Effects of Traditional Field Retting of Industrial Hemp (Cannabis sativa L.) on Soil Carbon and Soil Microbial Community Dynamics.

In Kentucky, and other states, there has been increasing interest in production of industrial hemp (Cannabis sativa L.) since the passage of the 2014 Farm Bill (section 7606). This legislation defined production standards for this crop at the federal level. Renewed production of hemp also brings new opportunities for agronomic research around this plant. Little research has been conducted on the soil quality impacts of hemp production. One practice used in hemp production for fiber is field retting, where harvested hemp stalks are placed on the soil surface and allowed to partially decompose. This process facilitates separation of fiber from other stalk components. Our research objective was to examine the effects of field retting of industrial hemp on soil carbon and soil microbial community dynamics. To reach this objective a field plot experiment was set up in 2015, 2016 and 2017 using a randomized complete block design that included four treatments: 1) control, 2) no retting (NR), 3) low density retting (LDR), and 4) high density retting (HDR). No hemp was grown or retted on control plots. Treatments 2-4 had hemp produced on the plots. Hemp stalks were retted in the LDR and HDR treatments by applying stalks to the soil surface at rates of 2120 kg/ha and 4240 kg/ha, respectively. The HDR treatment significantly increased total soil C levels over other treatments in 2015 and permanganate oxidizable carbon (POXC) levels in 2016. In 2015, analysis of other parameters showed (non-significant) trends that suggested that retting may also have a positive impact on POXC and microbial parameters including total biomass. Data from 2016 was still being analyzed at the time of this writing and a more thorough examination of effects on microbial community structure for 2015 and 2016 is being conducted.