265 research outputs found
Do cover crops increase or decrease nitrous oxide emissions? A meta-analysis
There are many environmental benefits to incorporating cover crops into crop rotations, such as their potential to decrease soil erosion, reduce nitrate (NO3) leaching, and increase soil organic matter. Some of these benefits impact other agroecosystem processes, such as greenhouse gas emissions. In particular, there is not a consensus in the literature regarding the effect of cover crops on nitrous oxide (N2O) emissions. Compared to site-specific studies, meta-analysis can provide a more general investigation into these effects. Twenty-six peer-reviewed articles including 106 observations of cover crop effects on N2O emissions from the soil surface were analyzed according to their response ratio, the natural log of the N2O flux with a cover crop divided by the N2O flux without a cover crop (LRR). Forty percent of the observations had negative LRRs, indicating a cover crop treatment which decreased N2O, while 60% had positive LRRs indicating a cover crop treatment which increased N2O. There was a significant interaction between N rate and the type of cover crop where legumes had higher LRRs at lower N rates than nonlegume species. When cover crop residues were incorporated into the soil, LRRs were significantly higher than those where residue was not incorporated. Geographies with higher total precipitation and variability in precipitation tended to produce higher LRRs. Finally, data points measured during cover crop decomposition had large positive LRRs and were larger than those measured when the cover crop was alive. In contrast, those data points measuring for a full year had LRRs close to zero, indicating that there was a balance between periods when cover crops increased N2O and periods when cover crops decreased emissions. Therefore, N2O measurements over the entire year may be needed to determine the net effect of cover crops on N2O. The data included in this meta-analysis indicate some overarching crop management practices that reduce direct N2O emissions from the soil surface, such as no soil incorporation of residues and use of nonlegume cover crop species. However, our results demonstrate that cover crops do not always reduce direct N2O emissions from the soil surface in the short term and that more work is needed to understand the full global warming potential of cover crop management
Comparison of Cellulosic Ethanol Yields from Midwestern Maize and Reconstructed Tallgrass Prairie Systems Managed for Bioenergy
Maize- and prairie-based systems were investigated as cellulosic feedstocks by conducting a 9 ha side-by-side comparison on fertile soils in the Midwestern United States. Maize was grown continuously with adequate fertilization over years both with and without a winter rye cover crop, and the 31-species reconstructed prairie was grown with and without spring nitrogen fertilization. Both maize stover and prairie biomass were harvested in the fall. We compared amounts of cellulosic biomass produced and harvested, carbohydrate contents as measured by both dietary and detergent methods, and estimated cellulosic ethanol yields per hectare. From 2009â2013, the cropping system with the largest non-grain biomass yield was fertilized prairie, averaging 10.4 Mg haâ1 yearâ1 aboveground biomass with average harvest removals of 7.8 Mg haâ1 yearâ1. The unfertilized prairie produced 7.4 Mg haâ1 yearâ1 aboveground biomass, with average harvests of 5.3 Mg haâ1 yearâ1. Lowest cellulosic (non-grain) biomass harvests were obtained from continuous maize systems, averaging 3.5 Mg haâ1 yearâ1 when grown with, and 3.7 Mg haâ1 yearâ1 when grown without a winter rye cover crop, respectively. Unfertilized prairie biomass and maize stover had equivalent dietary-determined potential biomass ethanol yields at 330 g ethanol kgâ1 dry biomass, but fertilized prairie was lower at 315. The detergent method did not accurately capture these differences. Over the five-year period of the experiment, unfertilized and fertilized prairie systems averaged 810 and 1,790 L potential cellulosic ethanol haâ1 yearâ1 more than the maize systems, respectively. Differences in harvested biomass accounted for \u3e90 % of ethanol yield variation
A nonlinear mixed-effects modeling approach for ecological data: Using temporal dynamics of vegetation moisture as an example
Increasingly, often ecologist collects data with nonlinear trends, heterogeneous variances, temporal correlation, and hierarchical structure. Nonlinear mixed-effects models offer a flexible approach to such data, but the estimation and interpretation of these models present challenges, partly associated with the lack of worked examples in the ecological literature. We illustrate the nonlinear mixed-effects modeling approach using temporal dynamics of vegetation moisture with field data from northwestern Patagonia. This is a Mediterranean-type climate region where modeling temporal changes in live fuel moisture content are conceptually relevant (ecological theory) and have practical implications (fire management). We used this approach to answer whether moisture dynamics varies among functional groups and aridity conditions, and compared it with other simpler statistical models. The modeling process is set out âstep-by-stepâ: We start translating the ideas about the system dynamics to a statistical model, which is made increasingly complex in order to include different sources of variability and correlation structures. We provide guidelines and R scripts (including a new self-starting function) that make data analyses reproducible. We also explain how to extract the parameter estimates from the R output. Our modeling approach suggests moisture