Sensing nitrogen and phosphorous concentrations in vegetation across different scales

Nitrogen (N) and Phosphorus (P) are essential nutrients for plant growth and have been linked to physiological
processes at the leaf and canopy scales. At the global scale, N and P are also strongly linked to carbon assimilation
by vegetation. Vegetation models predicting terrestrial carbon uptake have therefore integrated the N cycle, and
some of the models include the P cycle as well. Accessing information about the N- and P- cycles is thus essential
but plant trait data for these cycles, e.g. canopy N and P concentrations are often lacking. Remote sensing methods
offer the possibility to estimate canopy N and P concentrations at different scales. Our research focuses on sensing
these traits across different scales, from in situ experiments (N and P measurements) up to regional (N measure-
ments) and continental (N measurements) scale using vegetation indices (VIs). At the local scale, we tested and
developed VIs for canopy N and canopy N to P ratio (N:P) detection on a grass species, Holcus lanatus, grown
under three ratios and two levels of N:P supply. Canopy N concentration and canopy N:P were highly correlated
(R2 = 0.5 – 0.7) with VIs calculated using both the narrow band spectra and the spectra resampled to the spectral
properties of six satellites sensors (Sentinel 2 - MSI, Sentinel 3 - OLCI, MODIS, Landsat 8 - OLI, RapidEye and
Worldview 4). At the regional scale, we used the MERIS Terrestrial chlorophyll Index (MTCI) Level 3 product
from MERIS aboard ENVISAT (ESA) to study the relationship with canopy N. We analyzed 846 Mediterranean
forest plots located in Catalonia (north eastern Iberian Peninsula). Canopy N concentration was correlated with
MTCI (R2 = 0.32). The results suggested a strong influence of the plant functional type (PFT) of the plots and the
relationship for the deciduous broadleaf forest (DBF) plots was stronger than for any other PFTs. We will expand
this research to the continental scale of Europe using more than 700 temperate forest plots from the ICP forests
database. As these plots were measured repeatedly over 20 years, this will allow us to study the temporal variation
of canopy N.