Metamorphism-facilitated faulting in deforming orthopyroxene: Implications for global intermediate-depth seismicity.

SignificanceThe exothermic metamorphic reaction in orthopyroxene (Opx), a major component of oceanic lithospheric mantle, is shown to trigger brittle failure in laboratory deformation experiments under conditions where garnet exsolution takes place. The reaction product is an extremely fine-grained material, forming narrow reaction zones that are mechanically weak, thereby facilitating macroscopic faulting. Oceanic subduction zones are characterized by two separate bands of seismicity, known as the double seismic zone. The upper band of seismicity, located in the oceanic crust, is well explained by dehydration-induced mechanical instability. Our newly discovered metamorphism-induced mechanical instability provides an alternative physical mechanism for earthquakes in the lower band of seismicity (located in the oceanic lithospheric mantle), with no requirement of hydration/dehydration processes.

The strengthened impact of water availability at interannual and decadal time scales on vegetation GPP.

Water availability (WA) is a key factor influencing the carbon cycle of terrestrial ecosystems under climate warming, but its effects on gross primary production (EWA-GPP ) at multiple time scales are poorly understood. We used ensemble empirical mode decomposition (EEMD) and partial correlation analysis to assess the WA-GPP relationship (RWA-GPP ) at different time scales, and geographically weighted regression (GWR) to analyze their temporal dynamics from 1982 to 2018 with multiple GPP datasets, including near-infrared radiance of vegetation GPP, FLUXCOM GPP, and eddy covariance-light-use efficiency GPP. We found that the 3- and 7-year time scales dominated global WA variability (61.18% and 11.95%), followed by the 17- and 40-year time scales (7.28% and 8.23%). The long-term trend also influenced 10.83% of the regions, mainly in humid areas. We found consistent spatiotemporal patterns of the EWA-GPP and RWA-GPP with different source products: In high-latitude regions, RWA-GPP changed from negative to positive as the time scale increased, while the opposite occurred in mid-low latitudes. Forests had weak RWA-GPP at all time scales, shrublands showed negative RWA-GPP at long time scales, and grassland (GL) showed a positive RWA-GPP at short time scales. Globally, the EWA-GPP , whether positive or negative, enhanced significantly at 3-, 7-, and 17-year time scales. For arid and humid zones, the semi-arid and sub-humid zones experienced a faster increase in the positive EWA-GPP , whereas the humid zones experienced a faster increase in the negative EWA-GPP . At the ecosystem types, the positive EWA-GPP at a 3-year time scale increased faster in GL, deciduous broadleaf forest, and savanna (SA), whereas the negative EWA-GPP at other time scales increased faster in evergreen needleleaf forest, woody savannas, and SA. Our study reveals the complex and dynamic EWA-GPP at multiple time scales, which provides a new perspective for understanding the responses of terrestrial ecosystems to climate change.

Strengthen interactions among fungal and protistan taxa by increasing root biomass and soil nutrient in the topsoil than in the soil-rock mixing layer.

Soil depth plays a crucial role in shaping the interactions between soil microbes and nutrient availability. However, there is limited understanding of how bacterial, fungal, and protistan communities respond to different soil depths, particularly in the unique geological context and soil properties of karst regions. Organic matter, total nitrogen, and phosphorus, ammonium, nitrate, and plant root biomass, as well as bacterial and fungal abundances, bacterial and protistan diversity were higher in the 0-20 cm soil layer than those in the 20-40 cm and soil-rock mixing layers. In contrast, soil pH was higher in the 20-40 cm and soil-rock mixing layers than that in the 0-20 cm soil layer. The soil exchange of calcium, nitrate, and root biomass were identified as the primary factors regulating microbial assemblages across the depth transect. Moreover, co-occurrence network analysis revealed a greater degree of connectivity between protistan taxa and fungal taxa in the 0-20 cm soil layer than those in the 20-40 cm and soil-rock mixing layers. In contrast, the number of association links between protist-bacteria and bacteria-bacteria was higher in the soil-rock mixing layers compared to the 0-20 cm soil layer. Actinobacteria, Ascomycota, and unclassified protistan taxa were identified as keystones, displaying the highest number of connections with other microbial taxa. Collectively, these results suggested that the increased plant root biomass, coupled with sufficient available nutrient inputs in the upper 0-20 cm soil layer, facilitates strong interactions among fungal and protistan taxa, which play crucial roles in the topsoil. However, as nutrients become less available with increasing depth, competition among bacterial taxa and the predation between bacterial and protistan taxa intensify. Therefore, these findings indicate the interactions among keystone taxa at different soil depths has the potential to generate ecological implications during vegetation restoration in fragile ecosystems.

