Early accumulation of active fraction soil carbon in newly established cellulosic biofuel systems

We examined relative changes in soil C pools shortly after the establishment of six perennial and two annual bioenergy cropping systems that differed in diversity (monoculture vs. polyculture). Perennial systems included two monocultures (switchgrass, Panicum virgatum; and miscanthus, Miscanthus × giganteus) and four polycultures including hybrid poplar (Populus sp.) + herbaceous understory; mixed native grasses, successional vegetation, and restored prairie. Two annual systems included no-till continuous corn (Zea mays) and rotational corn (corn-soybean (Glycine max)-canola (Brassica napus)). Each crop was planted in a full factorial design at both a moderate fertility Alfisol and a high fertility Mollisol site. Relative differences in active, slow, and passive C pools in surface soils, where C changes are most likely to be detected early, were evaluated with 322-day laboratory incubations followed by acid hydrolysis to infer different pools from exponential decay curves. Five years post-establishment, active C pools under perennial polycultures at the Alfisol site were up to twice those under annual and perennial monocultures, and followed the order hybrid poplars (696 ± 216 μg C g− 1 soil, n = 5 replicate blocks) ≈ native grasses (656 ± 155) ≈ restored prairie (638 ± 44) > early successional (500 ± 54) ≫ continuous corn (237 ± 68) ≈ rotational corn (180 ± n.a.). Active C pools in perennial monocultures were similar to those in continuous corn: switchgrass (274 ± 29) ≈ miscanthus (299 ± 9). In contrast, differences in active C pools among crops at the more fertile Mollisol site were not detectable except for greater pools in the restored prairie and rotational corn systems. At both sites, slow and passive C pools differed little among systems except that slow pools were greater in the poplar system. That diversity rather than perenniality itself led to greater active C pools suggests that polycultures might be used to accelerate soil C accumulation in bioenergy and other perennial cropping systems.