Pore space and its geometrical character are crucial to soil transport phenomena, the soil’s response to alteration, and its interaction with other components of the biosphere. Porosity, the fraction of soil volume not occupied by solids, is relatively easy to conceptualize and measure. Pore size distribution is a complex topic, in part from the lack of a clear and unique concept of a soil pore as a discrete object. With conventions and operational definitions derived from hydraulic concepts, it is possible to construct representations of pore size distributions that have great value in assessing soil dynamics and transport properties.

/pdf/porosity-and-pore-size-distribution-1j39iqs9tf.pdf

Porosity and pore-size distribution

River networks represent the largest biogeochemical nexus between the continents, ocean and atmosphere. Our current understanding of the role of rivers in the global carbon cycle remains limited, which makes it difficult to predict how global change may alter the timing and spatial distribution of riverine carbon sequestration and greenhouse gas emissions. Here we review the state of river ecosystem metabolism research and synthesize the current best available estimates of river ecosystem metabolism. We quantify the organic and inorganic carbon flux from land to global rivers and show that their net ecosystem production and carbon dioxide emissions shift the organic to inorganic carbon balance en route from land to the coastal ocean. Furthermore, we discuss how global change may affect river ecosystem metabolism and related carbon fluxes and identify research directions that can help to develop better predictions of the effects of global change on riverine ecosystem processes. We argue that a global river observing system will play a key role in understanding river networks and their future evolution in the context of the global carbon budget. 

/pdf/river-ecosystem-metabolism-and-carbon-biogeochemistry-in-a-6o4rqy6j.pdf

River ecosystem metabolism and carbon biogeochemistry in a changing world

Environmental disturbances seem to be increasing in frequency and impact, yet we have little understanding of the belowground impacts of these events. Soil fauna, while widely acknowledged to be important drivers of biogeochemical function, soil structure and sustainability, and trophic interactions, are understudied compared to other belowground organisms such as archaea, bacteria, and fungi. In this review we summarize the current state of knowledge of soil fauna as it relates to and is influenced by various disturbances. We focus our review on three main natural and anthropogenic disturbance types: 1) natural disturbances, including damage from wind and flooding; 2) invasive species, including above and belowground flora and fauna; and 3) climate change impacts on the atmosphere and temperature. We do not address the impacts of wildfires, forestry, agricultural practices, mining, or human-caused pollution, as these topics have all been covered in other works. We highlight knowledge gaps and suggest future avenues of research, with hope that the importance of soil fauna and their influences on ecosystems will be given greater emphasis in future research.

/pdf/soil-fauna-responses-to-natural-disturbances-invasive-6litjgv5m9.pdf

Soil fauna responses to natural disturbances, invasive species, and global climate change: Current state of the science and a call to action

There is a need for a conceptual and analytical model to describe and interpret the common shrinkage curves of structured soil samples. In this work, we propose a new device for direct shrinkage measurement of unconfined structured soil samples and discuss the relevance of parametric models of the shrinkage curve to fit the experimental data. The experimental procedure consists of simultaneous and continuous measurements of the diameter, height, and weight of an initially saturated soil sample as it dries. The shrinkage measurement can be completed for the full moisture range in a short time (2-3 d), and all shrinkage phases can be established easily and accurately identified. The points of transition between the different shrinkage zones of the shrinkage curve are considered as characteristics of the shrinkage process. They are used as parameters to model the experimental shrinkage curves by linear shrinkage zones separated by curvilinear transition ones. The efficiency of two parametric models to fit the experimental data and to provide the characteristic points of the curve are compared and discussed. A new procedure for fitting the shrinkage curves of structured soil samples and for determining the best position of the characteristic points on the curves is proposed.

New Device and Method for Soil Shrinkage Curve Measurement and Characterization

The availability of methods for quasi-continuous measurements of soil shrinkage curves allowed the development of new models. The exponential (XP) model allows the calculation of the volume of two pore phases in the soil, namely macro and micropore volumes. The micropore volume is identified with the pore volume of the soil clay matrix according to the assumption that the maximum swelling of the clay matrix (MS) is the point of minimum water content of the structural shrinkage (Point D). This is discussed using undisturbed and repacked soil samples with various clay contents and clay types. The slope of the shrinkage curves as a function of equivalent saturated-pore radius show a transition in pore type around a 10-μm pore radius, where smaller and more deformable pores start to desaturate. This corresponds to the fitted D point and is close to the size of the largest pores in the clay matrix or clay–silt phase reported in the literature. The calculated micropore volume and micropore swelling properties are close to clay paste properties reported in the literature. At low water content, the specific micropore volume is independent from clay content. At Point D, the shrinking capacity of the specific micropore volume decreases with increasing clay content for clay contents below 40%. Our results show that Point D can be identified with the MS of the clay matrix, and that the XP model can be used to calculate the swelling properties of clays in the soil, without extraction.

