Nitrogen leaching from Douglas-fir forests after urea fertilization

Area of Science:

• Forestry
• Ecology
• Environmental Science

Background:

• Forest fertilization is a common practice to enhance timber production.
• Nitrogen (N) leaching after fertilization poses a risk to water quality.
• Understanding N cycling in fertilized forests is crucial for sustainable management.

Purpose of the Study:

• To quantify nitrogen leaching below the primary rooting zone in Douglas-fir forests after urea fertilization.
• To assess changes in nitrogen pools within trees, understory vegetation, and soil.
• To evaluate the environmental impact of forest fertilization on water quality.

Main Methods:

• Utilized tension lysimeters to measure nitrogen (N) leaching (NO(3)-N, NH(4)-N, dissolved organic N).
• Estimated soil water flux to quantify nutrient mobilization.
• Collected soil and vegetation samples at multiple time points (pre-fertilization, 1 and 6 months post-fertilization).

Main Results:

• Total N leaching beyond the primary rooting zone was 4.2 kg N ha(-1) in the year after fertilization, representing 2% of applied N.
• Peak nitrate-nitrogen (NO(3)-N) concentration leached was 0.2 mg NO(3)-N L(-1).
• Six months post-fertilization, 26% of applied N was found in overstory trees and 27% in the soil's O+A horizon.

Conclusions:

• Forest fertilization with urea resulted in small nitrogen leaching fluxes from the primary rooting zone in the first year.
• The majority of applied nitrogen was retained in the forest ecosystem (trees and soil).
• These findings suggest a low immediate risk of significant nitrogen pollution to ground and surface waters from urea fertilization in these forests.