Tree species diversity increases soil microbial carbon use efficiency in a subtropical forest.

Plant communities strongly influence soil microbial communities and, in turn, soil carbon (C) cycling. Microbial carbon use efficiency (CUE) is an important parameter for predicting soil C accumulation, yet how plant and soil microbial community traits influence microbial CUE remains poorly understood. Here, we determined how soil microbial CUE is influenced by plant and soil microbial community traits, by studying a natural gradient of plant species diversity in a subtropical forest. Our results showed that microbial CUE increased with increasing tree species diversity, suggesting a correlation between plant community traits and soil C storage. The specific soil properties that explained the greatest variation in microbial CUE were associated with microbial communities (biomass, enzyme activities and the ratio of oligotrophic to copiotrophic taxa); there were weaker correlations with plant-input properties, soil chemistry and soil organic C quality and its mineral protection. Overall, high microbial CUE was associated with soil properties correlated with increased tree species diversity: higher substrate availability (simple SOM chemical structures and weak mineral organic associations) and high microbial growth rates despite increased community dominance by oligotrophic strategists. Our results point to a mechanism by which increased tree species diversity may increase the forest C sink by affecting carbon use with the soil microbial community.

Elevated Mercury Deposition, Accumulation, and Migration in a Karst Forest.

The karst forest is one of the extremely sensitive and fragile ecosystems in southwest China, where the biogeochemical cycling of mercury (Hg) is largely unknown. In this study, we investigated the litterfall deposition, accumulation, and soil migration of Hg in an evergreen-deciduous broadleaf karst forest using high-resolution sampling and stable isotope techniques. Results show that elevated litterfall Hg concentrations and fluxes in spring are due to the longer lifespan of evergreen tree foliage exposed to atmospheric Hg0. The hillslope has 1-2 times higher litterfall Hg concentration compared to the low-lying land due to the elevated atmospheric Hg levels induced by topographical and physiological factors. The Hg isotopic model suggests that litterfall Hg depositions account for ∼80% of the Hg source contribution in surface soil. The spatial trend of litterfall Hg deposition cannot solely explain the trend of Hg accumulation in the surface soil. Indeed, soil erosion enhances Hg accumulation in soil of low-lying land, with soil Hg concentration up to 5-times greater than the concentration on the hillslope. The high level of soil Hg migration in the karst forest poses significant ecological risks to groundwater and downstream aquatic ecosystems.

Rheological separation of the megathrust seismogenic zone and episodic tremor and slip.

Episodic tremor and accompanying slow slip, together called ETS, is most often observed in subduction zones of young and warm subducting slabs. ETS should help us to understand the mechanics of subduction megathrusts, but its mechanism is still unclear. It is commonly assumed that ETS represents a transition from seismic to aseismic behaviour of the megathrust with increasing depth, but this assumption is in contradiction with an observed spatial separation between the seismogenic zone and the ETS zone. Here we propose a unifying model for the necessary geological condition of ETS that explains the relationship between the two zones. By developing numerical thermal models, we examine the governing role of thermo-petrologically controlled fault zone rheology (frictional versus viscous shear). High temperatures in the warm-slab environment cause the megathrust seismogenic zone to terminate before reaching the depth of the intersection of the continental Mohorovicic discontinuity (Moho) and the subduction interface, called the mantle wedge corner. High pore-fluid pressures around the mantle wedge corner give rise to an isolated friction zone responsible for ETS. Separating the two zones is a segment of semi-frictional or viscous behaviour. The new model reconciles a wide range of seemingly disparate observations and defines a conceptual framework for the study of slip behaviour and the seismogenesis of major faults.

Assessing Regional Ecosystem Conditions Using Geospatial Techniques-A Review.

