Research Objective

Project Description

To estimate amounts of N2 fixed by free-living organisms in soil under annual tillage and minimum-tillage; to determine how soil zone and soil texture affect free-living N2 fixation; to determine how community structure and activity of free-living N2 fixing bacteria is affected by tillage management; to evaluate if/how annual climate affects structure and activity of free-living N2 fixing bacteria; to evaluate how soil moisture affects free-living N2-fixing community structure.

The idea for this study arose from reports from a number of past nitrogen (N) balance studies conducted in Saskatchewan and Alberta. These studies consider all N inputs (fertilizer, crop residues, etc) and outputs (grain removal, soil erosion, etc) and calculated whether the soil has a net gain or a net loss of N. In all of these studies, a net loss of N from the system was predicted. However, when N was measured directly in the topsoil, a gain in N in the system was found. The gains were proportionate to cropping frequency – higher gains were found in continuously cropped systems, which generally are no-till, direct seeded systems. One input that is not considered in the N balance studies is N fixed by free living microorganisms. Unlike symbiotic N fixation that occurs in legume nodules, free-living microorganisms do not need a plant to fix N.


This study measured free-living nitrogen fixation and the microorganisms responsible, in soils that have been under no-tillage management for varying amounts of time. In contrast to what we expected, the amount of time in no-tillage management did not affect the amount of free-living N fixation, and rates of N fixation were very low, ranging from 0.9 to 2.9 kg N ha-1 y-1. Only after 50 years of no-tillage management did we see an indication that these soils supported higher N fixation rates. Indeed, the highest N fixation rate measured in an agricultural field was measured in an annually tilled organic field not in a no-tillage field. The community of organisms responsible for N fixation was very diverse. Soils immediately adjacent to one another but under different management had different communities of organisms. The diversity of microorganisms capable of fixing N means that the soils are capable of responding to environmental fluxes. While these small sustained levels of N fixation are undoubtedly important in natural ecosystems, they should have no impact on fertility decisions made by farmers. 

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