Study to assess stable isotope mapping to predict forest retention of fertilizer nitrogen

The potential contribution of forest fertilization to nutrient loading in water bodies has received considerable attention from state and federal regulators. Watershed and site-based research is needed to address fertilizer nutrient fate questions but is costly and time-consuming to implement and difficult to extrapolate beyond the sites investigated.

Fertilizer fate models provide broader-level assessments but are limited by a lack of comprehensive field data for their development and testing. Indices of fertilizer nutrient retention could complement information from field and modeling studies and lead to more efficient and effective fertilizer use.

NCASI is supporting a new approach called stable isotope mapping that could provide an index of fertilizer nitrogen retention or leaching in forest stands. Isotope mapping is based on the natural variation of the two stable isotopes of nitrogen in forest ecosystems, 15N and 14N, which respectively comprise 0.4% and 99.6% of nitrogen occurring in nature. Very slight differences in the relative abundance of the two isotopes from one site to another result in distinct 15N isotopic signatures of plant foliage and soil. These signatures have been identified as useful integrators of ecosystem N dynamics and have been correlated with the potential for N loss. For example, higher foliar 15N signatures have been linked to historical N loss and could be valuable to predicting potential retention and loss of fertilizer nitrogen. Coupling this approach to the use of remote sensing could allow isotope mapping to serve as a precision silviculture tool to guide forest fertilization decisions.

In this study, isotope mapping will be applied to loblolly pine and Douglas-fir stands to (i) quantify the relationship between 15N signatures and forest ecosystem N retention and fertilizer response, and (ii) predict this relationship remotely using foliar spectral reflectance data from a portable spectroradiometer. The study, led by Drs. Brian D. Strahm and Valerie A. Thomas of the Department of Forest Resources and Environmental Conservation at Virginia Tech, will link to a project recently initiated by the National Science Foundation’s Center for Advanced Forestry Systems (CAFS). The CAFS project will utilize direct measurements of 15N retention in loblolly pine and Douglas-fir stands following applications of 15N-labelled fertilizer.  

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