3,917 research outputs found
The fate of organic carbon and nitrogen in experimental marine sediment systems: Influence of bioturbation and anoxia
The decay rate of particulate organic carbon (POC) and nitrogen (PON) was followed during 94 days in three homogenized sediment microcosms: 1. With a natural density of the polychaete Nereis virens (NOx-cores); 2. Defaunated, with an aerobic water phase (Ox-cores); and 3. Defaunated, with an anaerobic water phase (An-cores). In all cores there was a marked preferential mineralization of PON compared to POC. The presence of Nereis increased the net decomposition of POC and PON 2.6 and 1.6 times relative to Ox-cores. Ventilation of burrow structures by the worms increased the flux of O2, TCO2 and DIN across the sediment-water interface 2.5–3.5 times. This significantly decreased the pore water concentrations of TCO2 and DIN. Similarly, nitrification and denitrification were stimulated 2.3–2.4 times due to nereid activity. Oxygen did not increase organic degradation: in fact, the decay of POC and PON was faster in An- than in Ox-cores, 1.5–1.6 and 1.2 times, respectively. Sulfate reduction, measured at the end of experiment, was surprisingly low in the aerobic NOx- and Ox-cores relative to An-cores. Net ammonium production measured at the end of the experiment agreed with the mean loss of PON for Ox- and An-cores, but was low for NOx-cores, suggesting that a high C:N substrate was being degraded in these cores at the end. An empirical model describing the temporal decay pattern of POC and PON is presented: the detritus in all cores were initially composed of two fractions (similar C:N); a readily degradable (∼43%) and a low degradable (∼57%) fraction. A substantial part of the degradable fraction in NOx-cores was used during the experiment, with nitrogen being mineralized preferentially. The mean C:N molar ratio of detritus used was 5.9, compared to a value of 15.5 determined at the end. The Ox- and An-cores, however, showed similar C:N ratios for the detritus used during the experiment (3.7 and 4.8) and that measured at the end (4.2 and 4.6). Presumably not all the low C:N detritus had yet been mineralized in these cores at the end of experiment
Catch Crops in Organic Farming Systems without Livestock Husbandry - Model Simulations
During the last years, an increasing number of stockless farms in Europe converted to organic farming practice without re-establishing a livestock. Due to the lack of animal manure as a nutrient input, the relocation and the external input of nutrients is limited in those organic cropping systems. The introduction of a one-year green manure fallow in a 4-year crop rotation, including clover-grass mixtures as a green manure crop is the classical strategy to solve at least some of the problems related to the missing livestock. The development of new crop rotations, including an extended use of catch crops and annual green manure (legumes) may be another possibility avoiding the economical loss during the fallow year.
Modelling of the C and N turnover in the soil-plant-atmosphere system using the soil-plant-atmosphere model DAISY is one of the tools used for the development of new organic crop rotations. In this paper, we will present simulations based on a field experiment with incorporation of different catch crops.
An important factor for the development of new crop rotations for stockless organic farming systems is the expected N mineralisation and immobilisation after incorporation of the plant materials. Therefore, special emphasise will be put on the simulation of N-mineralisation/-immobilisation and of soil microbial biomass N. Furthermore, particulate organic matter C and N as an indicator of remaining plant material under decomposition will be investigated
Far infrared CO and HO emission in intermediate-mass protostars
Intermediate-mass young stellar objects (YSOs) provide a link to understand
how feedback from shocks and UV radiation scales from low to high-mass star
forming regions. Aims: Our aim is to analyze excitation of CO and HO in
deeply-embedded intermediate-mass YSOs and compare with low-mass and high-mass
YSOs. Methods: Herschel/PACS spectral maps are analyzed for 6 YSOs with
bolometric luminosities of . The maps
cover spatial scales of AU in several CO and HO lines located
in the m range. Results: Rotational diagrams of CO show two
temperature components at K and
K, comparable to low- and high-mass protostars
probed at similar spatial scales. The diagrams for HO show a single
component at K, as seen in low-mass protostars, and
about K lower than in high-mass protostars. Since the uncertainties in
are of the same order as the difference between the
intermediate and high-mass protostars, we cannot conclude whether the change in
rotational temperature occurs at a specific luminosity, or whether the change
is more gradual from low- to high-mass YSOs. Conclusions: Molecular excitation
in intermediate-mass protostars is comparable to the central AU of
low-mass protostars and consistent within the uncertainties with the high-mass
protostars probed at AU scales, suggesting similar shock
