Fire changes the larger aggregate size classes in slash-and-burn agricultural systems

The slash-and-burn agricultural system is one of the oldest types of no-tillage soil management. However, farmers using fire may cause unintentional damage to soil structure in the slash-and-burn system. Herein, it is assumed that larger soil aggregates (≥2.0 mm to <8.0 mm) are the most affected by fire, with direct impacts to their structural stability. An experimental fire was carried out, and the rainfall simulation approach for a period of 30 min with a rainfall intensity of 58 mm h−1 was used to assess the effects of fire on soil aggregate stability. The macro-aggregate stability was evaluated for the test plot as well as an undisturbed soil sample using two aggregate sizes: 2-4 mm and 4-8 mm. The fire temperature measured at the soil surface was very high (673 ± 93 °C). Consequently, the soil structural stability under the slash-and-burn agricultural system rose significantly. Larger aggregates of ≥2.0 mm or ≥4.0 mm size indicated a clear influence of fire on soil physical properties. The fire temperature dramatically changed the distribution of the aggregates of ≥4 mm, since aggregates of this size were more frequent (51%) and stronger compared to unburned soil. The methodology is critical to detect the changes in the physical properties of aggregates affected by fire. Aggregate stability methods using a single sieve of ≥0.25 mm over rainfall simulation, but without consideration of the completely fragmented particles remaining in the sieve, were not able to detect the aggregation changes.