Performance Story: Addressing biological limitations on nutrient cycling in organic cropping systems
Dr. Reynald Lemke and Dr. Bobbi Helgason
Results from a recently completed long-term cropping system experiment conducted at the Agriculture & Agri-Food Canada Research Farm at Scott, Sk., showed that grain yields were consistently lower (25-40%) on organically managed (pesticides and synthetic fertilizers withheld) compared to conventionally managed systems. Each management system included two, six-year rotations comprised of either a mix of annual grain crops (oilseeds, pulses and cereals) or a mix of annual and perennial crops (oilseed, cereals and three years of alfalfa hay). The work also indicated that weed pressure had a minimal impact on yield, suggesting that one or more nutrient deficiencies – most likely nitrogen (N) or phosphorus (P) - was the most likely reason for the lower yields. However, it was also noted that the reduced crop residue returns on the organic systems had a negative impact on soil microbial functioning and this may also have been a factor contributing to the lower yields.
A follow up 3-year study was conducted to determine if the lower yields on the organic systems were due to a nutrient deficiency, and if so which one, or if the yield reductions were otherwise related to the observed impaired microbial functioning. Selected plots from each rotation (diversified annual grains and annual-perennial crops) of the original long-term study were utilized for the current study. The organic plots were “split” creating four sub-treatments, fertilizer N or P applied separately, N and P fertilizer applied together, and a check (no fertilizer). Plots from the comparable rotations but that had been under conventional management were included for comparison and wheat was grown as the test crop. A greenhouse study was also conducted, using soil collected from the field plots and applying the same fertility treatments - but with an additional treatment of fertilizer plus wheat residues to see if additional residue carbon would boost biological nutrient cycling and potentially, wheat yield.
Crop yield responses from this study confirmed that P was an important factor limiting yield on the organic systems, particularly in the annual grains rotation. This was supported by higher phosphatase production in these same soils, a response of the soil microbial community to P scarcity. Where an annual-perennial rotation had been maintained, soils showed less P limitation, a stronger response to N additions, and yields that were closer to the conventionally managed counterpart. Both fertilizer N and P additions were required to achieve a significant yield response compared to the check in the second and third year of the study, indicating that both N and P were limiting. After three consecutive years of N and P fertilizer applications, the yield gap between the organic and conventional systems was minimal, however tissue P concentrations indicated that P was still limiting on the organic systems. Results from the greenhouse studies indicated that the addition of a microbial energy source (wheat straw), particularly when combined with fertilizer N and P, improved the ability of the microbial community to cycle P and resulted in higher P uptake by the wheat on the organic soils. Overall, the study demonstrated that P was a major limitation on crop yields in this organic management system and in these soils, and that nitrogen was a secondary but important limiting factor. These deficiencies were amplified by the reduced nutrient cycling by the microbial community as a result of the negative influence of the low residue returns over the long-term. Restoring yields on these organic systems not only requires addressing the nutrient deficiencies but also will require measures to restore soil microbial functioning. Organic amendments that provide both nutrients and organic carbon may provide a means of accelerating recovery in fertility-impaired organic soils.