Seasonality of nitrogen balances in a Mediterranean climate watershed, Oregon, US

Authors: LIN, JIAJIA - U.S. ENVIRONMENTAL PROTECTION AGENCY (EPA); COMPTON, JANA - U.S. ENVIRONMENTAL PROTECTION AGENCY (EPA); LEIBOWITZ, SCOTT - U.S. ENVIRONMENTAL PROTECTION AGENCY (EPA); MUELLER WARRANT, GEORGE; MATTHEWS, WILLIAM - OREGON DEPARTMENT OF AGRICULTURE; SCHOENHOLTZ, STEPHEN - VIRGINIA TECH; EVANS, DANIEL - PLYMOUTH STATE UNIVERSITY; COULOMBE, ROB - DYNAMAC CORPORATION

Submitted to: Biogeochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/4/2018
Publication Date: 1/31/2019
Citation: Lin, J., Compton, J., Leibowitz, S.G., Mueller Warrant, G.W., Matthews, W., Schoenholtz, S.H., Evans, D.M., Coulombe, R.A. 2019. Seasonality of nitrogen balances in a Mediterranean climate watershed, Oregon, US. Biogeochemistry. 142(2):247-264. https://doi.org/10.1007/s10533-018-0532-0.
DOI: https://doi.org/10.1007/s10533-018-0532-0

Interpretive Summary: In the Calapooia River watershed, fertilization at an average rate of 80 kg N per ha per year accounted for 90% of the total N input. Nitrogen input was 50% in winter, 24% in spring, 23% in fall, and < 4% in summer, driven by fertilizer input. Stream loss of N was strongly correlated with total N input and net N input (r2 = 0.7 and 0.6, respectively). On average, annual stream export of N in the Calapooia River watershed was 19% of total N input (r2 = 0.7), and 31% of the net N input. Most of riverine export occurred in the wet winter, and the primary time of fertilizer application was mid-February to mid-March. Fractional N export increase exponentially with increasing runoff, partially explaining the high fractional export in CRW in comparison to other US watersheds. The annual crop harvest was equivalent to 41% of total N input, with 92% of the crop harvest happening in the summer. Retention of N was positively related to N inputs. Linear regression analysis of seasonal riverine yield showed that winter fertilization alone explained 60% of the spatial variation in winter stream yield, and continued to affect N export in later seasons: a portion of N entered streams in the spring was legacy from the previous winter. Also, an equivalent of 10% of winter plus spring fertilizer was exported via streams in the following fall. The analysis also revealed that a net removal of N in spring and summer via export and harvest did not prevent high fall and winter N loss to streams. The annual export positively correlated with summer crop yield, indicating intensive cultivation and management as well as strongly seasonal rainfall drives high loss of added N and the relatively low N use efficiency (41%) in the Calapooia River watershed. Disconnection between crop N requirements, fertilizer applications and hydrologic N losses creates challenges for N management.

Technical Abstract: We constructed a seasonal N budget for the year 2008 in the Calapooia River Watershed (CRW), an agriculturally dominated tributary of the Willamette River (Oregon, U.S.) under Mediterranean climate. Synthetic fertilizer application to agricultural land (dominated by grass seed crops) was the source of 90% of total N input to the CRW. Over 70% of the stream N export occurred during the wet winter, the primary time of fertilization and precipitation, and the lowest export occurred in the dry summer. Averaging across all 58 tributary subwatersheds, 19% of annual N inputs were exported by streams, and 41% by crop harvest. Regression analysis of seasonal stream export showed that winter fertilization was associated with 60% of the spatial variation in winter stream export, and this fertilizer continued to affect N export in later seasons. Annual N inputs were highly correlated with crop harvest N (r2=0.98), however, seasonal dynamics in N inputs and losses produced relatively low overall nutrient use efficiency (41%), suggesting that hydrologic factors may constrain improvements in nutrient management. The peak stream N export during fall and early winter creates challenges to reducing N losses to groundwater and surface waters. Construction of a seasonal N budget illustrated that the period of greatest N loss is disconnected from the period of greatest crop N uptake. Management practices that serve to reduce the N remaining in the system at the end of the growing season and prior to the fall and winter rains should be explored to reduce stream N export.