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 of cereal rye shoot biomass, and were injected with two mg N kg 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. Although cereal rye shoot N declined, shoots became enriched with N, indicating fungal transfer of soil N to shoots. Soil inorganic N declined by an average of 5 kg N ha. 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