Terrestrial net primary production (NPP) quantifies the amount of atmospheric carbon fixed by plants and accumulated as biomass. Previous studies have shown that climate constraints were relaxing with increasing temperature and solar radiation, allowing an upward trend in NPP from 1982 through 1999. The past decade (2000 to 2009) has been the warmest since instrumental measurements began, which could imply continued increases in NPP; however, our estimates suggest a reduction in the global NPP of 0.55 petagrams of carbon. Large-scale droughts have reduced regional NPP, and a drying trend in the Southern Hemisphere has decreased NPP in that area, counteracting the increased NPP over the Northern Hemisphere. A continued decline in NPP would not only weaken the terrestrial carbon sink, but it would also intensify future competition between food demand and proposed biofuel production.

Drought-Induced Reduction in Global Terrestrial Net Primary Production from 2000 Through 2009

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High-resolution X-ray computed tomography in geosciences: A review of the current technology and applications

This review discusses the causes and consequences of 'non-equilibrium' water flow and solute transport in large structural pores or macropores (root and earthworm channels, fissures and interaggregate voids). The experimental evidence suggests that pores larger than c. 0.3 mm in equivalent cylindrical diameter allow rapid non-equilibrium flow. Apart from their large size and continuity, this is also due to the presence of impermeable linings and coatings that restrict lateral mass exchange. Macropores also represent microsites in soil that are more biologically active, and often more chemically reactive than the bulk soil. However, sorption retardation during transport through such pores is weaker than in the bulk soil, due to their small surface areas and significant kinetic effects, especially in larger macropores. The potential for non-equilibrium water flow and solute transport at any site depends on the nature of the macropore network, which is determined by the factors of structure formation and degradation, including the abundance and activity of soil biota such as earthworms, soil properties (e.g. clay content), site factors (e.g. slope position, drying intensity, vegetation) and management (e.g. cropping, tillage, traffic). A conceptual model is proposed that summarizes these effects of site factors on the inherent potential for non-equilibrium water flow and solute transport in macropores. Initial and boundary conditions determine the extent to which this potential is realized. High rain intensities clearly increase the strength of non-equilibrium flow in macropores, but the effects of initial water content seem complex, due to the confounding effects of soil shrinkage and water repellency. The impacts of macropore flow on water quality are most significant for relatively immobile solutes that are foreign to the soil and whose effects on ecosystem and human health are pronounced even at small leached fractions (e.g. pesticides). The review concludes with a discussion of topics where process understanding is still lacking, and also suggests some potential applications of the considerable knowledge that has accumulated in recent decades.

A review of non-equilibrium water flow and solute transport in soil macropores: principles, controlling factors and consequences for water quality

. In this forum paper we discuss how soil scientists can help to reach the recently adopted UN Sustainable Development Goals (SDGs) in the most effective manner. Soil science, as a land-related discipline, has important links to several of the SDGs, which are demonstrated through the functions of soils and the ecosystem services that are linked to those functions (see graphical abstract in the Supplement). We explore and discuss how soil scientists can rise to the challenge both internally, in terms of our procedures and practices, and externally, in terms of our relations with colleague scientists in other disciplines, diverse groups of stakeholders and the policy arena. To meet these goals we recommend the following steps to be taken by the soil science community as a whole: (i) embrace the UN SDGs, as they provide a platform that allows soil science to demonstrate its relevance for realizing a sustainable society by 2030; (ii) show the specific value of soil science: research should explicitly show how using modern soil information can improve the results of inter- and transdisciplinary studies on SDGs related to food security, water scarcity, climate change, biodiversity loss and health threats; (iii) take leadership in overarching system analysis of ecosystems, as soils and soil scientists have an integrated nature and this places soil scientists in a unique position; (iii) raise awareness of soil organic matter as a key attribute of soils to illustrate its importance for soil functions and ecosystem services; (iv) improve the transfer of knowledge through knowledge brokers with a soil background; (v) start at the basis: educational programmes are needed at all levels, starting in primary schools, and emphasizing practical, down-to-earth examples; (vi) facilitate communication with the policy arena by framing research in terms that resonate with politicians in terms of the policy cycle or by considering drivers, pressures and responses affecting impacts of land use change; and finally (vii) all this is only possible if researchers, with soil scientists in the front lines, look over the hedge towards other disciplines, to the world at large and to the policy arena, reaching over to listen first, as a basis for genuine collaboration.

