The incorporation of legume cover crops into annual grain rotations remains limited, despite extensive evidence that they can reduce negative environmental impacts of agroecosystems while maintaining crop yields. Diversified grain rotations that include a winter cereal have a unique niche for interseeding cover crops. To understand how man- agement-driven soil fertility differences and inter- seeding with grains influenced red clover (Trifolium pratense)N2 fixation, we estimated biological N2 fixation (BNF) in 2006 and 2007, using the 15N natural abundance method across 15 farm fields characterized based on the reliance on BNF derived N inputs as a fraction of total N inputs. Plant treatments included winter grain with and without interseeded red clover, monoculture clover, mono- culture orchardgrass (Dactylis glomerata), and clo- ver-orchardgrass mixtures. Fields with a history of legume-based management had larger labile soil nitrogen pools and lower soil P levels. Orchardgrass biomass was positively correlated with the manage- ment-induced N fertility gradient, but we did not detect any relationship between soil N availability and clover N2 fixation. Interseeding clover with a winter cereal did not alter winter grain yield, however, clover production was lower during the establishment year when interseeded with taller winter grain varieties, most likely due to competition for light. Interseeding clover increased the % N from fixation relative to the monoculture clover (72% vs. 63%, respectively) and the average total N2 fixed at the end of the first growing season (57 vs. 47 kg N ha-1, respectively). Similar principles could be applied to develop more cash crop-cover crop complementary pairings that provide both an annual grain harvest and legume cover crop benefits.
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