Ecosystem conditions at the regional level are critical factors for environmental management, public awareness, and land use decision making. Regional ecosystem conditions may be examined from the perspectives of ecosystem health, vulnerability, and security, as well as other conceptual frameworks. Vigor, organization, and resilience (VOR) and pressure-stress-response (PSR) are two commonly adopted conceptual models for indicator selection and organization. The analytical hierarchy process (AHP) is primarily used to determine model weights and indicator combinations. Although there have been many successful efforts in assessing regional ecosystems, they remain affected by a lack of spatially explicit data, weak integration of natural and human dimensions, and uncertain data quality and analyses. In the future, regional ecosystem condition assessments may be advanced by incorporating recent improvements in spatial big data and machine learning to create more operative indicators based on Earth observations and social metrics. The collaboration between ecologists, remote sensing scientists, data analysts, and scientists in other relevant disciplines is critical for the success of future assessments.

Source Diversity of Intermediate Volatility n-Alkanes Revealed by Compound-Specific δ13C-δD Isotopes.

Intermediate volatility organic compounds (IVOCs) are important precursors of secondary organic aerosols, and their sources remain poorly defined. N-alkanes represent a considerable portion of IVOCs in atmosphere, which can be well identified and quantified out of the complex IVOC pool. To investigate the potential source diversity of intermediate volatility n-alkanes (IVnAs, nC12-nC20), we apportioned the sources of IVnAs in the atmosphere of four North China cities, based on their compound-specific δ13C-δD isotope compositions and Bayesian model analysis. The concentration level of IVnAs reached 1195 ± 594 ng/m3. The δ13C values of IVnAs ranged -32.3 to -27.6‰ and δD values -161 to -90‰. The δD values showed a general increasing trend toward higher carbon number alkanes, albeit a zigzag odd-even prevalence. Bayesian MixSIAR model using δ13C and δD compositions revealed that the source patterns of individual IVnAs were inconsistent; the relative contributions of liquid fossil combustion were higher for lighter IVnAs (e.g., nC12-nC13), while those of coal combustion were higher for heavier IVnAs (e.g., nC17-nC20). This result agrees with principal component analysis of the dual isotope data. Overall, coal combustion, liquid fossil fuel combustion, and biomass burning contributed about 47.8 ± 0.1, 35.7 ± 4.0, and 16.3 ± 4.2% to the total IVnAs, respectively, highlighting the importance of coal combustion as an IVnA source in North China. Our study demonstrates that the dual-isotope approach is a powerful tool for source apportionment of atmospheric IVOCs.

Asthenosphere rheology inferred from observations of the 2012 Indian Ocean earthquake.

The concept of a weak asthenospheric layer underlying Earth's mobile tectonic plates is fundamental to our understanding of mantle convection and plate tectonics. However, little is known about the mechanical properties of the asthenosphere (the part of the upper mantle below the lithosphere) underlying the oceanic crust, which covers about 60 per cent of Earth's surface. Great earthquakes cause large coseismic crustal deformation in areas hundreds of kilometres away from and below the rupture area. Subsequent relaxation of the earthquake-induced stresses in the viscoelastic upper mantle leads to prolonged postseismic crustal deformation that may last several decades and can be recorded with geodetic methods. The observed postseismic deformation helps us to understand the rheological properties of the upper mantle, but so far such measurements have been limited to continental-plate boundary zones. Here we consider the postseismic deformation of the very large (moment magnitude 8.6) 2012 Indian Ocean earthquake to provide by far the most direct constraint on the structure of oceanic mantle rheology. In the first three years after the Indian Ocean earthquake, 37 continuous Global Navigation Satellite Systems stations in the region underwent horizontal northeastward displacements of up to 17 centimetres in a direction similar to that of the coseismic offsets. However, a few stations close to the rupture area that had experienced subsidence of up to about 4 centimetres during the earthquake rose by nearly 7 centimetres after the earthquake. Our three-dimensional viscoelastic finite-element models of the post-earthquake deformation show that a thin (30-200 kilometres), low-viscosity (having a steady-state Maxwell viscosity of (0.5-10) × 1018 pascal seconds) asthenospheric layer beneath the elastic oceanic lithosphere is required to produce the observed postseismic uplift.

Water uptake depth is coordinated with leaf water potential, water-use efficiency and drought vulnerability in karst vegetation.

