Laboratory experiments were carried out to explore the effect of thermal shocks (as occur during fire) and simulated rainfall events on cation leaching dynamics in an organic rich Leptic Umbrisol soil. The soil samples were collected in the field using specially designed lysimeter boxes that allow sampling and application of thermal shock treatments and simulated rainfall while keeping the soil structure unaltered. The soil temperature during the thermal shocks and degree-hours of accumulated heat were determined, and cation (Na+, K+, Ca2+ and Mg2+) leaching was measured in surface runoff (0-cm depth) and subsurface flow (12-cm depth) samples collected from the lysimeter boxes. Important differences were found in cation leaching in relation to thermal shock: monovalent cation leaching from the soil above 200°C (68 degree-hours) and divalent cations leaching above 220°C (195 degree-hours) was higher than that seen in other treatments. In general, the amount of cations leached increased with the severity of the thermal shock; however, under moderate conditions, there was a decrease in cation leaching, mainly of monovalent ions. The exchangeable cation losses by leaching in the intense heat treatments were ~80%.
Important factors in the evaluation of fire severity are the duration of the soil exposition to a certain temperature as well as the factors that determine the thermal transmissivity on the soil (moisture, texture, organic matter content, etc.). The aim of this work was to apply the degree-hours method (DH) to characterize the thermal impact of forest fires in soils. Thermal treatments in the laboratory were conducted using soil samples in order to study the effects in the soil exchange complex. The results showed the effect of the supplied degree-hour (DH) on the cation exchange capacity (CEC), which was expressed by a continuous exponential decrease in the CEC. This function may better explain the process of the decreasing of CEC than only the maximum temperature values. The sum of cations extracted in relation to the thermal treatment gradually increased with temperature or DH, and tended to stabilize at high values. The concentration of the different cations extracted increased gradually with the intensity of heating, and when related to the DH appeared to fit an equation of the type y=a+bx<sup>c </sup>with a high degree of confidence. Analyses of the results show that the measurement of the heat supplied to the soil is a useful parameter with which to interpret pedologic changes, especially when those changes happen continuously over time.
Soil properties determining the thermal transmissivity, the heat duration and temperatures reached during soil heating are key factors driving the fire-induced changes in soil microbial communities. The aim of the present study is to analyze, under laboratory conditions, the impact of the thermal shock (infrared lamps reaching temperatures of 100 °C, 200 °C and 400 °C) and moisture level (0%, 25% and 50% per soil volume) on the microbial properties of three soil mixtures from different sites. The results demonstrated that the initial water content was a determinant factor in the response of the microbial communities to soil heating treatments. Measures of fire impact included intensity and severity (temperature, duration), using the degree-hours method. Heating temperatures produced varying thermal shock and impacts on biomass, bacterial activity and microbial community structure.
In this work, a revegetation experiment was performed on a forest soil artificially burnt in the laboratory, and the results of the soil without treatment were compared to those of the soil capped with an organic mulch. Plant biomass, as well as the variation of pH, electric conductivity (EC), and phytotoxicity with depth, were recorded. Only a very low plant growth was observed in the uncapped soil. The substrateamended soil had a higher production than the uncapped soil, despite showing an increased EC due to the mulch. The increase of pH or salinity during burning can be discarded as the cause of the different plant growth, because the values reached were not high enough to justify the negative effect observed. The determination of phytotoxicity after burning the soil at temperatures between 100ºC and 500ºC suggested that the problems observed for seed germination in the burnt soil might be linked to the formation of undetermined phytotoxic substances after soil heating at a temperature near 200ºC. Although polycyclic aromatic hydrocarbons followed the same trend as phytotoxicity with temperature, their concentrations in soil were too low to be the cause of the effects observed, so they can be discarded as playing an important role in phytotoxicity.
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