Spectral response analysis: An indirect and non-destructive methodology for the chlorophyll quantification of biocrusts

Chlorophyll a concentration (Chla) is a well-proven proxy of biocrust development, photosynthetic organisms' status, and recovery monitoring after environmental disturbances. However, laboratory methods for the analysis of chlorophyll require destructive sampling and are expensive and time consuming. Indirect estimation of chlorophyll a by means of soil surface reflectance analysis has been demonstrated to be an accurate, cheap, and quick alternative for chlorophyll retrieval information, especially in plants. However, its application to biocrusts has yet to be harnessed. In this study we evaluated the potential of soil surface reflectance measurements for non-destructive Chla quantification over a range of biocrust types and soils. Our results revealed that fromthe different spectral transformation methods and techniques, the first derivative of the reflectance and the continuum removal were the most accurate for Chla retrieval. Normalized difference values in the red-edge region and common broadband indexes (e.g., normalized difference vegetation index (NDVI)) were also sensitive to changes in Chla. However, such approaches should be carefully adapted to each specific biocrust type. On the other hand, the combination of spectral measurements with non-linear random forest (RF) models provided very good fits (R > 0.94) with a mean root mean square error (RMSE) of about 6.5 μg/g soil, and alleviated the need for a specific calibration for each crust type, opening a wide range of opportunities to advance our knowledge of biocrust responses to ongoing global change and degradation processes from anthropogenic disturbance.This research was funded by the RESUCI (CGL2014-59946-R), DINCOS (CGL2016-78075-P), and REBIOARID (RTI2018-101921-B-I00) projects funded by the Spanish National Plan for Research and the European Union ERDF funds, and the project H2020-MSCA-RISE-GYPWORLD (funded by the European Union’s Horizon 2020 research and innovation program under the Marie Slodowska-Curie Grant Agreement No. 777803). Field sampling was conducted within the framework of the project “Cuantificación de flujos de carbono y agua en zonas áridas a partir de información spectral” founded by Aerial Platforms for Research-ICTS (INTA) throughout the campaign “Investigaciones de Altura.” J.R.R. was funded by the FPU predoctoral fellowship from the Educational, Culture and Sports Ministry of Spain (FPU14/05806). E.R.-C. was supported by the Juan de la Cierva incorporación fellowship (IJCI-2016-29274) and by the Hipatia postdoctoral fellowship funded by the University of Almería. B.R.-R. was supported by the foundation Tatiana Pérez de Guzmán el Bueno under its predoctoral fellowship program and S.C. was supported by the Hipatia postdoctoral fellowship funded by the University of Almería