Effects of traditional field retting of hemp on soil organic carbon and the soil microbial community

Abstract

Retting is a process used with bast fiber crops such as industrial hemp (Cannabis sativa L.) to expedite the separation of fibers from the woody core of the stalk. Field retting involves partial decomposition of stalks on the soil surface and is facilitated by soil microorganisms. Effects of field retting on soil C have not been studied, and little information is available on how field retting affects the soil microbial community. We conducted an experiment to investigate effects of field retting on total organic soil C (TOC), permanganate oxidizable soil C (POXC), and relative abundance of microbial fatty acid methyl ester biomarkers (FAMEs). Four treatments were imposed:(a) no retting (NR), (b) low-density retting (LDR) (2,500 kg stalks ha -1 ), (c) highdensity retting (HDR) (5,000 kg stalks ha -1 ), and (d) control (CON). The NR, LDR, and HDR treatments were conducted on plots where hemp was grown, whereas the CON treatment excluded production and retting. Plots under HDR treatment generally had greater TOC and POXC than plots where no retting occurred. Analysis of FAMEs indicated that soil microbial communities were grouped according to retting treatments. Greater fungal abundance and higher fungi to bacteria ratios were associated with soils where retting occurred. Compared with retting methods where harvested stalks are immediately removed from the field, field retting may mitigate soil C losses associated with harvest and may be a useful component of maintaining soil health when used in concert with management practices that build soil C.