Examining the effect of rotation sequence on biological nitrogen fixation of pulse crops

Nitrogen (N) is often the most limiting nutrient in prairie crop production and is applied in the greatest quantity. Including pulse crops in rotations has become a popular option due to their ability to form symbiotic relationships with dinitrogen-fixing bacteria. This relationship means pulse crops can acquire a large proportion of their N needs from biological N fixation (BNF). In previous studies, mixed results of rotation effects on a pulse crop’s ability to fix N were reported at Scott, SK, Swift Current SK, and from a greenhouse experiment using soils from Central Butte, SK. These results led to questioning if BNF is affected by a previous crop in a rotation. To address this question, research was conducted at multiple locations across Saskatchewan. The natural abundance 15N isotope dilution method was used to estimate BNF in pulse crops grown on oilseed and cereal stubble in the Brown, Dark Brown, and Black soil zones. Soil samples were collected from each rotation to characterize sites and identify soil physical, chemical, and microbiological properties that may have affected BNF in pulse crops. Additionally, a controlled environment experiment was performed to determine if stubble quality (i.e., wheat and canola) affected N-mineralization potential before and after a pulse crop was grown. In the field study, an interaction between site and stubble affected BNF, where pulse crops grown on cereal stubble generally had higher BNF except at Biggar in 2017, and at Davidson and Theodore in 2018; BNF in these pulse crops was higher when grown on oilseed stubble. Inorganic N and available P contents may have affected BNF at some locations. A persistent pattern was observed in microbial biomass carbon (C) and phospholipid fatty acid (PLFA) biomarker results, where levels of each were higher in soil from pulse crops grown on oilseed stubble at Davidson, Theodore, and Springside. Climatic conditions also may have affected BNF at each location, especially in 2018, as conditions were hotter and drier compared to historical averages. In the controlled environment study, BNF was not affected by soil or stubble; however, soil affected N acquisition and yields. A similar pattern to field results for PLFA biomarkers was observed, where total biomarkers were higher in oilseed stubbles and in the Black soil. Gross mineralization and nitrification rates were not affected by stubble before or after field pea was grown. Based on the variable results from the field and controlled environment studies, seeding pulse crops on oilseed stubble in the Brown, Dark Brown, or Black soil zones is not recommended