Abstract
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 from the 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 (R2 > 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.
Highlights
Water scarcity restricts vegetation growth and productivity in most desert regions around the world
The highest values corresponded to cyanobacteria inoculations on soils from Las Amoladeras that showed very similar values to those found in natural cyanobacterial biocrust communities, whereas the artificially induced cyanoRbemaocteteSernisa. l20b1i9o, 1c1r,ux sFtOsRoPnEEtRhReEqVuIEaWrry substrates showed the lowest Chla amongst all th8eosfa2m4 ples analyzed in this study (Figure 2)
When the different biocrust communities were analyzed separately, we found that the correlation patterns between the surface reflectance, continuum removal (CR), and and Chla strongly varied amongst the biocrust communities (Figure 7a,b)
Summary
Water scarcity restricts vegetation growth and productivity in most desert regions around the world. Mainly dominated by photosynthesizing cyanobacteria, algae, lichens, or bryophytes that can survive during long drought periods in a dormant state and rapidly become active after small water pulses [2] These traits allow biocrusts to fix a large amount of atmospheric carbon and nitrogen [3,4], which becomes incorporated within the upper layer of the soil to be used by heterotrophic fungi, bacteria, archaea [5], and other soil surface inhabitants [6,7]. In a similar way to wet chemical methods used for the extraction of chlorophyll a from plant tissue, most of these techniques require extraction in a solvent followed by spectrophotometric determinations of the chlorophyll-related absorbance peaks in the supernatant, and conversion from absorbance to concentrations using standard published equations [25,26,27] Though accurate, these laboratory methods require destructive field sampling and are expensive and time consuming. Biocrusts have been described as one of the main photosynthetic communities in dryland regions around the world [30], there exist only a few rough estimations of their contribution to the total photosynthetic biomass pool at regional [31] or global scales [32,33,34] and these estimates rarely account for seasonal dynamics in biocrust biomass
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