Over the last decade, the incorporation into soils of pyrolised organic matter, often referred to as "biochar", has been promoted actively all over the globe as a “win-win” solution for sequestering carbon while improving soil fertility parameters. The topic has attracted significant attention among researchers. They have devoted considerable efforts to try to understand not only the technical aspects of biochar production and its impact on soil properties, soil ecology, and crop yields, but also the economics of incorporating this material in soils.

Concerns, which have yet to be fully addressed, have arisen in recent years relative to potential threats to human health caused by biochar, or possible changes in long-term bio-physico-chemical soil functioning and in land surface albedo associated with the addition of an essentially recalcitrant black body to soils. In addition, despite extensive field experiments it remains relatively difficult to predict under what conditions the addition of pyrolized organic matter to soils leads to higher crop yields, or stimulates microbial activity. One of the reasons for this unsatisfactory state of affairs may have to do with the fact that ten years is too short a time to run realistic field experiments. To make progress from that perspective, it is important to lobby decision-makers to fund long-term field trials. Unfortunately, the trend in most countries has been to de-emphasize such trials, even though there is little doubt among scientists that they are absolutely vital if we are to fully understand the consequences of biochar soil additions.

Another approach that may be useful in appraising the long-term, centennial and even millennial, impacts may come from a close interdisciplinary collaboration between soil scientists and archaeologists. Indeed, soils in most parts of the world contain often appreciable amounts of "black carbon" in the form of charcoal or soot, from a variety of sources. Whilst some sources of black carbon are natural (burnt wood remnants), often the source is anthropogenic, as is the case for example with the thousands of charcoal kiln or pit sites in Europe and Africa where charcoal was manufactured in domestic hearths, industrial furnaces, or in the vicinity of buildings that burned at some point in the past. Besides, 2 typical archaeological soils types resulting from long-term addition of charcoals are known and studied: Amerindian “Terra Preta”, and urban “Dark earth”. Instances of each of these events can be identified in the archaeological record over the past 10,000 years. Soil scientists have occasionally studied this aged black carbon, but in the past decade it has received far less attention than modern biochar.

Archaeologists and geoarchaeologist, on the other hand, have devoted much more attention to this aged black carbon, frequently using their observations of charcoal recovered from archaeological contexts to reach conclusions about the occupation and significance of the archaeological site. In addition, geoarchaeological studies lead to characterize the past soil environments of charcoals inputs, and this can give clues on the evolution of these soil components and soil properties.

It would therefore seem very worthwhile to combine archaeological and historical information about the origin of black carbon found in soils, with a typical soil scientists' analysis of its evolution until the present and of its effects on soil properties. From such a combination of knowledge, one could expect to gain increased understanding of the long-term fate of black carbon, and of its influence on soil chemistry and structure, crop yields, and many other aspects that are in question at the moment.

Keywords: Black carbon, biochar, soil processes, pedology, archaeology

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