Root access to bedrock water storage or groundwater is an important trait allowing plant survival in seasonally dry environments. However, the degree of coordination between water uptake depth, leaf-level water-use efficiency (WUEi) and water potential in drought-prone plant communities is not well understood. We conducted a 135-d rainfall exclusion experiment in a subtropical karst ecosystem with thin skeletal soils to evaluate the responses of 11 co-occurring woody species of contrasting life forms and leaf habits to a severe drought during the wet growing season. Marked differences in xylem water isotopic composition during drought revealed distinct ecohydrological niche separation among species. The contrasting behaviour of leaf water potential in coexisting species during drought was largely explained by differences in root access to deeper, temporally stable water sources. Smaller-diameter species with shallower water uptake, more negative water potentials and lower WUEi showed extensive drought-induced canopy defoliation and/or mortality. By contrast, larger-diameter species with deeper water uptake, higher leaf-level WUEi and more isohydric behaviour survived drought with only moderate canopy defoliation. Severe water limitation imposes strong environmental filtering and/or selective pressures resulting in tight coordination between tree diameter, water uptake depth, iso/anisohydric behaviour, WUEi and drought vulnerability in karst plant communities.

Prevalence of viscoelastic relaxation after the 2011 Tohoku-oki earthquake.

After a large subduction earthquake, crustal deformation continues to occur, with a complex pattern of evolution. This postseismic deformation is due primarily to viscoelastic relaxation of stresses induced by the earthquake rupture and continuing slip (afterslip) or relocking of different parts of the fault. When postseismic geodetic observations are used to study Earth's rheology and fault behaviour, it is commonly assumed that short-term (a few years) deformation near the rupture zone is caused mainly by afterslip, and that viscoelasticity is important only for longer-term deformation. However, it is difficult to test the validity of this assumption against conventional geodetic data. Here we show that new seafloor GPS (Global Positioning System) observations immediately after the great Tohoku-oki earthquake provide unambiguous evidence for the dominant role of viscoelastic relaxation in short-term postseismic deformation. These data reveal fast landward motion of the trench area, opposing the seaward motion of GPS sites on land. Using numerical models of transient viscoelastic mantle rheology, we demonstrate that the landward motion is a consequence of relaxation of stresses induced by the asymmetric rupture of the thrust earthquake, a process previously unknown because of the lack of near-field observations. Our findings indicate that previous models assuming an elastic Earth will have substantially overestimated afterslip downdip of the rupture zone, and underestimated afterslip updip of the rupture zone; our knowledge of fault friction based on these estimates therefore needs to be revised.

Matrix-Assisted Laser Desorption Ionization Imaging and Laser Ablation Sampling for Analysis of Fungicide Distribution in Apples.

A combination of matrix-assisted laser desorption ionization (MALDI) imaging and infrared (IR) laser ablation sampling with offline electrospray ionization mass spectrometry (ESI-MS) was used to determine the distribution of the fungicide imazalil in apples. MALDI images were used to determine the penetration depth of imazalil up to 7 days after its application. IR laser ablation sampling and ESI-MS were used to quantify the rate of penetration of the fungicide, which was determined to be approximately 1 mm per day. Imazalil concentration decreased in the apple skin over the course of the experiment, and after 7 days the fungicide was detected at 0.015 ppm 6 mm inside the apple. Approximately 60% of the pesticide remained in the skin after 7 days. This work demonstrates the utility of MALDI imaging for spatial localization of fungicide in fruit in combination with IR laser ablation and ESI-MS for quantitative analysis.

Deformation cycles of subduction earthquakes in a viscoelastic Earth.

Subduction zones produce the largest earthquakes. Over the past two decades, space geodesy has revolutionized our view of crustal deformation between consecutive earthquakes. The short time span of modern measurements necessitates comparative studies of subduction zones that are at different stages of the deformation cycle. Piecing together geodetic 'snapshots' from different subduction zones leads to a unifying picture in which the deformation is controlled by both the short-term (years) and long-term (decades and centuries) viscous behaviour of the mantle. Traditional views based on elastic models, such as coseismic deformation being a mirror image of interseismic deformation, are being thoroughly revised.

Soil gross nitrogen transformations in responses to land use conversion in a subtropical karst region.