dynamic to vary between grasses and shrubs, and between grasses facing different aridity conditions. Compared to more classical models, the nonlinear mixed-effects model showed greater goodness of fit and met statistical assumptions. While the mixed-effects approach accounts for spatial nesting, temporal dependence, and variance heterogeneity; the nonlinear function allowed to model the seasonal pattern. Parameters of the nonlinear mixed-effects model reflected relevant ecological processes. From an applied perspective, the model could forecast the time when fuel moisture becomes critical to fire occurrence. Due to the lack of worked examples for nonlinear mixed-effects models in the literature, our modeling approach could be useful to diverse ecologists dealing with complex data.Fil: Oddi, Facundo JosĂ©. Universidad Nacional de RĂo Negro. Sede Andina. Instituto de Investigaciones en Recursos Naturales, AgroecologĂa y Desarrollo Rural; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; ArgentinaFil: Miguez, Fernando E.. University of Iowa; Estados UnidosFil: Ghermandi, Luciana. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - Patagonia Norte. Instituto de Investigaciones en Biodiversidad y Medioambiente. Universidad Nacional del Comahue. Centro Regional Universidad Bariloche. Instituto de Investigaciones en Biodiversidad y Medioambiente; ArgentinaFil: Bianchi, Lucas Osvaldo. Universidad Nacional de RĂo Negro. Sede Andina. Instituto de Investigaciones en Recursos Naturales, AgroecologĂa y Desarrollo Rural; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; ArgentinaFil: Garibaldi, Lucas Alejandro. Universidad Nacional de RĂo Negro. Sede Andina. Instituto de Investigaciones en Recursos Naturales, AgroecologĂa y Desarrollo Rural; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; Argentin
Lowering the radioactivity of the photomultiplier tubes for the XENON1T dark matter experiment
The low-background, VUV-sensitive 3-inch diameter photomultiplier tube R11410
has been developed by Hamamatsu for dark matter direct detection experiments
using liquid xenon as the target material. We present the results from the
joint effort between the XENON collaboration and the Hamamatsu company to
produce a highly radio-pure photosensor (version R11410-21) for the XENON1T
dark matter experiment. After introducing the photosensor and its components,
we show the methods and results of the radioactive contamination measurements
of the individual materials employed in the photomultiplier production. We then
discuss the adopted strategies to reduce the radioactivity of the various PMT
versions. Finally, we detail the results from screening 216 tubes with
ultra-low background germanium detectors, as well as their implications for the
expected electronic and nuclear recoil background of the XENON1T experiment.Comment: 10 pages, 5 figure
Search for Event Rate Modulation in XENON100 Electronic Recoil Data
We have searched for periodic variations of the electronic recoil event rate
in the (2-6) keV energy range recorded between February 2011 and March 2012
with the XENON100 detector, adding up to 224.6 live days in total. Following a
detailed study to establish the stability of the detector and its background
contributions during this run, we performed an un-binned profile likelihood
analysis to identify any periodicity up to 500 days. We find a global
significance of less than 1 sigma for all periods suggesting no statistically
significant modulation in the data. While the local significance for an annual
modulation is 2.8 sigma, the analysis of a multiple-scatter control sample and
the phase of the modulation disfavor a dark matter interpretation. The
DAMA/LIBRA annual modulation interpreted as a dark matter signature with
axial-vector coupling of WIMPs to electrons is excluded at 4.8 sigma.Comment: 6 pages, 4 figure
Removing krypton from xenon by cryogenic distillation to the ppq level
The XENON1T experiment aims for the direct detection of dark matter in a
cryostat filled with 3.3 tons of liquid xenon. In order to achieve the desired
sensitivity, the background induced by radioactive decays inside the detector
has to be sufficiently low. One major contributor is the -emitter
Kr which is an intrinsic contamination of the xenon. For the XENON1T
experiment a concentration of natural krypton in xenon Kr/Xe < 200
ppq (parts per quadrillion, 1 ppq = 10 mol/mol) is required. In this
work, the design of a novel cryogenic distillation column using the common
McCabe-Thiele approach is described. The system demonstrated a krypton
reduction factor of 6.410 with thermodynamic stability at process
speeds above 3 kg/h. The resulting concentration of Kr/Xe < 26 ppq
is the lowest ever achieved, almost one order of magnitude below the
requirements for XENON1T and even sufficient for future dark matter experiments
using liquid xenon, such as XENONnT and DARWIN
Search for Two-Neutrino Double Electron Capture of Xe with XENON100
Two-neutrino double electron capture is a rare nuclear decay where two
electrons are simultaneously captured from the atomic shell. For Xe
this process has not yet been observed and its detection would provide a new
reference for nuclear matrix element calculations. We have conducted a search
for two-neutrino double electron capture from the K-shell of Xe using
7636 kgd of data from the XENON100 dark matter detector. Using a
Bayesian analysis we observed no significant excess above background, leading
to a lower 90 % credibility limit on the half-life
yr. We also evaluated the sensitivity of the XENON1T experiment, which is
currently being commissioned, and find a sensitivity of
yr after an exposure of 2 tyr.Comment: 6 pages, 4 figure
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