Effects of fire on the aggregate stability of clayey soils: A meta-analysis

Abstract

Fire is a natural phenomenon that affects the Earth's biogeochemical cycle. Wildland fires are increasing in frequency and magnitude, possibly owing to climate change. In addition, fire is applied as a tool in agro-ecosystem management. However, several detrimental effects on soil properties, particularly on soil aggregate stability, might occur. A general explanation is that “the effects of fire on soil aggregate stability in soil with high clay content, calcium carbonate, and Fe and Al oxides as the principal cementing substances cause gradual improvement in aggregate stability, following fire-heating severity.” In short, the higher the fire severity, the higher is the aggregate stability. A meta-analysis was conducted for assessing the laboratory and field studies to verify this concept. The Scopus® platform was used to search for the literature related to “soil aggregate stability” AND fire OR heating AND clay OR “clayey soil.” A total of 10 studies were selected with 16 soil types, 244 burned or heated samples, and 160 control samples. The meta-analysis was performed using RevMan 5.3 software. Meta-analysis was delineated according to the following assumptions: a) field and laboratory studies were tested as two independent subgroups, and a pooled effect was subsequently calculated; b) a random-effects model was used; c) mean differences with 95% confidence intervals were computed for continuous variables. Laboratory and field measurements showed dissimilar effects on soil aggregate stability and evidenced positive effects in the laboratory experiments compared with those in the field. Aggregate stability measured in the laboratory increased by 2.80% after heating. Fire effects measured in the field did not show a significant increase in soil aggregate stability (−0.16%). The overall effects of combined subgroups showed an aggregate stability improvement of 1.49% (confidence intervals 0.80% to 2.17%). However, the weight of the results from the laboratory studies played an important role in the overall effect. The soil clay content was found to be inversely associated with aggregate stability, i.e., the lower the clay content the higher the aggregate stability in soil subjected to fire heating. In conclusion, the aggregate stability of clayey soils rich in Al and Fe oxides does not necessarily improves following fire heating.