Use of coupled biochar and straw mulch as a technique to mitigate soil erosion and improve soil carbon stocks in burned soils of southern iberian peninsula

Abstract

Wildfires typically increase runoff as well as soil erosion due to exposure of soil to direct raindrop impact. In addition, wildfires may cause a drastic alteration of soil structure by decreasing organic matter content and quality aggregate stability and carbon cycling. Post-fire mulching is the most widely accepted measure to mitigate soil erosion effectively but unfortunately mulch materials and application rates can vary greatly affecting its effectiveness. Some mulch materials such as straw have been used widely but others such as biochar which can improve soil quality has never been used in burned areas and has the potential to mitigate soil losses and improve soil quality in burned areas. While the CESAM group at the University of Aveiro has extensive experience in evaluating methodologies for erosion control the Organic Matter group from IRNASCSIC institution has studied soil quality and soil conservation strategies. This work aims to show examples of tools applied to mitigate post-fire erosion and their effects on soil quality. A total of 40 microplots (1 m2) were installed in September 2018 in two areas where wildfires burned at high severity (sandy and loamy soils) of southern Portugal and Spain respectively. Soil and sediment samples are being collected to assess soil erosion and carbon stocks via TG and DSC analysis. Treatments are: Burned (controls) Brushed (ash and stones removed) Straw mulch (at a rate of 1 Mg ha-1) Straw+Biochar (at a rate of 1+15 Mg ha-1) Pine slash and Marine algae. Preliminary results show that Straw and Straw+Biochar mulch strongly reduced soil erosion in 67- 60% compared to Burned plots or the Brushed plots which did not differ from each other. These findings indicate that applying both Straw together with Biochar can be an effective way to increase not only the labile soil organic matter fraction (derived from mulch itself) but also retain more recalcitrant native soil organic matter fractions (derived from the lack of erosion) and possibly also increase recalcitrant organic matter derived from biochar