Browsing by Author "Yu, G."

Decomposition of soil organic matter (SOM) protected within aggregates can be accelerated via priming effect (PE) by the addition of fresh substrates. However, the knowledge of the sources of mineralization and PE in aggregate size classes is absent. We applied the three-source-partitioning isotopic (C-14 + delta C-13) approach to determine how aggregate size classes affect the contribution of three C sources (substrate added, recent and old SOM) to CO2 efflux and PE depending on the amount of added primer. Soil from a field with 3 years of maize cropping (C-4 plants) after long-term C-3 vegetation was used to differentiate between recent C (C-4-C; < 3 years) and old C (C-3 C; >3 years). Soil samples were separated into three aggregate size classes (>2 mm, 2-0.25 mm macroaggregates and <0.25 mm microaggregates) and were incubated for 49 days after being amended with two levels of C-14 labeled glucose. The proportion of glucose mineralized to CO2 increased with decreasing aggregate size, but C-14 incorporation into microbial biomass decreased, indicating higher C use efficiency in macroaggregates compared with microaggregates. The short-time PE was positive and was accompanied by a rapid reduction of dissolved organic C. After 49 days, the PE was higher in macro-versus microaggregates at both glucose levels. Positive PE induced by a low glucose level was observed only in large macroaggregates (>2 mm), but was observed in both macroaggregates (>0.25 mm) and microaggregates (<0.25 mm) after high glucose amendment. These results indicate that SOM pools are more decomposable in macro-versus microaggregates and that the SOM pools are involved in PE according to their biochemical availability. More primed CO2 originated from recent C-4-C than old C-3-C in larger macroaggregates under a low glucose level. The relative contribution of recent C-4-C to primed CO2 increased from macroaggregates (37.8%) to microaggregates (100%) after high glucose amendment. Therefore, increasing glucose addition stimulated the decomposition of old C-3-C in macroaggregates, but not in microaggregates. This indicates that microaggregates protect SOM against decomposition better than macroaggregates, and consequently, microaggregates can be considered as a potential reservoir for longterm C sequestration. Concluding, aggregate size is crucial for SOM decomposition, and it determines the source of PE and thus the protection of sequestrated C. The effects of the added primer on C sources involved in PE depend on the aggregate size. (C) 2016 Elsevier Ltd. All rights reserved.

Soil organic matter (SOM) pools, allocated within various aggregates, are characterized by different degradability and turnover rates that depend on the spatial accessibility of organics and their recalcitrance. Hence, to understand the processes and mechanisms of SOM cycling and stability, the contribution of individual aggregate size classes to the total CO2 efflux including extra mineralization via priming effect (PE) should be considered. In this study, we determined whether aggregate size classes and their disruption affected the mineralization of SOM and induced PE depending on the primer amount. Soil samples were separated into three aggregate size classes (>2mm, 2-0.25mm macroaggregates and <0.25mm microaggregates). Half of the samples within each class were left intact, whereas half were crushed. After the addition of two levels of C-14-labeled glucose, the amount of C-14 in CO2 efflux and microbial biomass were measured several times during the 49-day incubation. Cumulative SOM-derived CO2 production from the macroaggregates was 16-21% greater than the CO2 production from the microaggregates after 49 days. The percentage of glucose mineralized to CO2 increased with the level of glucose addition, but C-14 incorporation into microbial biomass decreased, indicating lower carbon (C) use efficiency at high substrate availability. Aggregate disruption had no effect on the cumulative total and SOM-derived CO2 production, but it increased glucose mineralization up to 11.2% while the percentage of added glucose incorporated into microbial biomass in macroaggregates decreased. The PE increased with an increased glucose level for the intact aggregates. Aggregate disruption increased the PE in all aggregates sizes under low glucose level. In summary, our findings demonstrate that the aggregate size class has clear effects on C mineralization while their disruption affects the added labile C decomposition and transformation, indicating the relevance of soil structure for SOM cycling in terms of priming and C sequestration. (C) 2015 Elsevier B.V. All rights reserved.

