Enhanced efficiency fertilizers (EEFs) do not increase nitrogen retention in Pacific Northwest Douglas-fir forest soils four weeks post-fertilization

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Enhanced efficiency fertilizers (EEFs) are used to minimize gaseous losses of applied nitrogen (N). Higher N retention resulting in increased uptake by plants is intended to offset the additional cost of these EEFs. Pacific Northwest Douglas-fir forests are N limited and assumed to have high N retention rates. Assuming N retention rates are high can lead to the over application of fertilizers which is both economically inefficient and detrimental to ecosystem health. We tested the hypothesis that EEF-fertilized plots have the highest ecosystem N recovery rates by tracing four urea-based fertilizers for four weeks after application in five commercially managed Douglas-fir forests in Oregon and Washington. Three EEFs, Environmentally Smart Nitrogen (ESN), N-(n-butyl) (NBPT), and Arborite coated urea fertilizer (CUF), were tested along with unformulated urea granules. These three particular EEFs are designed to reduce volatile losses of ammonia (NH3(gas)). Fertilizers were enriched with 15N, which facilitates the tracing of N as it moves between ecosystem pools. Retention rates were calculated for the forest floor and mineral soil to a depth of 20 cm. Retention rates were not significantly improved with the use of EEFs. On average, 30.2% of the applied N was lost from the system after four weeks. The lack of improvement in N retention with EEFs suggests that volatilization may not be a substantial loss pathway at these sites and therefore the application of these higher cost EEFs is not warranted.