Browsing by Author "Katul, GG"
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Item Open Access Analysis of soil carbon transit times and age distributions using network theories(Journal of Geophysical Research: Biogeosciences, 2009-01-01) Manzoni, S; Katul, GG; Porporato, AThe long-term soil carbon dynamics may be approximated by networks of linear compartments, permitting theoretical analysis of transit time (i.e., the total time spent by a molecule in the system) and age (the time elapsed since the molecule entered the system) distributions. We compute and compare these distributions for different network. configurations, ranging from the simple individual compartment, to series and parallel linear compartments, feedback systems, and models assuming a continuous distribution of decay constants. We also derive the transit time and age distributions of some complex, widely used soil carbon models (the compartmental models CENTURY and Rothamsted, and the continuous-quality Q-Model), and discuss them in the context of long-term carbon sequestration in soils. We show how complex models including feedback loops and slow compartments have distributions with heavier tails than simpler models. Power law tails emerge when using continuous-quality models, indicating long retention times for an important fraction of soil carbon. The responsiveness of the soil system to changes in decay constants due to altered climatic conditions or plant species composition is found to be stronger when all compartments respond equally to the environmental change, and when the slower compartments are more sensitive than the faster ones or lose more carbon through microbial respiration. Copyright 2009 by the American Geophysical Union.Item Open Access Boom and bust carbon-nitrogen dynamics during reforestation(Ecological Modelling, 2017-09-24) Parolari, AJ; Mobley, ML; Bacon, AR; Katul, GG; Richter, DDB; Porporato, A© 2017 Elsevier B.V. Legacies of historical land use strongly shape contemporary ecosystem dynamics. In old-field secondary forests, tree growth embodies a legacy of soil changes affected by previous cultivation. Three patterns of biomass accumulation during reforestation have been hypothesized previously, including monotonic to steady state, non-monotonic with a single peak then decay to steady state, and multiple oscillations around the steady state. In this paper, the conditions leading to the emergence of these patterns is analyzed. Using observations and models, we demonstrate that divergent reforestation patterns can be explained by contrasting time-scales in ecosystem carbon-nitrogen cycles that are influenced by land use legacies. Model analyses characterize non-monotonic plant-soil trajectories as either single peaks or multiple oscillations during an initial transient phase controlled by soil carbon-nitrogen conditions at the time of planting. Oscillations in plant and soil pools appear in modeled systems with rapid tree growth and low initial soil nitrogen, which stimulate nitrogen competition between trees and decomposers and lead the forest into a state of acute nitrogen deficiency. High initial soil nitrogen dampens oscillations, but enhances the magnitude of the tree biomass peak. These model results are supported by data derived from the long-running Calhoun Long-Term Soil-Ecosystem Experiment from 1957 to 2007. Observed carbon and nitrogen pools reveal distinct tree growth and decay phases, coincident with soil nitrogen depletion and partial re-accumulation. Further, contemporary tree biomass loss decreases with the legacy soil C:N ratio. These results support the idea that non-monotonic reforestation trajectories may result from initial transients in the plant-soil system affected by initial conditions derived from soil changes associated with land-use history.Item Open Access Causality across rainfall time scales revealed by continuous wavelet transforms(JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 2010-07-31) Molini, A; Katul, GG; Porporato, AItem Open Access Estimation of in-canopy ammonia sources and sinks in a fertilized Zea mays field.(Environ Sci Technol, 2010-03-01) Bash, JO; Walker, JT; Katul, GG; Jones, MR; Nemitz, E; Robarg, WPAn analytical model was developed to describe in-canopy vertical distribution of ammonia (NH(3)) sources and sinks and vertical fluxes in a fertilized agricultural setting using measured in-canopy mean NH(3) concentration and wind speed profiles. This model was applied to quantify in-canopy air-surface exchange rates and above-canopy NH(3) fluxes in a fertilized corn (Zea mays) field. Modeled air-canopy NH(3) fluxes agreed well with independent above-canopy flux estimates. Based on the model results, the urea fertilized soil surface was a consistent source of NH(3) one month following the fertilizer application, whereas the vegetation canopy was typically a net NH(3) sink with the lower portion of the canopy being a constant sink. The model results suggested that the canopy was a sink for some 70% of the estimated soil NH(3) emissions. A logical conclusion is that parametrization of within-canopy processes in air quality models are necessary to explore the impact of agricultural field level management practices on regional air quality. Moreover, there are agronomic and environmental benefits to timing liquid fertilizer applications as close to canopy closure as possible. Finally, given the large within-canopy mean NH(3) concentration gradients in such agricultural settings, a discussion about the suitability of the proposed model is also presented.Item Open Access Estimation of long-term basin scale evapotranspiration from streamflow time series(Water Resources Research, 2010-10-29) Palmroth, S; Katul, GG; Hui, D; McCarthy, HR; Jackson, RB; Oren, RWe estimated long-term annual evapotranspiration (ETQ) at the watershed scale by combining continuous daily streamflow (Q) records, a simplified watershed water balance, and a nonlinear reservoir model. Our analysis used Q measured from 11 watersheds (area ranged from 12 to 1386 km 2) from the uppermost section of the Neuse River Basin in North Carolina, USA. In this area, forests and agriculture dominate the land cover and the spatial variation in climatic drivers is small. About 30% of the interannual variation in the basin-averaged ETQ was explained by the variation in precipitation (P), while ETQ showed a minor inverse correlation with pan evaporation. The sum of annual Q and ETQ was consistent with the independently measured P. Our analysis shows that records of Q can provide