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The significance of soils and soil science towards realization of the United Nations sustainable development goals

http://research.engr.oregonstate.edu/immiscibles/sites/research.engr.oregonstate.edu.immiscibles/files/Papers/Wildenschild_Sheppard_AWR_2012.pdf

X-ray imaging and analysis techniques for quantifying pore-scale structure and processes in subsurface porous medium systems

Conventional models of liquid distribution, flow, and solute transport in partially saturated porous media are limited by the representation of media pore space as a bundle of cylindrical capillaries (BCC). Moreover, the capillary model ignores the dominant contribution of adsorptive surface forces and liquid films at low potentials. We propose two new complementary elements for improving our understanding of liquid configuration in porous media: (1) an approach for considering the individual contributions of adsorptive and capillary forces to the matric potential and (2) a more realistic model for pore space geometry. Modern interface science formalism is applied to determine the thickness of adsorbed liquid films as a function of thermodynamic conditions and specific surface area of the medium. The augmented Young-Laplace (AYL) equation provided the necessary framework for combining adsorptive and capillary processes. A new pore space geometry composed of an angular pore cross section (for capillary processes) connected to slit-shaped spaces with internal surface area (for adsorption processes) offers a more realistic representation of natural porous media with explicit consideration of surface area (absent in the standard BCC model). Liquid-vapor configuration, saturation, and liquid-vapor interfacial area were calculated for different potentials and pore (unit cell) dimensions. Pore dimensions may be easily related to measurable soil properties such as specific surface area and porosity. Rigorous calculations based on the AYL equation were simplified and led to the development of algebraic expressions relating saturation and interfacial area of liquid in the proposed pore space geometry to chemical potential. These simple expressions are amenable to upscaling procedures similar to those presently used with the BCC model.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999WR900098

Adsorption and capillary condensation in porous media: Liquid retention and interfacial configurations in angular pores

[1] Nondestructive imaging methods such as X-ray computed tomography (CT) yield high-resolution, three-dimensional representations of pore space and fluid distribution within porous materials. Steadily increasing computational capabilities and easier access to X-ray CT facilities have contributed to a recent surge in microporous media research with objectives ranging from theoretical aspects of fluid and interfacial dynamics at the pore scale to practical applications such as dense nonaqueous phase liquid transport and dissolution. In recent years, significant efforts and resources have been devoted to improve CT technology, microscale analysis, and fluid dynamics simulations. However, the development of adequate image segmentation methods for conversion of gray scale CT volumes into a discrete form that permits quantitative characterization of pore space features and subsequent modeling of liquid distribution and flow processes seems to lag. In this paper we investigated the applicability of various thresholding and locally adaptive segmentation techniques for industrial and synchrotron X-ray CT images of natural and artificial porous media. A comparison between directly measured and image-derived porosities clearly demonstrates that the application of different segmentation methods as well as associated operator biases yield vastly differing results. This illustrates the importance of the segmentation step for quantitative pore space analysis and fluid dynamics modeling. Only a few of the tested methods showed promise for both industrial and synchrotron tomography. Utilization of local image information such as spatial correlation as well as the application of locally adaptive techniques yielded significantly better results.

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2009WR008087

Segmentation of X‐ray computed tomography images of porous materials: A crucial step for characterization and quantitative analysis of pore structures

