Ash cover accelerates the mineralization of labile soil organic matter following experimental fire

After fires, the soil surface is covered by ash, which is important to the postfire environment because of its effect on hydrological behavior and its fertilizing properties. Ash effects are difficult to assess in the field but can be studied more readily through manipulative experiments in the laboratory. In this study, a laboratory burn was conducted to replicate the conditions of prescribed burning in the southern Pyrenees (Spain), and to evaluate the effects of ash on soil organic matter (SOM) and nutrients, with particular attention to particulate organic matter (POM) (> 50 μm), which was hypothesized to be more vulnerable to thermal and microbial degradation. Undisturbed soil blocks were covered with plant material and subjected to different burning treatments using a blowtorch. Part of the blocks were analyzed immediately after burning, and the rest, after removing the ash layer from half of them, were kept in a greenhouse for five months. Samples from 0 to 1 and 1–3 cm depths were analyzed for total SOM, POM and N and plant-available (Olsen) P, and POM was analyzed for fiber composition and N, P and K. The results show a large immediate loss of SOM in the surface soil when the blocks were subjected to high intensity burning treatments, with temperatures mainly ranging 400–550 °C. During the following months, only in ash-covered soils, an accelerated mineralization particularly affected POM and, within POM, its most labile components (nonstructural, hemicellulose and cellulose) compared to the more refractory ones (lignin-type), but did not significantly affect nonparticulate organic matter (NPOM). POM mineralization was boosted by nutrients such as P supplied by the ash, both in particulate organic and plant-available forms. Organic mulching and seeding of fast-growing grasses are proposed to help prevent ash-induced POM mineralization in post-fire environments.