Gross nitrogen (N) transformations can provide important information for assessing indigenous soil N supply capacity and soil nitrate leaching potential. The current study aimed to assess the variation of gross N transformations in response to conversion of maize-soybean fields to sugarcane, mulberry, and forage grass fields in a subtropical karst region of southwest China. Mature forests were included for comparison. Gross rates of N mineralization (GNM) were highest in the forests, intermediate in the maize-soybean and forage grass fields, and lowest in the sugarcane and mulberry fields, suggesting capacity of indigenous soil N supply derived from organic N mineralization was lowered after conversion to sugarcane and mulberry fields. The relative high indigenous soil N supply capacity in the maize-soybean fields was obtained at the cost of soil organic N depletion. Gross nitrification (GN) rates were highest in the forests, intermediate in the forage grass fields and lowest in the other three agricultural land use types. The nitrate retention capacity (24.1 ±â€¯2.0% on average) was similar among the five land use types, implying that nitrate leaching potential was not changed after land use conversion. Microbial biomass N exerted significant direct effects on the rates of N mineralization, nitrification, ammonium immobilization and nitrate immobilization. Soil organic carbon, total N and exchangeable magnesium had significant indirect effects on these N transformation rates. Our findings suggest that forage grass cultivation instead of other agricultural land uses should be recommended from the perspective of increasing indigenous soil N supply while not depleting soil organic N pool.

Responses of soil nutrients and microbial communities to three restoration strategies in a karst area, southwest China.

Ecological restoration is widespread in the karst region, southwest China, but the impacts of different restoration strategies on soil fertility indices have rarely been compared. Here soil nutrients and microbial communities were measured 16 years after agricultural abandonment in a karst area, southwest China. Three restoration strategies were included, i.e., i) restoration with an economic tree species Toona sinensis (TS), ii) restoration with Guimu-1 hybrid elephant grass (GG), iii) restoration with a combination of Zenia insignis and Guimu-1 hybrid elephant grass (ZG). Cropland under maize-soybean rotation (CR) was used as reference. Soil organic carbon level was more than doubled in TS, and that in GG and ZG was elevated by about 50% relative to CR. Soil total nitrogen concentration in GG was not significantly different from CR, but that in TS and ZG was increased by 93% and 55% relative to CR. Similar to nitrogen, soil total phosphorus concentration in GG was not changed relative to CR, but that in TS and ZG were significantly increased. Microbial biomass carbon and nitrogen concentrations were significantly increased in TS and GG by 124% and 82%, respectively, compared to CR, but those in ZG and CR were similar. The abundance of total PLFAs (phospholipid fatty acids) was significantly increased by 55-69% following agricultural abandonment, and there was no significant difference among the three restoration strategies. The patterns of the other microbial groups and the ratio of fungal to bacterial (F:B) PLFAs were largely similar to that of total PLFAs. Soil organic carbon was identified as the primary factor affecting the abundance of soil microbial communities. Our findings suggest that the three restoration strategies, particularly TS are efficient in improving soil fertility.

Infrared laser ablation sample transfer of tissue DNA for genomic analysis.

Infrared (IR) laser ablation was used to remove material from tissue sections mounted on microscope slides, with subsequent capture in a solvent-containing microcentrifuge tube. Experiments conducted with a 3200-bp double-stranded plasmid DNA template demonstrated IR-laser ablation transfer of intact DNA. The transfer efficiency and the molecular integrity of the captured DNA were evaluated using Sanger sequencing, gel electrophoresis, and fluorimetric analysis. The plasmid DNA was reproducibly transferred with an efficiency of 59 ± 3% at laser fluences of between 10 and 20 kJ/m2 at a wavelength of 3 µm. IR laser ablation sample transfer was then used to ablate and capture DNA from 50-µm-thick rat brain and kidney tissue sections. DNA was extracted from the captured material using five commercial DNA extraction kits that employed significantly divergent methodologies, with all kits recovering sufficient DNA for successful amplification by polymerase chain reaction (PCR). Four sets of primers were employed, targeting one region of the CYP 11b2 gene (376 bp) and three different regions of the Snn1g gene (298, 168, and 281 bp). The PCR results were not consistently reliable when using unpurified ablation samples; however, after extraction, all samples produced PCR products of the expected size. This work expands the sampling capabilities of IR laser ablation, demonstrating that DNA can be isolated from tissue samples for genomic assays. Due to the small size of the ablation regions (1 mm2), this technique will be useful for sampling discrete cell populations from tissue sections. Graphical abstract Infrared laser ablation transfer of intact DNA from a tissue section.

