Nitrogen Fixation by Legume Green Manures: Effect of Species, Planting Time, and Termination Time.

There is renewed interest within conventional agriculture to identify practices that improve and sustain soil fertility. For dryland cropping systems of the northern Great Plains, the incorporation of nitrogen (N)-fixing legume green manure (LGM) crops as a partial-fallow replacement crop can offset N fertilizer use and provide a number of agronomic and ecological benefits. Successful adoption of LGM crops hinges on the balance between N gain and soil water storage for the succeeding crop. Thus, important management considerations for optimizing benefits of dryland LGM crops include selection of legume species, planting time, and termination time. The impact of these management practices on N fixation, however, has not been quantified for LGM crops grown in Montana. Nitrogen fixation was measured in a plot-scale field study with the following treatments: two legume species (field pea and lentil), two planting times (spring and summer) and three termination times (flower, intermediate, and pod). Two methods, ¹⁵N natural abundance and N difference, were used to quantify N fixation for each treatment combination. Results for the ¹⁵N natural abundance method for both spring and summer planting times were imprecise due to high intra-site ¹⁵N variability. Nitrogen difference results for the spring-planted crops suggest that the quantity of N fixed did not differ among the three termination timings for pea, yet N fixed by lentil was significantly higher (P<0.05) at pod termination (77 kg N ha^-1) than at flower (33 kg N ha^-1) and intermediate (45 kg N ha^-1) terminations. There was no significant difference in N fixed between spring and summer plantings terminated at flower for either pea or lentil. The same parameters are being measured for a second year of the study. Findings from this two-year study should provide information that will allow LGM growers to determine the optimum balance between the quantity of N fixed and crop water use.