Continuum-removed absorption features estimate tropical savanna grass quality in situ

The remote sensing of grass quality as determined by nitrogen, phosphorous, potassium, calcium and magnesium concentration is critical for a better understanding of wildlife and livestock feeding patterns. Although remote sensing techniques have been proven useful to assess the concentration of foliar biochemicals under controlled laboratory conditions, more investigation is required to assess their capabilities at field level where inconsistent results have been obtained so far. We investigated the possibility of estimating the concentration of biochemicals in a savanna rangeland using spectral reflectance of five grass species. Canopy spectral measurements were taken in the field using a GER 3700 spectroradiometer. We tested the utility of using three methods: (i) continuum removed derivative reflectance (CRDR), (ii) band depth (BD) and (iii) band depth ratio (BDR) derived from continuum removed absorption features to estimate canopy N, P, K, Ca and Mg. Stepwise linear regression was used to select wavelengths from the absorption feature based methods. Using the training data set, the three methods could explain the variation in foliar nutrient concentration with R values ranging from 0.43 to 0.80. Results were highest from CRDR data, which yielded R values of 0.70, 0.80, 0.64, 0.50 and 0.68 for N, P, K, Ca and Mg, respectively. Predicting biochemicals on a test data set using regression models developed from a training data set resulted in R values ranging from 0.23 to 0.70 between the measured and predicted biochemicals. The method may be extended to data acquired by airborne and space borne imaging spectrometers to estimate and to ultimately map the concentration of macronutrients in tropical rangelands.