A review of nonstandardized applicators digitization in Nucletron™ HDR procedures.

The major errors in HDR procedures were failures to enter the correct treatment distance, which could be caused by either entering wrong transmission lengths or imprecisely digitizing the dwelling positions. Most of those errors were not easily avoidable by enhancing the HDR management level because they were caused by implementations of nonstandardized applicators utilizing transmission tubes of different lengths in standard HDR procedures. We performed this comprehensive study to include all possible situations with different nonstandardized applicators that frequently occurred in HDR procedures, provide corresponding situations with standard applicator as comparisons, list all possible errors and in planning, clarify the confusions in offsets setting, and provide mathematical and quantitative solutions for each given scenarios. Training on HDR procedures with nonstandardized applicators are normally not included in most residential program for medical physics, thus this study could be meaningful in both clinical and educational purpose. At precision of 1 mm, our study could be used as the essential and practical reference for finding the correct treatment length as well as locating the accurate dwelling positions in any HDR procedure with nonstandardized applicators.

Dynamics of soil organic carbon in density fractions during post-agricultural succession over two lithology types, southwest China.

Agricultural abandonment has been proposed as an effective way to enhance soil organic carbon (SOC) sequestration. Nevertheless, SOC sequestration in the long term is largely determined by whether the stable SOC fractions will increase. Here the dynamics of SOC fractions during post-agricultural succession were investigated in a karst region, southwest China using a space-for-time substitution approach. Cropland, grassland, shrubland and secondary forest were selected from areas underlain by dolomite and limestone, respectively. Density fractionation was used to separate bulk SOC into free light fraction (FLFC) and heavy fraction (HFC). FLFC contents were similar over dolomite and limestone, but bulk SOC and HFC contents were greater over limestone than over dolomite. FLFC content in the forest was greater than in the other vegetation types, but bulk SOC and HFC contents increased from the cropland through to the forest for areas underlain by dolomite. The contents of bulk SOC and its fractions were similar among the four vegetation types over limestone. The proportion of FLFC in bulk SOC was higher over dolomite than over limestone, but the case was inverse for the proportion of HFC, indicating SOC over limestone was more stable. However, the proportions of both FLFC and HFC were similar among the four vegetation types, implying that SOC stability was not changed by cropland conversion. Exchangeable calcium explained most of the variance of HFC content. Our study suggests that lithology not only affects SOC content and its stability, but modulates the dynamics of SOC fractions during post-agricultural succession.

Vertical distribution of soil saturated hydraulic conductivity and its influencing factors in a small karst catchment in Southwest China.

Saturated hydraulic conductivity (Ks) is one of the most important soil hydraulic parameters influencing hydrological processes. This paper aims to investigate the vertical distribution of Ks and to analyze its influencing factors in a small karst catchment in Southwest China. Ks was measured in 23 soil profiles for six soil horizons using a constant head method. These profiles were chosen in different topographical locations (upslope, downslope, and depression) and different land-use types (forestland, shrubland, shrub-grassland, and farmland). The influencing factors of Ks, including rock fragment content (RC), bulk density (BD), capillary porosity (CP), non-capillary porosity (NCP), and soil organic carbon (SOC), were analyzed by partial correlation analysis. The mean Ks value was higher in the entire profile in the upslope and downslope, but lower value, acting as a water-resisting layer, was found in the 10-20 cm soil depth in the depression. Higher mean Ks values were found in the soil profiles in the forestland, shrubland, and shrub-grassland, but lower in the farmland. These results indicated that saturation-excess runoff could occur primarily in the hillslopes but infiltration-excess runoff in the depression. Compared with other land-use types, surface runoff is more likely to occur in the farmlands. RC had higher correlation coefficients with Ks in all categories concerned except in the forestland and farmland with little or no rock fragments, indicating that RC was the dominant influencing factor of Ks. These results suggested that the vertical distributions of Ks and RC should be considered for hydrological modeling in karst areas.