approximate, continuous estimates of long-term ET and, thereby, bounds for modeling regional fluxes of water and of other closely coupled elements, such as carbon. Copyright 2010 by the American Geophysical Union.Item Open Access Predicting the dry deposition of aerosol-sized particles using layer-resolved canopy and pipe flow analogy models: Role of turbophoresis(Journal of Geophysical Research Atmospheres, 2010-01-01) Katul, GG; Grönholm, T; Launiainen, S; Vesala, TA number of synthesis activities, mathematical modeling, and experiments on dry deposition of aerosol-sized particles over forested surfaces point to three disjointed findings: (1) deposition velocities measured over tall forests do not support a clearly defined minimum for particle sizes in the range of 0.1-2 m; (2) when measurements of the normalized deposition velocity (V d+) are presented as a function of the normalized particle timescale (p+), where the normalizing variables are the friction velocity and air viscosity, a power law scaling in the form of V d+ ∼ (p+)2 emerges in the so-called inertial-impaction regime for many laboratory and crop experiments, but none of the forest measurements fall on this apparent scaling law; and (3) two recent models with entirely different assumptions about the representation of the particle deposition process reproduce common data sets for forests. We show that turbophoresis, when accounted for at the leaf scale in vertically resolved or multilayer models (MLMs), provides a coherent explanation for the first two findings and sheds light on the third. The MLM resolves the canopy vertical structure and its effects on both the flow statistics and the leaf particle collection mechanisms. The proposed MLM predictions agree with a recent two-level particle-resolving data set collected over 1 year duration for a Scots pine stand in Hyytil (southern Finland). Such an approach can readily proportion the particle deposition onto foliage and forest floor and can take advantage of recent advances in measurements of canopy structural properties derived from remote sensing platforms. Copyright 2010 by the American Geophysical Union.Item Open Access Role of microtopography in rainfall-runoff partitioning: An analysis using idealized geometry(Water Resources Research, 2010) Thompson, SE; Katul, GG; Porporato, AItem Open Access Scale-wise evolution of rainfall probability density functions fingerprints the rainfall generation mechanism(Geophysical Research Letters, 2010-01-01) Molini, A; Katul, GG; Porporato, A© 2010 by the American Geophysical Union.The cross-scale probabilistic structure of rainfall intensity records collected over time scales ranging from hours to decades at sites dominated by both convective and frontal systems is investigated. Across these sites, intermittency build-up from slow to fast time-scales is analyzed in terms of heavy tailed and asymmetric signatures in the scale-wise evolution of rainfall probability density functions (pdfs). The analysis demonstrates that rainfall records dominated by convective storms develop heavier-Tailed power law pdfs toward finer scales when compared with their frontal systems counterpart. Also, a concomitant marked asymmetry build-up emerges at such finer time scales. A scale-dependent probabilistic description of such fat tails and asymmetry appearance is proposed based on a modified q-Gaussian model, able to describe the cross-scale rainfall pdfs in terms of the nonextensivity parameter q, a lacunarity (intermittency) correction and a tail asymmetry coefficient, linked to the rainfall generation mechanism.Item Open Access The rainfall-no rainfall transition in a coupled land-convective atmosphere system(Geophysical Research Letters, 2010-07-01) Konings, AG; Katul, GG; Porporato, AA one-dimensional representation of the atmospheric boundary layer (ABL) depth is coupled to a soil moisture bucket model to dynamically explore the relative roles of surface and free atmospheric conditions on convective precipitation occurrence and resulting soil moisture states. This occurrence is taken to depend on the crossing of the ABL height and the lifting condensation level in the presence of pure convective instability. If rainfall occurs (unrealistically) whenever these conditions are met, and free atmospheric conditions are constant, the resulting system state evolves towards a limit cycle with precipitation every day or every few days, or to a completely dry state. The free atmospheric humidity profile has a larger effect on determining the stationary soil moisture state than the temperature profile. The effect of dry air entrainment on surface energy partitioning decreases soil moisture sensitivity to free atmospheric conditions. © 2010 by the American Geophysical Union.Item Open Access Transport in a coordinated soil-root-xylem-phloem leaf system(Advances in Water Resources, 2018-09) Huang, CW; Domec, JC; Palmroth, S; Pockman, WT; Litvak, ME; Katul, GG© 2018 Elsevier Ltd Links between the carbon and water economies of plants are coupled by combining the biochemical demand for atmospheric CO2with gas transfer through stomates, liquid water transport in the soil-xylem hydraulic system and sucrose export in the phloem. We formulated a model to predict stomatal conductance (gs), consistent with the maximum energy circulation concept of Lotka and Odum, by maximizing the sucrose flux out of photosynthesizing leaves. The proposed modeling approach recovers all prior results derived from stomatal optimization theories and profit-maximization arguments for the xylem hydraulic system aimed at predicting gs. The novel features of this approach are its ability to 1) predict the price of losing water in carbon units using xylem and phloem properties (i.e., the marginal water use efficiency) and 2) explain why water molecules become more expensive to exchange for CO2molecules when soil moisture becomes limiting or when plants acclimate to new elevated atmospheric CO2concentration. On short time-scales (sub-daily), predicted gsunder many environmental stimuli were consistent with measurements reported in the literature, including a general sensitivity of gsto vapor pressure deficit and leaf water potential. During progressive droughts, differences in the coordination among the leaf, xylem, and phloem functioning determine the isohydric-to-anisohydric behavior among plants.Item Open Access Vegetation-infiltration relationships across climatic and soil type gradients(Journal of Geophysical Research-Biogeosciences, 2010) Thompson, SE; Harman, CJ; Heine, P; Katul, GG