Nutrition, moisture and rhizobial strain influence isotopic fractionation during N 2, fixation in pasture legumes

Abstract

White clover ( Trifolium repens L.) and red clover ( T. pratense L.) were grown in a medium free of combined nitrogen (N-free) with Rhizobium leguminosarum by. trifolii strain PDD 2668 or a mixture of field isolates of rhizobia. The natural abundance of 15N was always lower in shoots than in roots and nodules. Whole plant N was depleted in 15N (−0.44 and −0.15% for white and red clovers, respectively) and unaffected by inoculation treatment. However, shoots were more depleted in 15N and roots and nodules were more enriched with the field isolates. With white clover, other factors examined were molybdenum (Mo) and phosphorus nutrition (low vs high levels), watering (regular vs less frequent) and water-logging. Whole plant N was more depleted in 15N with low Mo nutrition or with less frequent watering. Although these effects were small, they have not been accounted for in the past and indicate reduced precision in estimates of isotopic fractionation, and therefore of N 2 fixation using natural 15N abundance.