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

Authors: WILLIAMS ALWYN - University Of Queensland; WELLS, M - University Of Minnesota; DICKEY, DAVID - North Carolina State University; HU, SHUIJIN - North Carolina State University; Maul, Jude; RASKIN, DANIEL - University Of Minnesota; REBERG-HORTON, S - North Carolina State University; Mirsky, Steven

Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/24/2019
Publication Date: 3/12/2019
Citation: Williams, A., Wells, M.S., Dickey, D.A., Hu, S., Maul, J.E., Raskin, D.T., Reberg-Horton, S.C., Mirsky, S.B. 2019. Establishing the relationship of soil nitrogen immobilization to cereal rye residues in a mulched system. Plant and Soil. 426:95-107.

Interpretive Summary: Cover crops are crops that are planted to cover the ground between cash crops, when the ground would otherwise be bare. Cover crops provide many potential benefits such as decreased soil erosion and decreased loss of nutrients to the environment (e.g., nutrient pollution). The extent of the benefits provided depends on the amount of biomass produced (i.e., bigger plants take up more residual soil nitrogen leftover from the cash crop) and the carbon and nitrogen ratios of the biomass; more nitrogen per unit carbon means the nitrogen is less likely to be immobilized in the soil by microbes and thus temporarily unavailable to a following cash crop. Cereal rye is the most commonly planted cover crop in the US because it is winter hardy, the seed is inexpensive, and it can provide multiple benefits including weed control. Cereal rye has the potential to produce large amounts of biomass with a high C:N ratio. Because cereal rye can have a high C:N ratio, it is also well-known to potentially cause soil nitrogen immobilization after the rye is killed in the spring. There are several mechanisms through which cereal rye may suppress weeds – its high C:N ratio and subsequent soil N immobilization may deprive weed seeds of a trigger needed for germination by decreasing spring soil N levels. The thick mulch produced by its biomass may also prevent light from reaching weed seeds, preventing germination, or the mulch may smother weeds. The purpose of this study was to a) determine the amount of N immobilized under varying amounts of rye biomass and b) determine the effect of N immobilization on redroot pigweed, a common agricultural weed. After the cereal rye was killed in the spring, varying levels of cereal rye biomass were placed in microplots along with the application of 15N, which can be distinguished from N found in the environment using lab tests. Redroot pigweed seeds were planted in these microplots. Decomposition of the cover crop biomass, location of the 15N in the system, and redroot pigweed germination were tracked through the soybean growing season. Cereal rye decomposed fastest at biomass levels equal to or more than 5,000 kg ha-1. Pigweed produced less biomass, and contained less N, no matter the level of cereal rye biomass present (so long as biomass was present). Ultimately the authors concluded that previously-documented soil N immobilization is likely mostly the result of root-based N (not tested in this study), rather than shoot-based N. This work provides a useful data point for guiding further research (i.e., researchers should run similar experiments using rye root tissue). This work also demonstrates the value of cereal rye for providing weed suppression, a major concern for farmers.

Technical Abstract: Soil nitrogen (N) immobilization from cover crop residues may help suppress weeds. We established a gradient of cereal rye shoot biomass to determine the magnitude of N immobilization and its effect on redroot pigweed (Amaranthus retroflexus L.). 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, 2,000, 5,000, 8,000, 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. Maximum rates of cereal rye residue decomposition were observed at =5,000 kg ha-1. Although cereal rye residue N declined, residues became enriched with 15N, indicating a fungal transfer of soil N to shoots. Soil inorganic N declined by an average of 5.0 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. 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.