Despite the overwhelming importance of earthworm activity in the soil system, there are a limited number of studies that have examined the impact resulting from biochar addition to soil. Biochar is part of the black carbon continuum of chemo-thermal converted biomass. This review summarizes existing data pertaining to earthworms where biochar and other black carbon substances, including slash-and-burn charcoals and wood ash, have been applied. After analyzing existing studies on black carbon, we identified that these additions have a range from short-term negative impacts to long-term null effects on earthworm population density and total biomass. Documented cases of mortality were found with certain biochar-soil combinations; the cause is not fully understood, but hypothesized to be related to pH, whether the black carbon is premoistened, affects feeding behaviors, or other unknown factors. With wood ashes, negative impacts were overcome with addition of other carbon substrates. Given that field data is limited, soils amended with biochar did not appear to cause significant long-term impacts. However, this may indicate that the magnitude of short-term negative impacts on earthworm populations can be reduced with time. 1. Introduction The importance of earthworms in soil genesis (i.e., bioturbation) has long been recognized and dates back to the 1800’s with some of the initial work by Charles Darwin [1]. In his seminal publication, Darwin [2] noted that earthworm burrowing and casting activity together were the primary force in mixing soil layers and burying surface debris. Through this bioturbation, earthworms increase soil porosity affecting soil aeration as well as water infiltration. Earthworm casts are also important protective and dispersal vehicles for soil microbes and nutrients. Taken altogether, earthworms have been recognized as ecosystem engineers, or organisms that can have a profound influence on the structure and functioning of soils [3]. By way of function, earthworms have profound direct and indirect impacts on the availability of nutrients, particularly through increased decomposition of plant residues and turnover of soil organic matter. Thus, what positively or negatively affects soil biota [4] may indirectly affect soil function and plant growth. The functioning of intensively managed soil systems has increasingly become dependent on external inputs to maintain high levels of productivity. Management practices which degrade soil organic matter, including heavy tillage, degrade a soil’s inherent quality and reduce fertility [5, 6].
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