Closing the carbon loop: evaluating the potential of grapevine-derived biochar as a soil conditioner in warm climate vineyards

Abstract

Significant increases in anthropogenic carbon dioxide (CO2) emissions due to combustion of fossil fuels and intensive land management practices that release CO2 into the atmosphere have resulted in higher air temperatures due to the greenhouse effect. Agriculture offers tremendous potential to mitigate the negative impacts of climate change and global warming due to the CO2 sequestering ability of crops via photosynthesis, which ultimately build soil organic carbon (SOC) through root sequestration of carbon and decomposition of plant litter (Woolf et al. 2007). Another avenue to increase SOC is through the conversion of plant biomass, particularly woody biomass, into a recalcitrant carbon product known as biological charcoal (aka “biochar”). This conversion is typically done through pyrolysis, an anaerobic combustion, of lignocellulosic feedstock(s). Sequestering biochar into the soil can result in a negative carbon balance and reduce CO2 emissions by 12-84 % compared to using it as a fuel source (Lehmann 2007). As the wine industry seeks to meet their environmental, social and governance (ESG) targets, the use of biochar in vineyards as a carbon insetting tool could be a powerful tool in the viticulturist’s toolkit, particularly with the potential benefits offered by biochar for vineyards.

Several studies have reported on the use of biochar in vineyards located primarily in Europe, but also, to a lesser extent, in USA. In an early trial of biochar conducted in a Valais, Switzerland Pinot noir vineyard, top-soil application of biochar at 8 t ha-1, with and without compost, impacted neither grapevine vegetative and reproductive performance nor fruit composition (Schmidt et al. 2014). Similarly, a long term (4-year) study in Italy found no impact of biochar on grape composition, however, there was a significant and consistent increase in yield, by as much as 66%, compared to controls (Genesio et al. 2015). Biochars made with vineyard prunings as feedstock, as done in the current trial, had a positive impact on several soil physical parameters including bulk density (decreased), aggregate stability (increased), and plant available water (PAW; increased, but only at high soil matric potentials > -30 kPa) (Burrell et al. 2016). Oddly, the same study evaluated biochars made at two pyrolysis temperatures – 400 °C and 525 °C – and did not find differences in any of the aforementioned soil parameters despite the 525 °C biochar having higher porosity. Nevertheless, the study highlights the potential of using biochar to increase soil moisture retention, which would be advantageous in low rainfall seasons. Due to its highly porous structure, biochar could also provide habitats and niches for soil microorganisms including bacteria, fungi and Archaea (Li et al. 2018). The effect of biochar on soil microbial processes have not been reported in vineyards; in other cropping systems, addition of biochar to soil increased microbial abundance, which was favoured by less lignocellulosic feedstocks (Gul et al. 2015).