http://www.tinread.usarb.md:8888/tinread/fulltext/lal/soil_structure.pdf

Soil structure and management: a review

Summary
Mechanisms for C stabilization in soils have received much interest recently due to their relevance in the global C cycle. Here we review the mechanisms that are currently, but often contradictorily or inconsistently, considered to contribute to organic matter (OM) protection against decomposition in temperate soils: (i) selective preservation due to recalcitrance of OM, including plant litter, rhizodeposits, microbial products, humic polymers, and charred OM; (ii) spatial inaccessibility of OM against decomposer organisms due to occlusion, intercalation, hydrophobicity and encapsulation; and (iii) stabilization by interaction with mineral surfaces (Fe-, Al-, Mn-oxides, phyllosilicates) and metal ions. Our goal is to assess the relevance of these mechanisms to the formation of soil OM during different stages of decomposition and under different soil conditions. The view that OM stabilization is dominated by the selective preservation of recalcitrant organic components that accumulate in proportion to their chemical properties can no longer be accepted. In contrast, our analysis of mechanisms shows that: (i) the soil biotic community is able to disintegrate any OM of natural origin; (ii) molecular recalcitrance of OM is relative, rather than absolute; (iii) recalcitrance is only important during early decomposition and in active surface soils; while (iv) during late decomposition and in the subsoil, the relevance of spatial inaccessibility and organo-mineral interactions for SOM stabilization increases. We conclude that major difficulties in the understanding and prediction of SOM dynamics originate from the simultaneous operation of several mechanisms. We discuss knowledge gaps and promising directions of future research.

Stabilization of organic matter in temperate soils: mechanisms and their relevance under different soil conditions – a review

Many physical, chemical, mineralogical, and biological soil properties can be affected by forest fires. The effects are chiefly a result of burn severity, which consists of peak temperatures and duration of the fire. Climate, vegetation, and topography of the burnt area control the resilience of the soil system; some fire-induced changes can even be permanent. Low to moderate severity fires, such as most of those prescribed in forest management, promote renovation of the dominant vegetation through elimination of undesired species and transient increase of pH and available nutrients. No irreversible ecosystem change occurs, but the enhancement of hydrophobicity can render the soil less able to soak up water and more prone to erosion. Severe fires, such as wildfires, generally have several negative effects on soil. They cause significant removal of organic matter, deterioration of both structure and porosity, considerable loss of nutrients through volatilisation, ash entrapment in smoke columns, leaching and erosion, and marked alteration of both quantity and specific composition of microbial and soil-dwelling invertebrate communities. However, despite common perceptions, if plants succeed in promptly recolonising the burnt area, the pre-fire level of most properties can be recovered and even enhanced. This work is a review of the up-to-date literature dealing with changes imposed by fires on properties of forest soils. Ecological implications of these changes are described.

/pdf/effects-of-fire-on-properties-of-forest-soils-a-review-1ymllzm5e7.pdf

Effects of fire on properties of forest soils: a review

N itrogen emissions to the atmosphere due to human activity remain elevated in industrialized regions of the world and are accelerating in many developing regions (Galloway 1995). Although the deposition of sulfur has been reduced over much of the United States and Europe by aggressive environmental protection policies, current nitrogen deposition reduction targets in the US are modest. Nitrogen deposition remains relatively constant in the northeastern United States and is increasing in the Southeast and the West (Fenn et al. in press). The US acid deposition effects

/pdf/nitrogen-saturation-in-temperate-forest-ecosystems-29ixssxci4.pdf

Nitrogen Saturation in Temperate Forest Ecosystems

Better stewardship of land is needed to achieve the Paris Climate Agreement goal of holding warming to below 2 °C; however, confusion persists about the specific set of land stewardship options available and their mitigation potential. To address this, we identify and quantify "natural climate solutions" (NCS): 20 conservation, restoration, and improved land management actions that increase carbon storage and/or avoid greenhouse gas emissions across global forests, wetlands, grasslands, and agricultural lands. We find that the maximum potential of NCS-when constrained by food security, fiber security, and biodiversity conservation-is 23.8 petagrams of CO2 equivalent (PgCO2e) y-1 (95% CI 20.3-37.4). This is ≥30% higher than prior estimates, which did not include the full range of options and safeguards considered here. About half of this maximum (11.3 PgCO2e y-1) represents cost-effective climate mitigation, assuming the social cost of CO2 pollution is ≥100 USD MgCO2e-1 by 2030. Natural climate solutions can provide 37% of cost-effective CO2 mitigation needed through 2030 for a >66% chance of holding warming to below 2 °C. One-third of this cost-effective NCS mitigation can be delivered at or below 10 USD MgCO2-1 Most NCS actions-if effectively implemented-also offer water filtration, flood buffering, soil health, biodiversity habitat, and enhanced climate resilience. Work remains to better constrain uncertainty of NCS mitigation estimates. Nevertheless, existing knowledge reported here provides a robust basis for immediate global action to improve ecosystem stewardship as a major solution to climate change.

