Dryland Winter Wheat-Winter Pea Cover Crop Decreased Nitrate Leaching Compared to Winter Wheat-Fallow: Evidence from 15N natural abundance.

Authors: Gollany, Hero; Polumsky, Robert - Wayne

Submitted to: ASA-CSSA-SSSA Annual Meeting Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: 11/11/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: A widely accepted belief was that nitrate (NO3-) leaching didn't occur in the 12-16” precipitation zone of dryland winter wheat-fallow (WW-F) rotation with high synthetic nitrogen (N) fertilizer application rates in semiarid Eastern Oregon, U.S. The common belief that there was no nitrate (NO3-) leaching under dryland conditions. The objectives of this study were to determine: 1) nitrate leaching under dryland winter wheat-fallow rotation, 2) the effect of a winter pea cover crop on NO3- leaching, and 3) wheat yields and protein content at five N fertilizer rates. A long-term WW-F under no-till (NT) experiment was established in 1982 (NTA). A second WW-F under NT was established in 1997 (NTB) and converted to a winter wheat-winter pea cover crop (WW-WP) in 2010. Each experiment had four replications of five N fertilizer rates (0, 40, 80, 120, and 160 Ib/ac/yr) as subplots. Both phases of the rotation were present every year. In 2017, nitrogen tracer (15N) was applied to a 3.28 ft2 subplot of each plot at the end of April. By the end of the season in 2017, the 15N tracer had moved below 24” depth across each N inputs with or without cover crop. The 15N tracer moved > 5 ft depth by the end of season in 2019. Three years 15N tracer recoveries were 57.5% and 32.7% in the WW-WP and WW-F, respectively. Wheat grain protein content reached 10.5% in the WW-WP at ~ 61 lb N/ac, while in the WW-F, it needed ~86 lb N/ac application rate before reaching 10.5% protein content. Nitrogen contribution from pea cover crop could reduce N fertilizer application rate and carbon fertilizer footprint while reducing NO3- leaching and improving groundwater quality.

Technical Abstract: A widely accepted belief was that nitrate (NO3-) leaching didn't occur in the dryland winter wheat (Triticum aestivum L.)-fallow (WW-F) rotation with high synthetic nitrogen (N) fertilizer application rates in semiarid Eastern Oregon, U.S. The objectives of this study were to determine: 1) nitrate leaching under dryland winter wheat-fallow rotation, 2) the effect of a winter pea (Pisum sativum L.) cover crop on NO3- leaching, and 3) wheat yields and protein content at five N fertilizer rates. A long-term WW-F under no-till (NT) experiment was established in 1982 (NTA). A second WW-F under NT was established in 1997 (NTB) and converted to a winter wheat-winter pea cover crop (WW-WP) in 2010. Each experiment had four replications of five N fertilizer rates (0, 45, 90, 135, and 180 kg/ha/yr) as subplots. Both phases of the rotation were present every year. In 2017, 15N was applied to a 1.0 m2 subplot of each plot at the end of April. By the end of the season in 2017, the 15N had moved below 60 cm depth across each N treatments with or without cover crop. The 15N moved below 120 cm depth by 2019. Three years 15N recoveries were 57.5% and 32.7% in the WW-WP and WW-F, respectively. In 2022, average wheat grain yields in the NTB increased by 0.35, 1.28, 0.80, and 0.20 Mg/ha with cover crop compared to the NTA in the 0, 45, 90, and 135 kg N/ha fertilizer rates, respectively, although we had 8 out of 12 years below-average precipitation. Wheat grain protein content reached 10.5% in the WW-WP at ~68 kg N/ha compared with ~96 kg N/ha in the WW-F. Nitrogen contribution from pea cover crops could reduce N fertilizer application rates and carbon fertilizer footprint while reducing NO3--leaching and improving groundwater quality.