Establishing the relationship of soil nitrogen immobilization to cereal rye residues in a mulched system

Background and aims: Soil nitrogen (N) immobilization from cover crop residues may help suppress weeds. We established a gradient of cereal rye shoot biomass to determine the extent that soil N can be immobilized and its effect on redroot pigweed (Amaranthus retroflexus L.). Methods: A microplot study was conducted in no-till cereal rye (Secale cereale L.)—soybean (Glycine max L. (Merr.)) systems at two sites in eastern USA. Microplots received 0, 2000, 5000, 8000, 12,000 or 15,000 kg ha⁻¹ of cereal rye shoot biomass, and were injected with two mg ¹⁵N kg⁻¹ soil 5 cm below the soil surface. Pigweeds were sown and allowed to germinate. Results: Maximum rates of cereal rye shoot decomposition were observed at ≥5000 kg ha⁻¹. Although cereal rye shoot N declined, shoots became enriched with ¹⁵N, indicating fungal transfer of soil N to shoots. Soil inorganic N declined by an average of 5 kg N ha⁻¹. Pigweed tissue N and biomass were reduced in the presence of cereal rye. The magnitude of pigweed N reduction was similar across all shoot application rates. Conclusions: We found weak evidence for a cereal rye shoot-based N immobilization mechanism of weed suppression. Our results indicate N immobilization may be primarily due to root residues.