Abstract
To clarify the effect of the heat wave that accompanies the passage of a fire on some chemical parameters related to soil fertility and plant growth, we subjected two soils, classified as a sandy loam and a silty clay, respectively, to artificial heating under controlled conditions. We arranged the heating program according to the true thermal reactions occurring in the soils, which were previously detected by differential thermal analysis. The heating of soils had a variable effect on pH, with a decrease up to 220°C and a sharp increase after 700°C. Heating produced a continuous decrease of the cation exchange capacity (CEC) in both soils. The effect of the heat on organic matter content, total nitrogen, and organic phosphorus was similar, with a little decrease up to 220°C; beyond this value the organic matter was burned up, the total nitrogen was volatilized, and the organic phosphorus was mineralized and transformed into the inorganic form. N-NH4, on the contrary, increased up to 220°C and then decreased very sharply; at 460°C it was barely detectable. Similar behavior was shown by the available P, which peaked at 460°C. Water-extractable cations showed various behaviors. Ca increased to a peak at 220°C; after a decrease at 460°C Ca increased drastically at 900°C. Mg showed an increase to a peak at 220°C followed by a continuous decrease to complete unextractability at 700° and 900°C. K extractability increased progressively, with a maximum peak at 700°C. The extractability of Na increased to a maximum at 220°C and then decreased continuously to a very low level at 900°C. Heating to 170°C had no effect on plant and root growth and dry matter production. On the contrary, heating to 220° and 460°C, respectively, improved plant height and dry matter production of both tops and roots but not root extension. Further heating at 700° and 900°C resulted in a reduction in plant height, root extension, and dry matter production.
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