Multi-Air Pollutant Impacts on Forage Yields: A Case Study of Alfalfa Responses in West Central Alberta, Canada

Sagar Krupa1, M. Nosal2, E. Ryl3 and A.H. Legge4

Under ambient conditions, the dynamics of multi-air pollutant exposures and chronic crop (yield) responses are stochastic by their nature. However, virtually all our knowledge of that subject is based on univariate studies in artificial field exposures. To address the consequent major limitation in our understanding, during 1998-2002, alfalfa yield responses to multiple gaseous air pollutants and climate parameters was examined under real world conditions using a cropping system as practiced by the local farmers. Ambient concentrations of sulfur and nitrogen dioxides at the study sites (emissions from oil and natural gas production) did not violate the National Ambient Air Quality Standards or Objectives in the US or in Canada. However, during 2002, one-hour average ozone levels exceeded 80 ppb (~twice the so-called current global background). Combining data from all study sites, years and the two harvests/year, 60% of the yearly alfalfa yields from harvest # 1 and 30% from # 2 were higher than their median value (+17 to +118%), while 40% from harvest # 1 and 70% from # 2 had yields lower than the median (-15 to -83%). Based on air quality and climatology, a multi-variant, growth season time series empirical forecasting model was applied and it correctly predicted 88% and 92% respectively of the time when alfalfa yields would be “low” or high. A global statistical model (“low” and “high” yield data combined) identified air quality (ozone + sulfur dioxide + oxides of nitrogen) as a major factor influencing alfalfa yields. The model could explain 68 to 76% of the variation in the combined yields, but only 43 to 57% in low alfalfa yields. Possible reasons for that appear to be the complex interactions resulting from, (1) low to moderate variations in air pollution concentrations, (2) significant genetic variability in the crop cultivar, (3) nutrient availability, (4) diseases and insects and (5) crop-weed competition. Air quality (ozone + sulfur dioxide + oxides of nitrogen) influenced ~50% of the accounted variation in all of the combined alfalfa yields with ozone accounting for one-half of it. The remaining 50% was due to variations in the climate and due to parameters that were not quantified (e.g., insect infestation). In summary, the results show the critical need for multi-variant ambient studies, as opposed to the continued reliance on univariate controlled exposure experiments and incomplete definition of the crop response surface. Currently the air quality-climate-forage model is being extended to other crops.

1University of Minnesota, St. Paul, MN, USA. Phone: 651-483-9218,E-mail:
2Statscon, Calgary, Alberta, Canada
3Agronomist, Seba Beach, Alberta, Canada
4Biosphere Solutions, Calgary, Alberta, Canada