https://www.pnas.org/content/pnas/114/44/11645.full.pdf

Natural climate solutions

[1] Hydrologic models use relatively simple mathematical equations to conceptualize and aggregate the complex, spatially distributed, and highly interrelated water, energy, and vegetation processes in a watershed. A consequence of process aggregation is that the model parameters often do not represent directly measurable entities and must therefore be estimated using measurements of the system inputs and outputs. During this process, known as model calibration, the parameters are adjusted so that the behavior of the model approximates, as closely and consistently as possible, the observed response of the hydrologic system over some historical period of time. In practice, however, because of errors in the model structure and the input (forcing) and output data, this has proven to be difficult, leading to considerable uncertainty in the model predictions. This paper surveys the limitations of current model calibration methodologies, which treat the uncertainty in the input-output relationship as being primarily attributable to uncertainty in the parameters and presents a simultaneous optimization and data assimilation (SODA) method, which improves the treatment of uncertainty in hydrologic modeling. The usefulness and applicability of SODA is demonstrated by means of a pilot study using data from the Leaf River watershed in Mississippi and a simple hydrologic model with typical conceptual components.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2004WR003059

Improved treatment of uncertainty in hydrologic modeling: Combining the strengths of global optimization and data assimilation

An investigation is being made into the relationship between forest fire and hillslope hydrology in the Selva region of Catalonia over a three year period at sites with a known fire history. The major sites are on granite, granodiorite or on sediments derived from these rocks at locations between Santa Christina d'Aro and Santa Coloma de Farnes. It appears that there is a general relationship between the type of organic profile found in a forest and the impact of fire in terms of loss of organic matter, soil erosion hillslope hydrology and on the regeneration of the forest. This paper summarises the results of about 150 rainfall simulation experiments and presents an overview of work carried out so far. A major conclusion is that runoff processes are discontinuous and spatially structured due mainly to the effects of water repellency. As a result of surface properties (macropores, hydrophobic behaviour and swelling), four types of infiltration curve can be recognised. At many locations infiltration increases with time during the early part of an experiment, reaches a maximum value after about 30 minutes therupon subsequently decreasing. This type of relationship can not be modelled with standard infiltration equations. Illustrations are given of sections through both burnt and unburnt soils to show how the hydrophobic effect influences water penetration. It can be seen that infiltration patterns are similar for both burnt and unburnt soils and that micropores filled with organic matter are very important in conducting water through a water repellent surface layer. The hydrophobic layer traps water in the BC horizon and prevents evaporation and loss by upwards capillary movement. Although the hydrophobic effects mean that small areas sometimes have a high runoff coefficient, on larger areas there are many macropores and shrubs, beneath which the soil is permeable, and these trap runoff generated on the water repellent areas. The main effect of fire is to illiminate the storage of water in the organic horizons (several cm). At first infiltration rates remain high but later these decrease as there is a reduction in porosity. The sites having a moder type profile have better water retention properties, more organic matter that survives the fire and a faster rate of vegetation recovery. Sites with a mor type profile can be degraded and erode if they are subject to disturbance. The study provides another example of how, on seemingly uniform slopes, hydrological processes are structured by interactions with vegetation. The patterns produced by these interactions survive the effects of fire.

The effects of fire and water repellency on infiltration and runoff under Mediterranean type forest

Soil water content measurements at different scales: accuracy of time domain reflectometry and ground-penetrating radar

Dissolved organic matter, aluminium and iron interactions: precipitation induced by metal/carbon ratio, pH and competition.

Time domain reflectometry (TDR) can be used to study temporal variations in volumetric soil water content (θ) and bulk soil electrical conductivity (σ a ). The variations in σ a are associated with changes in θ and the soil water composition. Laboratory and field experiments were conducted to verify if TDR can be used to monitor the temporal variation in the soil water composition between solution sampling occasions. Effects of cable length and temperature on the σ a measurement were evaluated. Including the series resistance of the cable and connectors in the analysis improves measurements at high electrical conductivity levels. The temperature factor of the bulk soil appears to be similar to the temperature factor of soil extracts. Laboratory experiments showed that the theoretical model giving σ a as function of θ and the electrical conductivity of the soil solution (σ w ) combined with the water retention function was capable of describing σ w measured on soil solution extracted with ceramic cup solution samplers under static water flow conditions. After optimization of a single parameter, the model was able to describe σ w values of the soil solution obtained in the laboratory, whereas literature values were sufficient for field data. Concentrations of a number of solutes in a field data set spanning 3 yr were positively correlated with σ w . Site-specific regressions between solute concentration and σ w combined with automated TDR measurements of σ a and θ enable a more meaningful interpretation of the temporal variation of the concentration of major solutes present in the soil solution between sampling occasions.

Jacobus M. Verstraten

Papers

Improved treatment of uncertainty in hydrologic modeling: Combining the strengths of global optimization and data assimilation

The effects of fire and water repellency on infiltration and runoff under Mediterranean type forest

Soil water content measurements at different scales: accuracy of time domain reflectometry and ground-penetrating radar

Dissolved organic matter, aluminium and iron interactions: precipitation induced by metal/carbon ratio, pH and competition.

Assessing temporal variations in soil water composition with time domain reflectometry