conditions in all those sources.Comment: Accepted to Astronomy & Astrophysics. 4 pages, 5 figures, 3 table
APEX-CHAMP+ high-J CO observations of low-mass young stellar objects: II. Distribution and origin of warm molecular gas
The origin and heating mechanisms of warm (50<T<200 K) molecular gas in
low-mass young stellar objects (YSOs) are strongly debated. Both passive
heating of the inner collapsing envelope by the protostellar luminosity as well
as active heating by shocks and by UV associated with the outflows or accretion
have been proposed. We aim to characterize the warm gas within protosteller
objects, and disentangle contributions from the (inner) envelope, bipolar
outflows and the quiescent cloud. High-J CO maps (12CO J=6--5 and 7--6) of the
immediate surroundings (up to 10,000 AU) of eight low-mass YSOs are obtained
with the CHAMP+ 650/850 GHz array receiver mounted on the APEX telescope. In
addition, isotopologue observations of the 13CO J=6--5 transition and [C I]
3P_2-3P_1 line were taken. Strong quiescent narrow-line 12CO 6--5 and 7--6
emission is seen toward all protostars. In the case of HH~46 and Ced 110 IRS 4,
the on-source emission originates in material heated by UV photons scattered in
the outflow cavity and not just by passive heating in the inner envelope. Warm
quiescent gas is also present along the outflows, heated by UV photons from
shocks. Shock-heated warm gas is only detected for Class 0 flows and the more
massive Class I sources such as HH~46. Outflow temperatures, estimated from the
CO 6--5 and 3--2 line wings, are ~100 K, close to model predictions, with the
exception of the L~1551 IRS 5 and IRAS 12496-7650, for which temperatures <50 K
are found. APEX-CHAMP+ is uniquely suited to directly probe a protostar's
feedback on its accreting envelope gas in terms of heating, photodissociation,
and outflow dispersal by mapping 1'x1' regions in high-J CO and [C I] lines.Comment: 18 pages, accepted by A&A, A version with the figures in higher
quality can be found on my website: http://www.cfa.harvard.edu/~tvankemp
[OI]63micron jets in class 0 sources detected by Herschel
We present Herschel PACS mapping observations of the [OI]63 micron line
towards protostellar outflows in the L1448, NGC1333-IRAS4, HH46, BHR71 and
VLA1623 star forming regions. We detect emission spatially resolved along the
outflow direction, which can be associated with a low excitation atomic jet. In
the L1448-C, HH46 IRS and BHR71 IRS1 outflows this emission is kinematically
resolved into blue- and red-shifted jet lobes, having radial velocities up to
200 km/s. In the L1448-C atomic jet the velocity increases with the distance
from the protostar, similarly to what observed in the SiO jet associated with
this source. This suggests that [OI] and molecular gas are kinematically
connected and that this latter could represent the colder cocoon of a jet at
higher excitation. Mass flux rates (\.M(OI)) have been measured from
the [OI]63micron luminosity adopting two independent methods. We find values in
the range 1-4 10 Mo/yr for all sources but HH46, for which an order of
magnitude higher value is estimated. \.M(OI) are compared with mass
accretion rates (\.M) onto the protostar and with \.M derived
from ground-based CO observations. \.M(OI)/\.M ratios are in
the range 0.05-0.5, similar to the values for more evolved sources.
\.M(OI) in HH46 IRS and IRAS4A are comparable to \.M(CO), while
those of the remaining sources are significantly lower than the corresponding
\.M(CO). We speculate that for these three sources most of the mass
flux is carried out by a molecular jet, while the warm atomic gas does not
significantly contribute to the dynamics of the system.Comment: 37 pages and 12 figures, accepted for publication on Astrophysical
Journa
Bias and temperature dependence of the 0.7 conductance anomaly in Quantum Point Contacts
The 0.7 (2e^2/h) conductance anomaly is studied in strongly confined, etched
GaAs/GaAlAs quantum point contacts, by measuring the differential conductance
as a function of source-drain and gate bias as well as a function of
temperature. We investigate in detail how, for a given gate voltage, the
differential conductance depends on the finite bias voltage and find a
so-called self-gating effect, which we correct for. The 0.7 anomaly at zero
bias is found to evolve smoothly into a conductance plateau at 0.85 (2e^2/h) at
finite bias. Varying the gate voltage the transition between the 1.0 and the
0.85 (2e^2/h) plateaus occurs for definite bias voltages, which defines a gate
voltage dependent energy difference . This energy difference is
compared with the activation temperature T_a extracted from the experimentally
observed activated behavior of the 0.7 anomaly at low bias. We find \Delta =
k_B T_a which lends support to the idea that the conductance anomaly is due to
transmission through two conduction channels, of which the one with its subband
edge \Delta below the chemical potential becomes thermally depopulated as the
temperature is increased.Comment: 9 pages (RevTex) with 9 figures (some in low resolution
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