Productivity and soil quality of organic forage, quinoa, and grain cropping systems in the dryland Pacific Northwest, USA

Abstract

Organic management is rapidly expanding as an alternative to conventional agriculture, but maintaining sufficient available soil nitrogen (N) supply is a challenge for organic systems in the semiarid regions of the US Pacific Northwest (PNW). Quinoa (Chenopodium quinoa Willd.) is a potential new crop for such systems, but there are many knowledge gaps regarding impacts of quinoa in crop rotations in the US. Here we tested the effects of quinoa in organic crop rotations for the Palouse region of the PNW. Specifically, we evaluated eight 3-year grain crop sequences, as part of a longer rotation with alfalfa, on the following metrics: crop productivity, soil N budgets, soil microbial parameters, arbuscular mycorrhizae, and soil aggregate size distribution. Organic crop yields averaged 495, 2736, and 2403 kg ha−1 for chickpea, barley, and wheat, respectively (33 %, 79 % and 77 % of the respective crop county averages). Yields were affected by weather across growing seasons, as were the amount and quality of residues, which in turn influenced soil N cycling. Overall soil N supply decreased slightly through the 3-yr sequences. Cereal grain yields were correlated with inorganic N (IN), and the correlation was stronger for subsurface (30−150 cm) depths than the surface (0−30 cm); IN was sufficient to support high cereal yields even in the later years when weather was suitable, but cereal yields overall may have been N-limited. Crop yields were more highly correlated with spring surface soil potentially mineralizable N than with spring surface IN. More inorganic N was available in surface (0−30 cm) post-harvest soil following quinoa (13.1 kg N ha-1 more) compared to wheat, but quinoa yields were low, with a 4-yr average of 104 kg ha−1. Soil microbial biomass and activity were not affected by sequence treatments. Although plants of the family Amaranthaceae are often considered “non-mycorrhizal”, moderate rates of arbuscular mycorrhizal fungi (AMF) colonization were observed in quinoa. However, AMF colonization was lower in quinoa compared to other grains in the sequences, and the colonization of crops following quinoa was lower than following wheat. Soil structure was finer after quinoa, with a higher proportion of aggregates in the smaller size classes. Our results show that quinoa affects soil properties, and that organic management of 3-yr dryland grain sequences in rotation with alfalfa are possible in this area, given efficient N cycling through crop residues.