1,802 research outputs found
Calcium-mediated stabilisation of soil organic carbon
Soils play an essential role in the global cycling of carbon and understanding the stabilisation mechanisms behind the preservation of soil organic carbon (SOC) pools is of globally recognised significance. Until recently, research into SOC stabilisation has predominantly focused on acidic soil environments and the interactions between SOC and aluminium (Al) or iron (Fe). The interactions between SOC and calcium (Ca) have typically received less attention, with fewer studies conducted in alkaline soils. Although it has widely been established that exchangeable Ca (CaExch) positively correlates with SOC concentration and its resistance to oxidation, the exact mechanisms behind this relationship remain largely unidentified. This synthesis paper critically assesses available evidence on the potential role of Ca in the stabilisation of SOC and identifies research topics that warrant further investigation. Contrary to the common view of the chemistry of base cations in soils, chemical modelling indicates that Ca2+ can readily exchange its hydration shell and create inner sphere complexes with organic functional groups. This review therefore argues that both inner- and outer-sphere bridging by Ca2+ can play an active role in the stabilisation of SOC. Calcium carbonate (CaCO3) can influence occluded SOC stability through its role in the stabilisation of aggregates; however, it could also play an unaccounted role in the direct sorption and inclusion of SOC. Finally, this review highlights the importance of pH as a potential predictor of SOC stabilisation mechanisms mediated by Al- or Fe- to Ca, and their respective effects on SOC dynamics
A cascading influence of calcium carbonate on the biogeochemistry and pedogenic trajectories of subalpine soils, Switzerland
Evidence linking calcium to increased organo-mineral association in soils
Geochemical indicators are emerging as important predictors of soil organic carbon (SOC) dynamics, but evidence concerning the role of calcium (Ca) is scarce. This study investigates the role of Ca prevalence in SOC accumulation by comparing otherwise similar sites with (CaCO-bearing) or without carbonates (CaCO-free). We measured the SOC content and indicators of organic matter quality (C stable isotope composition, expressed as δC values, and thermal stability) in bulk soil samples. We then used sequential sonication and density fractionation (DF) to separate two occluded pools from free and mineral-associated SOC. The SOC content, mass, and δC values were determined in all the fractions. X-ray photoelectron spectroscopy was used to investigate the surface chemistry of selected fractions. Our hypothesis was that occlusion would be more prevalent at the CaCO-bearing site due to the influence of Ca on aggregation, inhibiting oxidative transformation, and preserving lower δC values. Bulk SOC content was twice as high in the CaCO-bearing profiles, which also had lower bulk δC values, and more occluded SOC. Yet, contrary to our hypothesis, occlusion only accounted for a small proportion of total SOC (< 10%). Instead, it was the heavy fraction (HF), containing mineral-associated organic C, which accounted for the majority of total SOC and for the lower bulk δC values. Overall, an increased Ca prevalence was associated with a near-doubling of mineral-associated SOC content. Future investigations should now aim to isolate Ca-mediated complexation processes that increase organo-mineral association and preserve organic matter with lower δC values
Decoupling of topsoil and subsoil controls on organic matter dynamics in the Swiss Alps
Spatial modelling of soil water holding capacity improves models of plant distributions in mountain landscapes
Aims
The aims of this study were: 1) to test a new methodology to overcome the issue of the predictive capacity of soil water availability in geographic space due to measurement scarcity, 2) to model and generalize soil water availability spatially to a whole region, and 3) to test its predictive capacity in plant SDMs.
Methods
First, we modelled the measured Soil Water Holding Capacity (SWHC at different pFs) of 24 soils in a focal research area, using a weighted ensemble of small bivariate models (ESM). We then used these models to predict 256 locations of a larger region and used the differences in these pF predictions to calculate three different indices of soil water availability for plants (SWAP. These SWAP variables were added one by one to a set of conventional topo-climatic predictors to model 104 plant species distributions.
Results
We showed that adding SWAP to the SDMs could improve our ability to predict plant species distributions, and more specifically, pF1.8–pF4.2 became the third most important predictor across all plant models.
Conclusions
Soil water availability can contribute a significant increase in the predictive power of plant distribution models, by identifying important additional abiotic information to describe plant ecological niches
On the Angular Resolution of the AGILE gamma-ray imaging detector
We present a study of the Angular Resolution of the AGILE gamma-ray imaging
detector (GRID) that is operational in space since April 2007. The AGILE
instrument is made of an array of 12 planes each equipped with a Tungsten
converter and Silicon micros trip detectors and is sensitive in the energy
range 50 MeV - 10 GeV. Among the space instruments devoted to gamma-ray
astrophysics, AGILE uniquely exploits an analog readout system with dedicated
electronics coupled with Silicon detectors. We show the results of Monte Carlo
simulations carried out to reproduce the gamma-ray detection by the GRID, and
we compare them to in-flight data. We use the Crab (pulsar + Nebula) system for
discussion of real data performance, since its E^{-2} energy spectrum is
representative of the majority of gamma-ray sources. For Crab-like spectrum
sources, the GRID angular resolution (FWHM of ~4deg at 100 MeV; ~0.8deg at 1
GeV; ~0.9deg integrating the full energy band from 100 MeV to tens of GeV) is
stable across a large field of view, being characterized by a flat response up
to 30deg off-axis. A comparison of the angular resolution obtained by the two
operational gamma-ray instruments, AGILE-GRID and Fermi-LAT, is interesting in
view of future gamma-ray missions, that are currently under study. The two
instruments exploit different detector configurations affecting the angular
resolution: the former being optimized in the readout and track reconstruction
especially in the low-energy band, the latter in terms of converter thickness
and power consumption. We show that, despite these differences, the angular
resolution of both instruments is very similar between 100 MeV and a few GeV.Comment: 19 pages, 8 figures, accepted for publication in Ap
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