Incorporation of Leaf Nitrogen Observations for
Biochemical and Environmental
Modeling of Photosynthesis and Evapotranspiration
E. Boegh, B. Gjetterman, P. Abrahamsen, S. Hansen, N. Jensen, K. Schelde, H.
Soegaard, A. Thomsen
Lack of knowledge concerning regional and global nitrogen (N) availability and
its impact on CO2 sequestration processes complicates the modeling of carbon
cycle feedbacks to climate change. The use of remote sensing constitutes a
valuable data source to quantify and investigate impacts of bulk leaf N
contents, however information on the vertical leaf N distribution and its
relation to photosynthetic (Rubisco) capacity should also be known to quantify
leaf N impacts on canopy photosynthesis. In this study, impacts of the amount
and vertical distribution of leaf N contents on canopy photosynthesis were
investigated by combining field measurements and photosynthesis modelling.
While most canopy photosynthesis models assume an exponential vertical profile
of leaf N contents in the canopy, the field measurements showed that
well-fertilized fields may have a uniform or exponential profile, and senescent
canopies have reduced levels of N contents in upper leaves. The sensitivity of
simulated canopy photosynthesis to the different (observed) N profiles was
examined using a multi-layer sun/shade biochemically based photosynthesis model
and found to be important; ie. for a well-fertilized barley field, the use of
exponential instead of uniform vertical N profiles increased the annual
assimilate by 12 %. Perspectives for remote sensing based estimation of leaf N
contents for photosynthesis modeling are finally discussed.