Assessment of long-lived Carbon permanence in agricultural soil: Unearthing 15 years-old biochar from long-term field experiment in vineyard

Abstract

Carbon persistence in soil is a key issue in the context of Carbon Dioxide Removal (CDR) policies and regulations: Soil Carbon Accumulation (SCA) is also included in the latest EU regulations on sustainable biofuels, and gaining attention at international level within ICAO and IMO. The long-lived nature of the durable carbon share in biochar can meet the most sever criteria set by relevant and ambitious CDR policies: however, the possibility to quantitatively assess the persistent carbon fraction in biochar has been highly debated in recent years. While lab-scale incubation experiments are intrinsically limited in providing information on long-term permanence, they do not address actual farm-scale persistence under real cultivation management practices. The deployment and combined use of recent analytical techniques allows instead to identify and quantitatively assess the persistence of the durable carbon fractions in biochar, and thus compliance of this carbon removal with the targets of CDR policies. The present work builds on one of the longest, almost unique, biochar experiments in the EU, originally developed for assessing the agronomic performances of biochar amended agricultural soil: for the first time, biochar distributed in a vineyard soil at 22 t/ha scale in 2009 was unearthed in 2024 and collected for full characterization. The agricultural soil was subject to conventional agricultural practices over the 15 years of vineyard cultivation. The scope of this research is to assess the permanence of biochar under these conditions. The present work shows the complexity of unearthing biochar from soil, applying a focused method to recover and clean the material before its characterization, without altering its chemical and physical properties. Both unearthed and original (i.e. before deployment) biochars were washed with water under same condition and procedures, and fully characterized. In addition to analytical practices commonly adopted for biochar characterization, FT-IR, SEM EDX, and Random Reflectance (R_o) techniques were used, quantifying the amount of the inertinite carbon component in biochar. Despite the dilution from the inclusion of exogeneous organic and inorganic matter from soil in the original biochar, the ratio of fixed carbon (Cfix) to total carbon (Ctot) showed minor variations (∼8 %). Moreover, the inertinite and semi-inertinite fractions in the washed original and unearthed biochars remained almost unchanged over 15 years of active use in agricultural soil, confirming the permanent nature of the inertinite share of carbon in biochar. This result, together with other recent findings in literature, provides scientific evidence supporting Biochar Carbon Removals (BCRs) as permanent removal in Carbon Dioxide Removal (CDR) regulations.