PS 40-100 - Tile Drainage and Greenhouse Gas Emissions – Preliminary Results in a Ryegrass Field

Abstract: Tile drainage is a common practice in seed production fields in the Willamette Valley, OR. By lowering the water table, tile drainage aids crop productivity by increasing rooting zone depth, expanding crop choices, and allowing earlier field access. Changes in soil hydrologic and plant physiological traits will alter microbial activities and nutrient cycling. Soil denitrification is particularly sensitive to shifts in water status. When O₂ is limiting, nitrate-N is transformed by soil microbes during the process of denitrification to various gases, mainly nitrous oxide (N₂O) and dinitrogen gas (N₂). Complete denitrification to N₂ is preferred because N₂ is an inert gas and makes up nearly 80% of our atmosphere. In contrast, N₂O is a potent greenhouse gas (GHG), with nearly 300x the heat trapping power of CO₂. Understanding how agronomic practices, such as tile drainage, affect N₂O release from soils is an important step in combating climate change. The overall objective of our study was to determine how tile drainage impacts CO₂ and N₂O fluxes from soil in an annual ryegrass field, where three tile drainage treatments had previously been established. The three treatments were: installation of new tile drainage (< 6 years), old tile drainage (installed > 15 years ago), and untiled (no history of tile drainage). We hypothesized that the poorly drained (untiled) field would have greater N₂O production compared to the tile drained fields by increasing the duration that soil is anaerobic, decreasing rooting depth, and nitrate-N uptake. Three transects were established in each treatment in areas of the field under similar soil type (Vertic Albaqualfs, which are naturally poorly drained). Three soil collars were installed in each transect in each treatment. Soil CO₂ and N₂O fluxes were measured using LiCOR 870 and 7829 field gas analyzers, respectively. Fluxes were measured from April-June 2022 just prior to harvest and from September 2022 until present. Preliminary analyses indicate significant CO₂ fluxes occur during fall/winter months despite relatively cool temperatures, which suggests microbial activity is relatively high during this period. Despite wet conditions during fall 2022-winter 2023, N₂O fluxes were low compared to early spring 2022 when nitrogen fertilization likely resulted in pulses of N₂O. In untiled fields, lower amounts of N₂O fluxes were detected, suggesting that complete denitrification is occurring in soils that are presumably more anaerobic. Ongoing sampling campaigns will continue to refine these trends to further understand how artificial drainage contributes to or mitigates GHG emissions.