Seasonality of nitrogen balances in a Mediterranean climate watershed, Oregon, US
Citation:
Lin, J., J. Compton, S. Leibowitz, G. Mueller-Warrant, W. Matthews, S. Schoenholz, D. Evans, AND R. Coulombe. Seasonality of nitrogen balances in a Mediterranean climate watershed, Oregon, US. BIOGEOCHEMISTRY. Springer, New York, NY, 142(2):247-264, (2019). https://doi.org/10.1007/s10533-018-0532-0
Impact/Purpose:
The Calapooia River is a major tributary of the Willamette River Basin in Oregon, previously identified as having high N concentrations relative to many other Willamette tributaries. Here EPA scientists work with local agricultural researchers and managers to apply the best available local and regional data to assemble a comprehensive, seasonal N input-output budget for the Calapooia River Watershed (CRW) in 2008. Quantified inputs include agricultural, industrial, human and animal waste, and natural sources; exports include crop harvest and hydrologic export. Objectives of the study were to (1) quantify contributions of various N sources in the CRW and its subwatersheds for the year 2008 using locally-derived, crop-specific land-use information; (2) estimate fractional N export via stream export and crop harvest; (3) quantify amount of remainder and surplus N within the watershed; (4) study spatial and temporal variations in N inputs and exports, and (5) explore dominant factors that drive these variations. The study found that management to reduce N inputs to surface and groundwater will require improved nutrient management that carefully accounts for the seasonality of precipitation and N surplus remaining in the soil after harvest. This work is useful to numerous local environmental, public health and agricultural management entities.
Description:
We constructed a seasonal nitrogen (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 nitrogen 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.
URLs/Downloads:
DOI: Seasonality of nitrogen balances in a Mediterranean climate watershed, Oregon, US