rsquo; relaxation cannot. At high ϕ SiO 2 , the polymer chains were bound to the nanoparticles due to the physical adsorption effect, leading to the decrease of relaxation unit concentration involved in &alpha; relaxation. However, the dissociation of those bonds with heating and the concentration heterogeneity of polymer chains result in the increase of peak height for &alpha;&rsquo; relaxation

Abnormal segmental dynamics of poly(methyl methacrylate)/poly(vinylidene fluoride) blends by mechanical spectroscopy

The molecular relaxation dynamics of PMMA/PVDF blends above the glass transition temperature (Tg) over a wide composition range are studied by mechanical spectroscopy combined with differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) measurements. The mechanical spectra of the blends reveal the existence of two relaxation modes: α, ascribed to the glass transition, and α′, related to the softening dispersion composed of the sub-Rouse modes and the Rouse modes. At ϕPVDF = 40%, both the α and α′ relaxation processes shift to low-temperatures and are accelerated, which is due to the formation of the interphase and unfavourable interchain entanglements in the intermediate composition. The abnormal dynamics of the blend at ϕPVDF = 40% is further confirmed by the observed weak interaction between PMMA and PVDF from FTIR measurements and an obvious drop of the intermolecular coupling from the Coupling Model. However, the longer α′ relaxation shows a different dynamic behavior from the α relaxation for the blends with increasing the PVDF content at ϕPVDF ≤ 40%, which is due to the structure evolution and the change of chains entanglement with heating. This work enriches the understanding of the complex relaxation dynamics and the structure evolution in PMMA/PVDF blends

Frozen-to-jamming-to-fluid Transition of Weakly Sheared Granular Systems by Low-frequency Mechanical Spectroscopy

<div><p>Granular matter usually displays frozen, jamming and fluidized states when submitted to an external vibration with increasing intensity. The dissipation properties of granular systems with three different millimeter-size glass grains (0.1, 0.5 and 1.9 mm) have been investigated by a modified low-frequency inverted torsion pendulum under a shear strain and an external pressure. With increasing the immersed depth of the oscillating probe, all the systems show the frozen, jamming and fluidized behaviors. Furthermore, the critical depth at which the transition occurs increases with increasing grain size, but decreases with the application of pressure. A qualitative explanation is tentatively proposed to understand the underlying mechanism of complex viscoelastic properties of the glass particle systems.</p></div