Ana Catarina Bastos

Wildfires in Mediterranean pine forests intensify post-fire runoff and soil erosion, accelerating losses of soil and organic resources and delaying recovery. We tested whether pine-wood-derived biochar can enhance soil “sponge function” and reduce erosion in a repeatedly burned pine plantation soil from Central Portugal. In an outdoor box-lysimeter experiment (Oct 2019–Sept 2020; 11 rainfall events; 25% slope), we compared control, with 2% (B2) and 4% (B4) biochar-amended soils on a dystric/humic (skeletic) Leptosol, all sown with a biodiverse seed mixture. Both biochar treatments reduced runoff (B2 −19%; B4 −17%) and erosion (B2 −67%; B4 −49%) and increased vegetation cover (B2 +141%; B4 +113%). Using sediment organic matter, estimated soil organic carbon (SOC) loss declined by 58% under B2 and 13% under B4 versus the control. The stronger performance of 2% indicates an efficiency threshold relevant to post-fire soil conservation and land management.

Soil erosion by water poses major environmental challenges to the European viticulture sector. Biochar is recognised as a sustainable tool for combating land degradation, but few studies have been conducted in Mediterranean vineyards with hilly terrain and heavy rainfall. This study assesses the potential of biochar to support soil conservation by enhancing sponge function, i.e. water retention and infiltration, and reducing erodibility in sloping sandy loam soil under natural rainfall conditions. An 18-month outdoor box lysimeter experiment was conducted using bare soil, including soil amended with 4% (w/w) biochar from a Portuguese vineyard. Over the monitoring period, biochar application significantly (p−1 yr⁻¹). Improved soil structure led to a 7% reduction in bulk density, an average increase of 73% in stored water, and a 28% increase in infiltration rate. During drier periods, the biochar-amended soil stored 171% to 303% more water than the control soil. We recommend a minimum monitoring period of a full hydrological cycle under natural rainfall to comprehensively capture the effect of biochar on the soil sponge function. Observed seasonal trends and atmospheric river (AR) events suggest that studies using rainfall simulations without considering antecedent soil moisture and AR variations will yield skewed data on effects.

Biochar application to soil has been promoted as a means of increasing crop yields. However, the results of single-study analyses often show contradictory results, and so the development of a single framework within which to explore the range of published results is required. Fourteen meta-analyses have been developed on plant growth effects since biochar first became promoted as a soil amendment. Here we aim to review these meta-analyses while drawing on individual studies to explore mechanisms where warranted. There is broad agreement across meta-analyses showing that biochar application to soil increases crop yields by an average of ~10–15%. A comprehensive contextualization of such results is presented here. The strongest effects are observed when nutrient-rich biochars (e.g., made from manures or composts) are applied to unfertile soils. Analyses suggest that nutrient provision, liming, and water retention are the main drivers of yield increases, but other site-specific factors can also play a part as discussed in this chapter. While biochar has a strong potential to increase crop yields following application to soil, it is necessary to match biochar characteristics with soil properties, climate, and plant requirements to maximize the possibility of yield increases and to minimize the occurrence of trade-offs.

Biochar research is extensive and there are many pot and laboratory studies carried out in Europe to investigate the mechanistic understanding that govern its impact on soil processes. A survey was conducted in order to find out how representative these studies under controlled experimental conditions are of actual environmental conditions in Europe and biomass availability and conversion technologies. The survey consisted of various key questions related to types of soil and biochar used, experimental conditions and effects of biochar additions on soil chemical, biological and physical properties. This representativeness study showed that soil texture and soil organic carbon contents used by researchers are well reflected in the current biochar research in Europe (through comparison with published literature), but less so for soil pH and soil type. This study provides scope for future work to complement existing research findings, avoiding unnecessary repetitions and highlighting existing research gaps. Keyword : biochar, soil, Europe, pot experiments, laboratory scale, representativeness This work is licensed under a Creative Commons Attribution 4.0 International License.

A representativeness survey of existing European Biochar field experiments within the Biochar COST Action TD1107 was conducted to gather key information for setting up future experiments and collaborations, and to minimise duplication of efforts amongst European researchers. Woody feedstock biochar, applied without organic or inorganic fertiliser appears over-represented compared to other categories, especially considering the availability of crop residues, manures, and other organic waste streams and the efforts towards achieving a zero waste economy. Fertile arable soils were also over-represented while shallow unfertile soils were under-represented. Many of the latter are likely in agroforestry or forest plantation land use. The most studied theme was crop production. However, other themes that can provide evidence of mechanisms, as well as potential undesired side-effects, were relatively well represented. Biochar use for soil contamination remediation was the least represented theme; further work is needed to identify which specific contaminants, or mixtures of contaminants, have the potential for remediation by different biochars.