Geospatial Modeling of Nitrogen and Carbon Content and Stock in the Forest Litter Horizons Based on Sentinel-2 Multi-Seasonal Satellite Imagery

摘要

The capabilities of Sentinel-2 optical multispectral satellite data for modeling nitrogen (N) and carbon (C) contents, their ratio (C : N), and stocks in the litter horizons of forest soils were assessed. The study was conducted in the Bryansk Forest Nature Reserve and its buffer zone. The organic horizon samples were taken on 33 plots selected with due account for the tree species diversity of the reserve's forests. Two layers of the organic horizon-L and FH-were sampled separately. The main variables for geospatial modeling were derived from a time series of eight Sentinel-2 multi-seasonal satellite images. Basic terrain characteristics and pixel coordinates were also added to variables' stack. We used random forest to build regression models and the corresponding standard methods to assess their performance. The best results were obtained for the C : N ratio: the coefficient of determination R-2 = 0.71 with a scaled root-mean-square error RMSE = 12.5% in the L layer, and R-2 = 0.83 with RMSE = 10.6% in the FH layer. For other models, the values of R-2 ranged from 0.23 to 0.61, and the RMSE ranged from 15.8 to 48.6% with the least reliable results for the N and C stocks. Satellite-based variables were most informative for the contents of N and C, and, notably, for the C : N ratio. The most significant periods in the time series were early spring, summer, and snowy winter. To conclude, Sentinel-2 satellite imagery can be successfully used for estimation and mapping the contents and stocks of N and C in the forest soil organic horizon as a free and relevant alternative to thematic data on the species composition and related properties of stands.