Assessing legacy effects of a 12-year irrigated cropping systems study with a post-hoc bioassay

A healthy soil resource is vital to the continued success of irrigated agriculture in southern Alberta. A 12-yr (2000-2011) irrigated cropping systems study was followed with a dry bean (Phaseolus vulgaris L.) field bioassay in 2012, to assess legacy effects of preceding management. Specifically, a comparison of conventional (CONV) and conservation (CONS) management (reduced tillage, cover crops, compost addition, narrow-row dry bean) legacies was sought. However, rotational legacies such as preceding phase, length, preceding crop, and interval since previous dry bean were also assessed by the fully phased experimental setup. Only 1 of 18 possible soil management contrasts (CONV vs. CONS, 2000-2011) was significant for the dry bean bioassay in 2012, despite overwhelming evidence of improved soil health (microbial biomass C and beta-glucosidase activity) under CONS management. Monoculture wheat (Triticum aestivum L.) from 2000 to 2011 led to 2 d earlier maturity and higher disease incidence in bioassay dry bean. With wheat as a preceding crop, bioassay dry bean was significantly shorter (4 cm), earlier maturing (2 d), and lower yielding (by 21-35%) than with dry bean, potato (Solanum tuberosum L.), or sugar beet (Beta vulgaris L.) as preceding crops, largely due to volunteer wheat competition. Significantly enhanced bioassay yields (9-13%) with shorter intervals since previous dry bean demonstrated a "legume effect." Overall, the dry bean bioassay was less effective at assessing soil management legacies (CONV vs. CONS) than rotational legacies such as preceding crop or interval since previous dry bean.