Anthropogenic charcoal-rich soils of the XIX century reveal that biochar leads to enhanced fertility and fodder quality of alpine grasslands

International audienceSoil incorporation of charcoal (biochar) has been suggested as practice to sequester carbon, improve soil properties and crop yields but most studies have been done in the short term. Old anthropogenic charcoal-rich soils in the Alps enable to explore the long-term impact of charcoal addition to alpine grassland on seed germination, fertility and fodder nutritive value. A germination test and a growth experiment in pots with Festuca nigrescens Lam. and Trifolium pratense L. were performed using three different substrates: control soil (i.e. sandy-loam brown acid soils with some podsolization), charcoal hearth soil (i.e. charcoal-enriched anthropogenic soils derived from the carbonization of larch wood on flat terraces) and control soil mixed with a fraction of fresh larch wood charcoal to reach the soil-charcoal ratio of 0.6. Both aged and fresh charcoal improved germination and markedly increased plant growth of the two plant species. The addition of fresh charcoal had an initial detrimental effect that disappeared in the second and third growth cycles. Plant Nitrogen:Phosphorus ratio revealed that growth was N-limited in the anthropogenic soils and P-limited in the control and freshly amended soils demonstrating that biochar aging is critical to obtain a significant growth stimulation. Plant nutrient contents revealed an improved fodder quality in both the charcoal amended soils. Despite the occurrence of limited toxic effects on seedlings, larch wood charcoal appears to have positive effects on fertility and fodder quality of alpine grasslands in the long term