A new model for liquid configuration in angular pore space considering both capillary and adsorptive contributions was proposed as an alternative to the conventional bundle of capillaries representation. In this study we develop a statistical framework for upscaling pore-scale processes to represent a sample-scale response of variably saturated porous medium. The representation of pore size distribution by the gamma distribution enables derivation of closed-form expressions for sample-scale liquid retention and liquid-vapor interfacial area. The statistical framework calculates the expected values of liquid configuration as a function of pore geometry and chemical potential considerations. Media properties are used to estimate upscaling parameters by matching model predictions with measured retention data subject to specific surface area constraint. Additionally, a method for estimating liquid-solid adsorption behavior for the medium is proposed. Model predictions compare favorably with measured retention data, yielding a similar close fit as obtained with the van Genuchten parametric model. Liquid-vapor interfacial area as a function of chemical potential is readily calculated using the estimated retention parameters. Model calculations of liquid-vapor interfacial area for sand show reasonable agreement with measurements obtained with surface-active tracers. The contribution of liquid films dominates the total liquid-vapor interfacial area and often surpasses the capillary contribution (curved menisci) by several orders of magnitude. This illustrates potential limitations in using cylindrical pore network modeling of interfacial area for multiphase flow predictions. The detailed picture of liquid vapor interfaces provides a sound basis for unsaturated hydraulic conductivity calculations in the sample cross section (i.e., neglecting network effects) and offers insights into microbial habitats and related exchange processes in partially saturated porous media.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999WR900262

Liquid Retention and Interfacial Area in Variably Saturated Porous Media: Upscaling from Single Pore to Sample Scale Model

Many models for hydraulic conductivity of partially saturated porous media rely on oversimplified representation of the pore space as a bundle of cylindrical capillaries and disregard flow in liquid films. Recent progress in modeling liquid behavior in angular pores of partially saturated porous media offers an alternative framework. We assume that equilibrium liquid-vapor interfaces provide well-defined and stable boundaries for slow laminar film and corner flow regimes in pore space comprised of angular pores connected to slit-shaped spaces. Knowledge of liquid configuration in the assumed geometry facilitates calculation of average liquid velocities in films and corners and enables derivation of pore-scale hydraulic conductivity as a function of matric potential. The pore-scale model is statistically upscaled to represent hydraulic conductivity for a sample of porous medium. Model parameters for the analytical sample-scale expressions are estimated from measured liquid retention data and other measurable medium properties. Model calculations illustrate the important role of film flow, whose contribution dominates capillary flow (in full pores and corners) at relatively high matric potentials (approximately −100 to −300 J kg−1, or −1 to 3 bars). The crossover region between film and capillary flow is marked by a significant change in the slope of the hydraulic conductivity function as often observed in measurements. Model predictions are compared with the widely applied van Genuchten–Mualem model and yield reasonable agreement with measured retention and hydraulic conductivity data over a wide range of soil textural classes.

Hydraulic conductivity of variably saturated porous media: Film and corner flow in angular pore space

[1] Individual contributions of capillarity and adsorptive surface forces to the matric potential are seldom differentiated in determination of soil water characteristic (SWC) curves. Typically, capillary forces dominate at the wet end, whereas adsorptive surface forces dominate at the dry end of a SWC where water is held as thin liquid films. The amount of adsorbed soil water is intimately linked to soil specific surface area (SA) and plays an important role in various biological and transport processes in arid environments. Dominated by van der Waals adsorptive forces, surface-water interactions give rise to a nearly universal scaling relationship for SWC curves at low water contents. We demonstrate that scaling measured water content at the dry end by soil specific surface area yields remarkable similarity across a range of soil textures and is in good agreement with theoretical predictions based on van der Waals interactions. These scaling relationships are important for accurate description of SWC curves in dry soils and may provide rapid and reliable estimates of soil specific surface area from SWC measurements for matric potentials below ‒10 MPa conveniently measured with the chilled-mirror dew point technique. Surface area estimates acquired by fitting the scaling relationship to measured SWC data were in good agreement with SA data measured by standard methods. Preliminary results suggest that the proposed method could provide reliable SA estimates for natural soils with hydratable surface areas smaller than 200 m2/g.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005WR004142

Markus Tuller

Papers

Adsorption and capillary condensation in porous media: Liquid retention and interfacial configurations in angular pores

Segmentation of X‐ray computed tomography images of porous materials: A crucial step for characterization and quantitative analysis of pore structures

Liquid Retention and Interfacial Area in Variably Saturated Porous Media: Upscaling from Single Pore to Sample Scale Model

Hydraulic conductivity of variably saturated porous media: Film and corner flow in angular pore space

Water films and scaling of soil characteristic curves at low water contents