The elucidation of the pseudogap phenomenon of the high-transition-temperature (high-T-c) copper oxides-a set of anomalous physical properties below the characteristic temperature T and above T-c-has been a major challenge in condensed matter physics for the past two decades(1). Following initial indications of broken time-reversal symmetry in photoemission experiments(2), recent polarized neutron diffraction work demonstrated the universal existence of an unusual magnetic order below T (refs 3, 4). These findings have the profound implication that the pseudogap regime constitutes a genuine new phase of matter rather than a mere crossover phenomenon. They are furthermore consistent with a particular type of order involving circulating orbital currents, and with the notion that the phase diagram is controlled by a quantum critical point(5). Here we report inelastic neutron scattering results for HgBa2CuO4+delta that reveal a fundamental collective magnetic mode associated with the unusual order, and which further support this picture. The mode's intensity rises below the same temperature T and its dispersion is weak, as expected for an Ising-like order parameter(6). Its energy of 52-56 meV renders it a new candidate for the hitherto unexplained ubiquitous electron-boson coupling features observed in spectroscopic studies(7-10).

Patterns in the spatial distribution of soil microorganisms and the factors that determine them provide important information about the mechanisms regulating diversity and function of terrestrial ecosystems. The spatial heterogeneity of metabolic functional diversity of soil microorganisms was studied across a 30 x 40 m plot and at two soil depths (0-10 cm and 10-20 cm) in a natural, mixed broad-leaved Korean pine (Pinus koraiensis) forest soil in the Changbai Mountains. In addition, we assessed the importance of the quantity and quality (indicated by labile soil organic matter fractions) of soil organic matter in small-scale structuring of soil microbial metabolic functional diversity. Microbial metabolic functional diversity was characterized based on the Biolog profile. The results showed that metabolic activity exhibited moderate spatial dependence, while functional diversity had a much stronger spatial dependence. All soil organic matter fractions including total soil organic matter, dissolved organic matter, particulate organic matter explained 15-27% of the variance in microbial functional diversity in the two soil layers. Among all soil organic matter fractions, the labile dissolved organic carbon accounted for the largest amount of variation. Overall, the significant relationship between soil microorganisms and organic matter fractions allows for better understanding the ecological functions governing C cycling and microbial communities in forest ecosystems. (C) 2014 Elsevier Masson SAS. All rights reserved.

Inelastic neutron-scattering measurements of single-CuO2-layer HgBa2CuO4+delta reveal an antiferromagnetic resonance with energy omega(r) = 56 meV (approximate to 6.8k(B)T(c)) below the superconducting transition temperature T-c approximate to 96 K. The resonance is energy-resolution limited and exhibits an intrinsic momentum width of about 0.2 angstrom(-1), consistent with prior work on several other cuprates. The rather large value of omega(r) is identical to the characteristic energy of the electron-boson spectral density obtained from recent optical conductivity work, consistent with the notion that the charge carriers are strongly coupled to magnetic fluctuations.

Neutron scattering for Nd2-xCexCuO4+delta(x approximate to 0.155, T-c=25 K) reveals two distinct magnetic energy scales in the superconducting state: omega(1)approximate to 6.4 meV and omega(2)approximate to 4.5 meV. These magnetic energies agree quantitatively with the B-1g/B-2g and A(1g) features observed in electronic Raman scattering, where the former is believed to indicate the maximum superconducting gap and the origin of the latter has remained unexplained. The data are inconsistent with previous claims of the existence of a magnetic resonance mode near 10 meV, but consistent with a resonance at omega(2) and with the recently established universal ratio of resonance energy to superconducting gap in unconventional superconductors [G. Yu et al., Nat. Phys. 5, 873 (2009)].

There exists increasing evidence that the phase diagram of the high-transition temperature (T-c) cuprate superconductors is controlled by a quantum critical point. According to one distinct theoretical proposal, on decreasing the hole-carrier concentration a transition occurs to an ordered state with two circulating orbital currents per CuO2 square. Below the 'pseudogap' temperature T (T > T-c), the theory predicts a discrete order parameter and two weakly-dispersive magnetic excitations in structurally simple compounds which should be measurable by neutron scattering. Indeed, novel magnetic order and one such excitation were recently observed. Here, we demonstrate for tetragonal HgBa2CuO4+delta the existence of a second excitation with local character, consistent with the theory. The excitations mix with conventional antiferromagnetic fluctuations, which points towards a unifying picture of magnetism in the cuprates that will probably require a multi-band description.