Designing Organic Cropping Systems for Improved Nutrient Cycling and Soil Health.

Organic field crop systems are characterized by more complex rotations with high spatial and temporal vegetative diversity, an enhanced use of legumes, and reduced external nutrient (nitrogen (N) and phosphorus (P)) use. These distinct production systems provide benefits with respect to reduced energy use and reduced farm nutrient surpluses, in combination with maintenance of organic matter and improved soil health, although the latter may be consistently true only for larger soil organisms.  Results from studies on long-term trials and commercial organic farms in Canada examining the productivity of organic potato, forage, grain and small fruit systems, and impact of management strategies on organic matter, N and P dynamics, and soil microbial and macrofaunal abundance and diversity will be presented. Low P inputs and soil P status documented across many of these organic cropping systems, however, may potentially be compromising organic farm long-term sustainability. Results of recent research, employing molecular and isotopic techniques, to examine the impact of soil P status on crop yields and soil P availability and P uptake, legume biological N₂ fixation, bacterial and mycorrhizal community diversity, and biochemical hydrolysis of soil organic P within these production systems will also be presented. Results provide further evidence of the resilience of the soil microbial community with respect to functionality, if not diversity of microbial community composition. This may challenge core organic agriculture premises of farming system benefits to soil health via enhanced microbial diversity, but in its place lead to an improved understanding of how plant-soil microbe interactions that influence nutrient dynamics are impacted by specific beneficial cropping practices.