Relationship between Clay Content, Clay Type, and Shrinkage Properties of Soil Samples

Carbon monitoring is critical for the reporting and verification of carbon stocks and change. Remote sensing is a tool increasingly used to estimate the spatial heterogeneity, extent and change of carbon stocks within and across various systems. We designate the use of the term wet carbon system to the interconnected wetlands, ocean, river and streams, lakes and ponds, and permafrost, which are carbon-dense and vital conduits for carbon throughout the terrestrial and aquatic sections of the carbon cycle. We reviewed wet carbon monitoring studies that utilize earth observation to improve our knowledge of data gaps, methods, and future research recommendations. To achieve this, we conducted a systematic review collecting 1622 references and screening them with a combination of text matching and a panel of three experts. The search found 496 references, with an additional 78 references added by experts. Our study found considerable variability of the utilization of remote sensing and global wet carbon monitoring progress across the nine systems analyzed. The review highlighted that remote sensing is routinely used to globally map carbon in mangroves and oceans, whereas seagrass, terrestrial wetlands, tidal marshes, rivers, and permafrost would benefit from more accurate and comprehensive global maps of extent. We identified three critical gaps and twelve recommendations to continue progressing wet carbon systems and increase cross system scientific inquiry.

A review of carbon monitoring in wet carbon systems using remote sensing

Soil-Volume Changes and Accompanying Moisture and Pore-Space Relationships1

Inland wetlands are critical carbon reservoirs storing 30% of global soil organic carbon (SOC) within 6% of the land surface. However, forested regions contain SOC-rich wetlands that are not included in current maps, which we refer to as 'cryptic carbon'. Here, to demonstrate the magnitude and distribution of cryptic carbon, we measure and map SOC stocks as a function of a continuous, upland-to-wetland gradient across the Hoh River Watershed (HRW) in the Pacific Northwest of the U.S., comprising 68,145 ha. Total catchment SOC at 30 cm depth (5.0 TgC) is between estimates from global SOC maps (GSOC: 3.9 TgC; SoilGrids: 7.8 TgC). For wetland SOC, our 1 m stock estimates are substantially higher (Mean: 259 MgC ha-1; Total: 1.7 TgC) compared to current wetland-specific SOC maps derived from a combination of U.S. national datasets (Mean: 184 MgC ha-1; Total: 0.3 TgC). We show that total unmapped or cryptic carbon is 1.5 TgC and when added to current estimates, increases the estimated wetland SOC stock to 1.8 TgC or by 482%, which highlights the vast stores of SOC that are not mapped and contained in unprotected and vulnerable wetlands. 

Revealing the hidden carbon in forested wetland soils

Accurate, un-biased wetland inventories are critical to monitor and protect wetlands from future harm or land conversion. 10 However, most wetland inventories are constructed through manual image interpretation or automated classification of multi-band imagery and are biased towards wetlands that are easy to detect directly in aerial and satellite imagery. Wetlands that are obscured by forest canopy, occur ephemerally, and those without visible standing water are, therefore, often missing from wetland maps. To aid in detection of these cryptic wetlands, we developed the Wetland Intrinsic Potential tool, based on a wetland indicator framework commonly used on the ground to detect wetlands through the presence of hydrophytic vegetation, 15 hydrology, and hydric soils. Our tool uses a random forest model with spatially explicit input variables that represent all three wetland indicators, including novel multi-scale topographic indicators that represent the processes that drive wetland formation, to derive a map of wetland probability. With the ability to include multi-scale topographic indicators, the WIP tool can identify areas conducive to wetland formation and provides a flexible approach that can be adapted to diverse landscapes. For a study area in the Hoh River Basin in Western Washington, USA, classification of the output probability with a threshold 20 of 0.5 provided an overall accuracy of 91.97%. Compared to the National Wetland Inventory, the classified WIP-tool output increased areas classified as wetland by 160% and reduced errors of omission from 47.5% to 14.1%, but increased errors of commission from 1.9% to 10.5%. The WIP tool is implemented using a combination of R and python scripts in ArcGIS.

The Wetland Intrinsic Potential tool: Mapping wetland intrinsic potential through machine learning of multi-scale remote sensing proxies of wetland indicators

Cryptic carbon: The hidden carbon in forested wetland soils

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